[[websocket]]
= WebSocket Support
This part of the reference documentation covers Spring Framework's support for
WebSocket-style messaging in web applications including use of STOMP as an
application level WebSocket sub-protocol.

<<websocket-intro>> establishes a frame of mind in which to think about
WebSocket, covering adoption challenges, design considerations, and thoughts on
when it is a good fit.

<<websocket-server>> reviews the Spring WebSocket API on the server-side, while
<<websocket-fallback>> explains the SockJS protocol and shows how to configure
and use it.

<<websocket-stomp-overview>> introduces the STOMP messaging protocol.
<<websocket-stomp-enable>> demonstrates how to configure STOMP support in Spring.
<<websocket-stomp-handle-annotations>> and the following sections explain how to
write annotated message handling methods, send messages, choose message broker
options, as well as work with the special "user" destinations. Finally,
<<websocket-stomp-testing>> lists three approaches to testing STOMP/WebSocket
applications.



[[websocket-intro]]
== Introduction
The WebSocket protocol http://tools.ietf.org/html/rfc6455[RFC 6455] defines an important
new capability for web applications: full-duplex, two-way communication between client
and server. It is an exciting new capability on the heels of a long history of
techniques to make the web more interactive including Java Applets, XMLHttpRequest,
Adobe Flash, ActiveXObject, various Comet techniques, server-sent events, and others.

A proper introduction to the WebSocket protocol is beyond the scope of this
document. At a minimum however it's important to understand that HTTP is used only for
the initial handshake, which relies on a mechanism built into HTTP to request
a protocol upgrade (or in this case a protocol switch) to which the server can respond with
HTTP status 101 (switching protocols) if it agrees. Assuming the handshake succeeds
the TCP socket underlying the HTTP upgrade request remains open and both client and
server can use it to send messages to each other.

Spring Framework 4 includes a new `spring-websocket` module with comprehensive
WebSocket support. It is compatible with the Java WebSocket API standard
(http://jcp.org/en/jsr/detail?id=356[JSR-356])
and also provides additional value-add as explained in the rest of the introduction.



[[websocket-into-fallback-options]]
=== WebSocket Fallback Options
An important challenge to adoption is the lack of support for WebSocket in some
browsers. Notably the first Internet Explorer version to support WebSocket is
version 10 (see http://caniuse.com/websockets for support by browser versions).
Furthermore, some restrictive proxies may be configured in ways that either
preclude the attempt to do an HTTP upgrade or otherwise break connection after
some time because it has remained opened for too long. A good overview on this
topic from Peter Lubbers is available in the InfoQ article
http://www.infoq.com/articles/Web-Sockets-Proxy-Servers["How HTML5 Web Sockets Interact With Proxy Servers"].

Therefore to build a WebSocket application today, fallback options are required in
order to simulate the WebSocket API where necessary. The Spring Framework provides
such transparent fallback options based on the https://github.com/sockjs/sockjs-protocol[SockJS protocol].
These options can be enabled through configuration and do not require modifying the
application otherwise.



[[websocket-intro-architecture]]
=== A Messaging Architecture
Aside from short-to-midterm adoption challenges, using WebSocket
brings up important design considerations that are important to recognize
early on, especially in contrast to what we know about building web applications today.

Today REST is a widely accepted, understood, and supported
architecture for building web applications. It is an architecture that relies
on having many URLs (__nouns__), a handful of HTTP methods (__verbs__), and
other principles such as using hypermedia (__links__), remaining stateless, etc.

By contrast a WebSocket application may use a single URL only for the
initial HTTP handshake. All messages thereafter share and flow on the
same TCP connection. This points to an entirely different, asynchronous,
event-driven, messaging architecture. One that is much closer
to traditional messaging applications (e.g. JMS, AMQP).

Spring Framework 4 includes a new `spring-messaging` module with key
abstractions from the
http://projects.spring.io/spring-integration/[Spring Integration] project
such as `Message`, `MessageChannel`, `MessageHandler`, and others that can serve as
a foundation for such a messaging architecture. The module also includes a
set of annotations for mapping messages to methods, similar to the Spring MVC
annotation based programming model.



[[websocket-intro-sub-protocol]]
=== Sub-Protocol Support in WebSocket
WebSocket does imply a __messaging architecture__ but does not mandate the
use of any specific __messaging protocol__. It is a very thin layer over TCP
that transforms a stream of bytes into a stream of messages
(either text or binary) and not much more. It is up to applications
to interpret the meaning of a message.

Unlike HTTP, which is an application-level protocol, in the WebSocket protocol
there is simply not enough information in an incoming message for a framework
or container to know how to route it or process it. Therefore WebSocket is arguably
too low level for anything but a very trivial application. It can be done, but
it will likely lead to creating a framework on top. This is comparable to how
most web applications today are written using a web framework rather than the
Servlet API alone.

For this reason the WebSocket RFC defines the use of
http://tools.ietf.org/html/rfc6455#section-1.9[sub-protocols].
During the handshake, the client and server can use the header
`Sec-WebSocket-Protocol` to agree on a sub-protocol, i.e. a higher, application-level
protocol to use. The use of a sub-protocol is not required, but
even if not used, applications will still need to choose a message
format that both the client and server can understand. That format can be custom,
framework-specific, or a standard messaging protocol.

The Spring Framework provides support for using
http://stomp.github.io/stomp-specification-1.2.html#Abstract[STOMP] -- a simple, messaging protocol
originally created for use in scripting languages with frames inspired
by HTTP. STOMP is widely supported and well suited for use over
WebSocket and over the web.



[[websocket-intro-when-to-use]]
=== Should I Use WebSocket?
With all the design considerations surrounding the use of WebSocket, it is
reasonable to ask, "When is it appropriate to use?".

The best fit for WebSocket is in web applications where the client and
server need to exchange events at high frequency and with low latency. Prime
candidates include, but are not limited to, applications in finance, games,
collaboration, and others. Such applications are both very sensitive to time
delays and also need to exchange a wide variety of messages at a high
frequency.

For other application types, however, this may not be the case.
For example, a news or social feed that shows breaking news as it becomes
available may be perfectly okay with simple polling once every few minutes.
Here latency is important, but it is acceptable if the news takes a
few minutes to appear.

Even in cases where latency is crucial, if the volume of messages is
relatively low (e.g. monitoring network failures) the use of
https://spring.io/blog/2012/05/08/spring-mvc-3-2-preview-techniques-for-real-time-updates[long polling]
should be considered as a relatively simple alternative that
works reliably and is comparable in terms of efficiency (again assuming the volume of
messages is relatively low).

It is the combination of both low latency and high frequency of messages that can make
the use of the WebSocket protocol critical. Even in such applications,
the choice remains whether all client-server
communication should be done through WebSocket messages as opposed to using
HTTP and REST. The answer is going to vary by application; however, it is likely
that some functionality may be exposed over both WebSocket and as a REST API in
order to provide clients with alternatives. Furthermore, a REST API call may need
to broadcast a message to interested clients connected via WebSocket.

The Spring Framework allows `@Controller` and `@RestController` classes to have both
HTTP request handling and WebSocket message handling methods.
Furthermore, a Spring MVC request handling method, or any application
method for that matter, can easily broadcast a message to all interested
WebSocket clients or to a specific user.




[[websocket-server]]
== WebSocket API
The Spring Framework provides a WebSocket API designed to adapt to various WebSocket engines.
Currently the list includes WebSocket runtimes such as Tomcat 7.0.47+, Jetty 9.1+,
GlassFish 4.1+, WebLogic 12.1.3+, and Undertow 1.0+ (and WildFly 8.0+). Additional support
may be added as more WebSocket runtimes become available.

[NOTE]
====
As explained in the <<websocket-intro-sub-protocol,introduction>>, direct use of a
WebSocket API is too low level for applications -- until assumptions are made about the
format of a message there is little a framework can do to interpret messages or route
them via annotations. This is why applications should consider using a sub-protocol
and Spring's <<websocket-stomp,STOMP over WebSocket>> support.

When using a higher level protocol, the details of the WebSocket API become less
relevant, much like the details of TCP communication are not exposed to applications
when using HTTP. Nevertheless this section covers the details of using WebSocket
directly.
====



[[websocket-server-handler]]
=== Create and Configure a WebSocketHandler
Creating a WebSocket server is as simple as implementing `WebSocketHandler` or more
likely extending either `TextWebSocketHandler` or `BinaryWebSocketHandler`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.socket.WebSocketHandler;
	import org.springframework.web.socket.WebSocketSession;
	import org.springframework.web.socket.TextMessage;

	public class MyHandler extends TextWebSocketHandler {

		@Override
		public void handleTextMessage(WebSocketSession session, TextMessage message) {
			// ...
		}

	}
----

There is dedicated WebSocket Java-config and XML namespace support for mapping the above
WebSocket handler to a specific URL:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.socket.config.annotation.EnableWebSocket;
	import org.springframework.web.socket.config.annotation.WebSocketConfigurer;
	import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry;

	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(myHandler(), "/myHandler");
		}

		@Bean
		public WebSocketHandler myHandler() {
			return new MyHandler();
		}

	}
----

XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers>
			<websocket:mapping path="/myHandler" handler="myHandler"/>
		</websocket:handlers>

		<bean id="myHandler" class="org.springframework.samples.MyHandler"/>

	</beans>
----

The above is for use in Spring MVC applications and should be included in the
configuration of a <<mvc-servlet,DispatcherServlet>>. However, Spring's WebSocket
support does not depend on Spring MVC. It is relatively simple to integrate a `WebSocketHandler`
into other HTTP serving environments with the help of
{javadoc-baseurl}/org/springframework/web/socket/server/support/WebSocketHttpRequestHandler.html[WebSocketHttpRequestHandler].



[[websocket-server-handshake]]
=== Customizing the WebSocket Handshake
The easiest way to customize the initial HTTP WebSocket handshake request is through
a `HandshakeInterceptor`, which exposes "before" and "after" the handshake methods.
Such an interceptor can be used to preclude the handshake or to make any attributes
available to the `WebSocketSession`. For example, there is a built-in interceptor
for passing HTTP session attributes to the WebSocket session:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(new MyHandler(), "/myHandler")
				.addInterceptors(new HttpSessionHandshakeInterceptor());
		}

	}
----

And the XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers>
			<websocket:mapping path="/myHandler" handler="myHandler"/>
			<websocket:handshake-interceptors>
				<bean class="org.springframework.web.socket.server.support.HttpSessionHandshakeInterceptor"/>
			</websocket:handshake-interceptors>
		</websocket:handlers>

		<bean id="myHandler" class="org.springframework.samples.MyHandler"/>

	</beans>
----

A more advanced option is to extend the `DefaultHandshakeHandler` that performs
the steps of the WebSocket handshake, including validating the client origin,
negotiating a sub-protocol, and others. An application may also need to use this
option if it needs to configure a custom `RequestUpgradeStrategy` in order to
adapt to a WebSocket server engine and version that is not yet supported
(also see <<websocket-server-deployment>> for more on this subject).
Both the Java-config and XML namespace make it possible to configure a custom
`HandshakeHandler`.



[[websocket-server-decorators]]
=== WebSocketHandler Decoration
Spring provides a `WebSocketHandlerDecorator` base class that can be used to decorate
a `WebSocketHandler` with additional behavior. Logging and exception handling
implementations are provided and added by default when using the WebSocket Java-config
or XML namespace. The `ExceptionWebSocketHandlerDecorator` catches all uncaught
exceptions arising from any WebSocketHandler method and closes the WebSocket
session with status `1011` that indicates a server error.



[[websocket-server-deployment]]
=== Deployment Considerations
The Spring WebSocket API is easy to integrate into a Spring MVC application where
the `DispatcherServlet` serves both HTTP WebSocket handshake as well as other
HTTP requests. It is also easy to integrate into other HTTP processing scenarios
by invoking `WebSocketHttpRequestHandler`. This is convenient and easy to
understand. However, special considerations apply with regards to JSR-356 runtimes.

The Java WebSocket API (JSR-356) provides two deployment mechanisms. The first
involves a Servlet container classpath scan (Servlet 3 feature) at startup; and
the other is a registration API to use at Servlet container initialization.
Neither of these mechanism makes it possible to use a single "front controller"
for all HTTP processing -- including WebSocket handshake and all other HTTP
requests -- such as Spring MVC's `DispatcherServlet`.

This is a significant limitation of JSR-356 that Spring's WebSocket support
addresses by providing a server-specific `RequestUpgradeStrategy` even when
running in a JSR-356 runtime.

[NOTE]
====
A request to overcome the above limitation in the Java WebSocket API has been
created and can be followed at
https://java.net/jira/browse/WEBSOCKET_SPEC-211[WEBSOCKET_SPEC-211].
Also note that Tomcat and Jetty already provide native API alternatives that
makes it easy to overcome the limitation. We are hopeful that more servers
will follow their example regardless of when it is addressed in the
Java WebSocket API.
====

A secondary consideration is that Servlet containers with JSR-356 support are expected
to perform a `ServletContainerInitializer` (SCI) scan that can slow down application
startup, in some cases dramatically. If a significant impact is observed after an
upgrade to a Servlet container version with JSR-356 support, it should
be possible to selectively enable or disable web fragments (and SCI scanning)
through the use of the `<absolute-ordering />` element in `web.xml`:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<web-app xmlns="http://java.sun.com/xml/ns/javaee"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://java.sun.com/xml/ns/javaee
			http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
		version="3.0">

		<absolute-ordering/>

	</web-app>
----

You can then selectively enable web fragments by name, such as Spring's own
`SpringServletContainerInitializer` that provides support for the Servlet 3
Java initialization API, if required:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<web-app xmlns="http://java.sun.com/xml/ns/javaee"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://java.sun.com/xml/ns/javaee
			http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
		version="3.0">

		<absolute-ordering>
			<name>spring_web</name>
		</absolute-ordering>

	</web-app>
----

[[websocket-server-runtime-configuration]]
=== Configuring the WebSocket Engine

Each underlying WebSocket engine exposes configuration properties that control
runtime characteristics such as the size of message buffer sizes, idle timeout,
and others.

For Tomcat, WildFly, and GlassFish add a `ServletServerContainerFactoryBean` to your
WebSocket Java config:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Bean
		public ServletServerContainerFactoryBean createWebSocketContainer() {
			ServletServerContainerFactoryBean container = new ServletServerContainerFactoryBean();
			container.setMaxTextMessageBufferSize(8192);
			container.setMaxBinaryMessageBufferSize(8192);
			return container;
		}

	}
----

or WebSocket XML namespace:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<bean class="org.springframework...ServletServerContainerFactoryBean">
			<property name="maxTextMessageBufferSize" value="8192"/>
			<property name="maxBinaryMessageBufferSize" value="8192"/>
		</bean>

	</beans>
----

[NOTE]
====
For client side WebSocket configuration, you should use `WebSocketContainerFactoryBean`
(XML) or `ContainerProvider.getWebSocketContainer()` (Java config).
====

For Jetty, you'll need to supply a pre-configured Jetty `WebSocketServerFactory` and plug
that into Spring's `DefaultHandshakeHandler` through your WebSocket Java config:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(echoWebSocketHandler(),
				"/echo").setHandshakeHandler(handshakeHandler());
		}

		@Bean
		public DefaultHandshakeHandler handshakeHandler() {

			WebSocketPolicy policy = new WebSocketPolicy(WebSocketBehavior.SERVER);
			policy.setInputBufferSize(8192);
			policy.setIdleTimeout(600000);

			return new DefaultHandshakeHandler(
					new JettyRequestUpgradeStrategy(new WebSocketServerFactory(policy)));
		}

	}
----

or WebSocket XML namespace:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers>
			<websocket:mapping path="/echo" handler="echoHandler"/>
			<websocket:handshake-handler ref="handshakeHandler"/>
		</websocket:handlers>

		<bean id="handshakeHandler" class="org.springframework...DefaultHandshakeHandler">
			<constructor-arg ref="upgradeStrategy"/>
		</bean>

		<bean id="upgradeStrategy" class="org.springframework...JettyRequestUpgradeStrategy">
			<constructor-arg ref="serverFactory"/>
		</bean>

		<bean id="serverFactory" class="org.eclipse.jetty...WebSocketServerFactory">
			<constructor-arg>
				<bean class="org.eclipse.jetty...WebSocketPolicy">
					<constructor-arg value="SERVER"/>
					<property name="inputBufferSize" value="8092"/>
					<property name="idleTimeout" value="600000"/>
				</bean>
			</constructor-arg>
		</bean>

	</beans>
----

[[websocket-server-allowed-origins]]
=== Configuring allowed origins

As of Spring Framework 4.1.5, the default behavior for WebSocket and SockJS is to accept
only _same origin_ requests. It is also possible to allow _all_ or a specified list of origins.
This check is mostly designed for browser clients. There is nothing preventing other types
of clients from modifying the `Origin` header value (see
https://tools.ietf.org/html/rfc6454[RFC 6454: The Web Origin Concept] for more details).

The 3 possible behaviors are:

 * Allow only same origin requests (default): in this mode, when SockJS is enabled, the
   Iframe HTTP response header `X-Frame-Options` is set to `SAMEORIGIN`, and JSONP
   transport is disabled since it does not allow to check the origin of a request.
   As a consequence, IE6 and IE7 are not supported when this mode is enabled.
 * Allow a specified list of origins: each provided _allowed origin_ must start with `http://`
   or `https://`. In this mode, when SockJS is enabled, both IFrame and JSONP based
   transports are disabled. As a consequence, IE6 through IE9 are not supported when this
   mode is enabled.
 * Allow all origins: to enable this mode, you should provide `{asterisk}` as the allowed origin
   value. In this mode, all transports are available.

WebSocket and SockJS allowed origins can be configured as shown bellow:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.socket.config.annotation.EnableWebSocket;
	import org.springframework.web.socket.config.annotation.WebSocketConfigurer;
	import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry;

	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(myHandler(), "/myHandler").setAllowedOrigins("http://mydomain.com");
		}

		@Bean
		public WebSocketHandler myHandler() {
			return new MyHandler();
		}

	}
----

XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers allowed-origins="http://mydomain.com">
			<websocket:mapping path="/myHandler" handler="myHandler" />
		</websocket:handlers>

		<bean id="myHandler" class="org.springframework.samples.MyHandler"/>

	</beans>
----


[[websocket-fallback]]
== SockJS Fallback Options
As explained in the <<websocket-into-fallback-options,introduction>>, WebSocket is not
supported in all browsers yet and may be precluded by restrictive network proxies.
This is why Spring provides fallback options that emulate the WebSocket API as close
as possible based on the https://github.com/sockjs/sockjs-protocol[SockJS protocol]
(version 0.3.3).

[[websocket-fallback-sockjs-overview]]
=== Overview of SockJS

The goal of SockJS is to let applications use a WebSocket API but fall back to
non-WebSocket alternatives when necessary at runtime, i.e. without the need to
change application code.

SockJS consists of:

* The https://github.com/sockjs/sockjs-protocol[SockJS protocol]
defined in the form of executable
http://sockjs.github.io/sockjs-protocol/sockjs-protocol-0.3.3.html[narrated tests].
* The https://github.com/sockjs/sockjs-client/[SockJS JavaScript client] - a client library for use in browsers.
* SockJS server implementations including one in the Spring Framework `spring-websocket` module.
* As of 4.1 `spring-websocket` also provides a SockJS Java client.

SockJS is designed for use in browsers. It goes to great lengths
to support a wide range of browser versions using a variety of techniques.
For the full list of SockJS transport types and browsers see the
https://github.com/sockjs/sockjs-client/[SockJS client] page. Transports
fall in 3 general categories: WebSocket, HTTP Streaming, and HTTP Long Polling.
For an overview of these categories see
https://spring.io/blog/2012/05/08/spring-mvc-3-2-preview-techniques-for-real-time-updates/[this blog post].

The SockJS client begins by sending `"GET /info"` to
obtain basic information from the server. After that it must decide what transport
to use. If possible WebSocket is used. If not, in most browsers
there is at least one HTTP streaming option and if not then HTTP (long)
polling is used.

All transport requests have the following URL structure:
----
http://host:port/myApp/myEndpoint/{server-id}/{session-id}/{transport}
----

* `{server-id}` - useful for routing requests in a cluster but not used otherwise.
* `{session-id}` - correlates HTTP requests belonging to a SockJS session.
* `{transport}` - indicates the transport type, e.g. "websocket", "xhr-streaming", etc.

The WebSocket transport needs only a single HTTP request to do the WebSocket handshake.
All messages thereafter are exchanged on that socket.

HTTP transports require more requests. Ajax/XHR streaming for example relies on
one long-running request for server-to-client messages and additional HTTP POST
requests for client-to-server messages. Long polling is similar except it
ends the current request after each server-to-client send.

SockJS adds minimal message framing. For example the server sends the letter +o+
("open" frame) initially, messages are sent as +a["message1","message2"]+
(JSON-encoded array), the letter +h+ ("heartbeat" frame) if no messages flow
for 25 seconds by default, and the letter +c+ ("close" frame) to close the session.

To learn more, run an example in a browser and watch the HTTP requests.
The SockJS client allows fixing the list of transports so it is possible to
see each transport one at a time. The SockJS client also provides a debug flag
which enables helpful messages in the browser console. On the server side enable
`TRACE` logging for `org.springframework.web.socket`.
For even more detail refer to the SockJS protocol
http://sockjs.github.io/sockjs-protocol/sockjs-protocol-0.3.3.html[narrated test].


[[websocket-fallback-sockjs-enable]]
=== Enable SockJS
SockJS is easy to enable through Java configuration:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(myHandler(), "/myHandler").withSockJS();
		}

		@Bean
		public WebSocketHandler myHandler() {
			return new MyHandler();
		}

	}
----

and the XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers>
			<websocket:mapping path="/myHandler" handler="myHandler"/>
			<websocket:sockjs/>
		</websocket:handlers>

		<bean id="myHandler" class="org.springframework.samples.MyHandler"/>

	</beans>
----

The above is for use in Spring MVC applications and should be included in the
configuration of a <<mvc-servlet,DispatcherServlet>>. However, Spring's WebSocket
and SockJS support does not depend on Spring MVC. It is relatively simple to
integrate into other HTTP serving environments with the help of
{javadoc-baseurl}/org/springframework/web/socket/sockjs/support/SockJsHttpRequestHandler.html[SockJsHttpRequestHandler].

On the browser side, applications can use the
https://github.com/sockjs/sockjs-client/[sockjs-client] (version 1.0.x) that
emulates the W3C WebSocket API and communicates with the server to select the best
transport option depending on the browser it's running in. Review the
https://github.com/sockjs/sockjs-client/[sockjs-client] page and the list of
transport types supported by browser. The client also provides several
configuration options, for example, to specify which transports to include.

[[websocket-fallback-xhr-vs-iframe]]
=== HTTP Streaming in IE 8, 9: Ajax/XHR vs IFrame

Internet Explorer 8 and 9 are and will remain common for some time. They are
a key reason for having SockJS. This section covers important
considerations about running in those browsers.

The SockJS client supports Ajax/XHR streaming in IE 8 and 9 via Microsoft's
http://blogs.msdn.com/b/ieinternals/archive/2010/05/13/xdomainrequest-restrictions-limitations-and-workarounds.aspx[XDomainRequest].
That works across domains but does not support sending cookies.
Cookies are very often essential for Java applications.
However since the SockJS client can be used with many server
types (not just Java ones), it needs to know whether cookies matter.
If so the SockJS client prefers Ajax/XHR for streaming or otherwise it
relies on a iframe-based technique.

The very first `"/info"` request from the SockJS client is a request for
information that can influence the client's choice of transports.
One of those details is whether the server application relies on cookies,
e.g. for authentication purposes or clustering with sticky sessions.
Spring's SockJS support includes a property called `sessionCookieNeeded`.
It is enabled by default since most Java applications rely on the `JSESSIONID`
cookie. If your application does not need it, you can turn off this option
and the SockJS client should choose `xdr-streaming` in IE 8 and 9.

If you do use an iframe-based transport, and in any case, it is good to know
that browsers can be instructed to block the use of IFrames on a given page by
setting the HTTP response header `X-Frame-Options` to `DENY`,
`SAMEORIGIN`, or `ALLOW-FROM <origin>`. This is used to prevent
https://www.owasp.org/index.php/Clickjacking[clickjacking].

[NOTE]
====
Spring Security 3.2+ provides support for setting `X-Frame-Options` on every
response. By default the Spring Security Java config sets it to `DENY`.
In 3.2 the Spring Security XML namespace does not set that header by default
but may be configured to do so, and in the future it may set it by default.

See http://docs.spring.io/spring-security/site/docs/3.2.2.RELEASE/reference/htmlsingle/#headers[Section 7.1. "Default Security Headers"]
of the Spring Security documentation for details on how to configure the
setting of the `X-Frame-Options` header. You may also check or watch
https://jira.spring.io/browse/SEC-2501[SEC-2501] for additional background.
====

If your application adds the `X-Frame-Options` response header (as it should!)
and relies on an iframe-based transport, you will need to set the header value to
`SAMEORIGIN` or `ALLOW-FROM <origin>`. Along with that the Spring SockJS
support also needs to know the location of the SockJS client because it is loaded
from the iframe. By default the iframe is set to download the SockJS client
from a CDN location. It is a good idea to configure this option to
a URL from the same origin as the application.

In Java config this can be done as shown below. The XML namespace provides a
similar option via the `<websocket:sockjs>` element:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/portfolio").withSockJS()
					.setClientLibraryUrl("http://localhost:8080/myapp/js/sockjs-client.js");
		}

		// ...

	}
----

[NOTE]
====
During initial development, do enable the SockJS client `devel` mode that prevents
the browser from caching SockJS requests (like the iframe) that would otherwise
be cached. For details on how to enable it see the
https://github.com/sockjs/sockjs-client/[SockJS client] page.
====

[[websocket-fallback-sockjs-heartbeat]]
=== Heartbeat Messages

The SockJS protocol requires servers to send heartbeat messages to preclude proxies
from concluding a connection is hung. The Spring SockJS configuration has a property
called `heartbeatTime` that can be used to customize the frequency. By default a
heartbeat is sent after 25 seconds assuming no other messages were sent on that
connection. This 25 seconds value is in line with the following
http://tools.ietf.org/html/rfc6202[IETF recommendation] for public Internet applications.

[NOTE]
====
When using STOMP over WebSocket/SockJS, if the STOMP client and server negotiate
heartbeats to be exchanged, the SockJS heartbeats are disabled.
====

The Spring SockJS support also allows configuring the `TaskScheduler` to use
for scheduling heartbeats tasks. The task scheduler is backed by a thread pool
with default settings based on the number of available processors. Applications
should consider customizing the settings according to their specific needs.

[[websocket-fallback-sockjs-servlet3-async]]
=== Servlet 3 Async Requests

HTTP streaming and HTTP long polling SockJS transports require a connection to remain
open longer than usual. For an overview of these techniques see
https://spring.io/blog/2012/05/08/spring-mvc-3-2-preview-techniques-for-real-time-updates/[this blog post].

In Servlet containers this is done through Servlet 3 async support that
allows exiting the Servlet container thread processing a request and continuing
to write to the response from another thread.

A specific issue is that the Servlet API does not provide notifications for a client
that has gone away, see https://java.net/jira/browse/SERVLET_SPEC-44[SERVLET_SPEC-44].
However, Servlet containers raise an exception on subsequent attempts to write
to the response. Since Spring's SockJS Service supports sever-sent heartbeats (every
25 seconds by default), that means a client disconnect is usually detected within that
time period or earlier if messages are sent more frequently.

[NOTE]
====
As a result network IO failures may occur simply because a client has disconnected, which
can fill the log with unnecessary stack traces. Spring makes a best effort to identify
such network failures that represent client disconnects (specific to each server) and log
a minimal message using the dedicated log category `DISCONNECTED_CLIENT_LOG_CATEGORY`
defined in `AbstractSockJsSession`. If you need to see the stack traces, set that
log category to TRACE.
====

[[websocket-fallback-cors]]
=== CORS Headers for SockJS

If you allow cross-origin requests (see <<websocket-server-allowed-origins>>), the SockJS protocol
uses CORS for cross-domain support in the XHR streaming and polling transports. Therefore
CORS headers are added automatically unless the presence of CORS headers in the response
is detected. So if an application is already configured to provide CORS support, e.g.
through a Servlet Filter, Spring's SockJsService will skip this part.

It is also possible to disable the addition of these CORS headers via the
`suppressCors` property in Spring's SockJsService.

The following is the list of headers and values expected by SockJS:

* `"Access-Control-Allow-Origin"` - initialized from the value of the "Origin" request header.
* `"Access-Control-Allow-Credentials"` - always set to `true`.
* `"Access-Control-Request-Headers"` - initialized from values from the equivalent request header.
* `"Access-Control-Allow-Methods"` - the HTTP methods a transport supports (see `TransportType` enum).
* `"Access-Control-Max-Age"` - set to 31536000 (1 year).

For the exact implementation see `addCorsHeaders` in `AbstractSockJsService` as well
as the `TransportType` enum in the source code.

Alternatively if the CORS configuration allows it consider excluding URLs with the
SockJS endpoint prefix thus letting Spring's `SockJsService` handle it.


[[websocket-fallback-sockjs-client]]
=== SockJS Client

A SockJS Java client is provided in order to connect to remote SockJS endpoints without
using a browser. This can be especially useful when there is a need for bidirectional
communication between 2 servers over a public network, i.e. where network proxies may
preclude the use of the WebSocket protocol. A SockJS Java client is also very useful
for testing purposes, for example to simulate a large number of concurrent users.

The SockJS Java client supports the "websocket", "xhr-streaming", and "xhr-polling"
transports. The remaining ones only make sense for use in a browser.

The `WebSocketTransport` can be configured with:

* `StandardWebSocketClient` in a JSR-356 runtime
* `JettyWebSocketClient` using the Jetty 9+ native WebSocket API
* Any implementation of Spring's `WebSocketClient`

An `XhrTransport` by definition supports both "xhr-streaming" and "xhr-polling" since
from a client perspective there is no difference other than in the URL used to connect
to the server. At present there are two implementations:

* `RestTemplateXhrTransport` uses Spring's `RestTemplate` for HTTP requests.
* `JettyXhrTransport` uses Jetty's `HttpClient` for HTTP requests.

The example below shows how to create a SockJS client and connect to a SockJS endpoint:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
  List<Transport> transports = new ArrayList<>(2);
  transports.add(new WebSocketTransport(StandardWebSocketClient()));
  transports.add(new RestTemplateXhrTransport());

  SockJsClient sockJsClient = new SockJsClient(transports);
  sockJsClient.doHandshake(new MyWebSocketHandler(), "ws://example.com:8080/sockjs");
----

[NOTE]
====
SockJS uses JSON formatted arrays for messages. By default Jackson 2 is used and needs
to be on the classpath. Alternatively you can configure a custom implementation of
`SockJsMessageCodec` and configure it on the `SockJsClient`.
====

To use the SockJsClient for simulating a large number of concurrent users you will
need to configure the underlying HTTP client (for XHR transports) to allow a sufficient
number of connections and threads. For example with Jetty:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
HttpClient jettyHttpClient = new HttpClient();
jettyHttpClient.setMaxConnectionsPerDestination(1000);
jettyHttpClient.setExecutor(new QueuedThreadPool(1000));
----

Consider also customizing these server-side SockJS related properties (see Javadoc for details):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Configuration
public class WebSocketConfig extends WebSocketMessageBrokerConfigurationSupport {

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/sockjs").withSockJS()
            .setStreamBytesLimit(512 * 1024)
            .setHttpMessageCacheSize(1000)
            .setDisconnectDelay(30 * 1000);
    }

    // ...

}
----




[[websocket-stomp]]
== STOMP Over WebSocket Messaging Architecture
The WebSocket protocol defines two types of messages, text and binary, but their
content is undefined. It's expected that the client and server may agree on using
a sub-protocol (i.e. a higher-level protocol) to define message semantics.
While the use of a sub-protocol with WebSocket is completely optional either way
client and server will need to agree on some kind of protocol to help interpret
messages.



[[websocket-stomp-overview]]
=== Overview of STOMP
http://stomp.github.io/stomp-specification-1.2.html#Abstract[STOMP] is a simple
text-oriented messaging protocol that was originally created for scripting languages
such as Ruby, Python, and Perl to connect to enterprise message brokers. It is
designed to address a subset of commonly used messaging patterns. STOMP can be
used over any reliable 2-way streaming network protocol such as TCP and WebSocket.
Although STOMP is a text-oriented protocol, the payload of messages can be
either text or binary.

STOMP is a frame based protocol whose frames are modeled on HTTP. The structure
of a STOMP frame:

----
COMMAND
header1:value1
header2:value2

Body^@
----

Clients can use the +SEND+ or +SUBSCRIBE+ commands to send or subscribe for
messages along with a +"destination"+ header that describes what the
message is about and who should receive it. This enables a simple
publish-subscribe mechanism that can be used to send messages through the broker
to other connected clients or to send messages to the server to request that
some work be performed.

When using Spring's STOMP support, the Spring WebSocket application acts
as the STOMP broker to clients. Messages are routed to `@Controller` message-handling
methods or to a simple, in-memory broker that keeps track of subscriptions and
broadcasts messages to subscribed users. You can also configure Spring to work
with a dedicated STOMP broker (e.g. RabbitMQ, ActiveMQ, etc) for the actual
broadcasting of messages. In that case Spring maintains
TCP connections to the broker, relays messages to it, and also passes messages
from it down to connected WebSocket clients. Thus Spring web applications can
rely on unified HTTP-based security, common validation, and a familiar programming
model message-handling work.

Here is an example of a client subscribing to receive stock quotes which
the server may emit periodically e.g. via a scheduled task sending messages
through a `SimpMessagingTemplate` to the broker:

----
SUBSCRIBE
id:sub-1
destination:/topic/price.stock.*

^@
----

Here is an example of a client sending a trade request, which the server
may handle through an `@MessageMapping` method and later on, after the execution,
broadcast a trade confirmation message and details down to the client:

----
SEND
destination:/queue/trade
content-type:application/json
content-length:44

{"action":"BUY","ticker":"MMM","shares",44}^@
----

The meaning of a destination is intentionally left opaque in the STOMP spec. It can
be any string, and it's entirely up to STOMP servers to define the semantics and
the syntax of the destinations that they support. It is very common, however, for
destinations to be path-like strings where `"/topic/.."` implies publish-subscribe
(__one-to-many__) and `"/queue/"` implies point-to-point (__one-to-one__) message
exchanges.

STOMP servers can use the +MESSAGE+ command to broadcast messages to all subscribers.
Here is an example of a server sending a stock quote to a subscribed client:

----
MESSAGE
message-id:nxahklf6-1
subscription:sub-1
destination:/topic/price.stock.MMM

{"ticker":"MMM","price":129.45}^@
----

It is important to know that a server cannot send unsolicited messages. All messages
from a server must be in response to a specific client subscription, and the
+"subscription-id"+ header of the server message must match the +"id"+ header of the
client subscription.

The above overview is intended to provide the most basic understanding of the
STOMP protocol. It is recommended to review the protocol
http://stomp.github.io/stomp-specification-1.2.html[specification] in full.

The benefits of using STOMP as a WebSocket sub-protocol:

* No need to invent a custom message format
* Use existing https://github.com/jmesnil/stomp-websocket[stomp.js] client in the browser
* Ability to route messages to based on destination
* Option to use full-fledged message broker such as RabbitMQ, ActiveMQ, etc. for broadcasting

Most importantly the use of STOMP (vs plain WebSocket) enables the Spring Framework
to provide a programming model for application-level use in the same way that
Spring MVC provides a programming model based on HTTP.



[[websocket-stomp-enable]]
=== Enable STOMP over WebSocket
The Spring Framework provides support for using STOMP over WebSocket through
the +spring-messaging+ and +spring-websocket+ modules.
Here is an example of exposing a STOMP WebSocket/SockJS endpoint at the URL path
`/portfolio` where messages whose destination starts with "/app" are routed to
message-handling methods (i.e. application work) and messages whose destinations
start with "/topic" or "/queue" will be routed to the message broker (i.e.
broadcasting to other connected clients):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker;
	import org.springframework.web.socket.config.annotation.StompEndpointRegistry;

	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/portfolio").withSockJS();
		}

		@Override
        public void configureMessageBroker(MessageBrokerRegistry config) {
            config.setApplicationDestinationPrefixes("/app");
            config.enableSimpleBroker("/topic", "/queue");
        }

	}
----

and in XML:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker application-destination-prefix="/app">
			<websocket:stomp-endpoint path="/portfolio">
				<websocket:sockjs/>
			</websocket:stomp-endpoint>
			<websocket:simple-broker prefix="/topic, /queue"/>
		</websocket:message-broker>

	</beans>
----

[NOTE]
====
The "/app" prefix is arbitrary. You can pick any prefix. It's simply meant to differentiate
messages to be routed to message-handling methods to do application work vs messages
to be routed to the broker to broadcast to subscribed clients.

The "/topic" and "/queue" prefixes depend on the broker in use. In the case of the simple,
in-memory broker the prefixes do not have any special meaning; it's merely a convention
that indicates how the destination is used (pub-sub targetting many subscribers or
point-to-point messages typically targeting an individual recipient).
In the case of using a dedicated broker, most brokers use "/topic" as
a prefix for destinations with pub-sub semantics and "/queue" for destinations
with point-to-point semantics. Check the STOMP page of the broker to see the destination
semantics it supports.
====


On the browser side, a client might connect as follows using
https://github.com/jmesnil/stomp-websocket[stomp.js] and the
https://github.com/sockjs/sockjs-client[sockjs-client]:

[source,javascript,indent=0]
[subs="verbatim,quotes"]
----
	var socket = new SockJS("/spring-websocket-portfolio/portfolio");
	var stompClient = Stomp.over(socket);

	stompClient.connect({}, function(frame) {
	}
----

Or if connecting via WebSocket (without SockJS):

[source,javascript,indent=0]
[subs="verbatim,quotes"]
----
	var socket = new WebSocket("/spring-websocket-portfolio/portfolio");
	var stompClient = Stomp.over(socket);

	stompClient.connect({}, function(frame) {
	}
----

Note that the `stompClient` above does not need to specify `login` and `passcode` headers.
Even if it did, they would be ignored, or rather overridden, on the server side. See the
sections <<websocket-stomp-handle-broker-relay-configure>> and
<<websocket-stomp-authentication>> for more information on authentication.


[[websocket-stomp-message-flow]]
=== Flow of Messages

When a STOMP endpoint is configured, the Spring application acts as the STOMP broker
to connected clients. This section provides a big picture overview of how messages flow
within the application.

The `spring-messaging` module provides the foundation for asynchronous message processing.
It contains a number of abstractions that originated in the
https://spring.io/spring-integration[Spring Integration] project and are intended
for use as building blocks in messaging applications:

* {javadoc-baseurl}/org/springframework/messaging/Message.html[Message] --
a message with headers and a payload.
* {javadoc-baseurl}/org/springframework/messaging/MessageHandler.html[MessageHandler] --
a contract for handling a message.
* {javadoc-baseurl}/org/springframework/messaging/MessageChannel.html[MessageChannel] --
a contract for sending a message enabling loose coupling between senders and receivers.
* {javadoc-baseurl}/org/springframework/messaging/SubscribableChannel.html[SubscribableChannel] --
extends `MessageChannel` and sends messages to registered `MessageHandler` subscribers.
* {javadoc-baseurl}/org/springframework/messaging/support/ExecutorSubscribableChannel.html[ExecutorSubscribableChannel] --
a concrete implementation of `SubscribableChannel` that can deliver messages
asynchronously via a thread pool.

The `@EnableWebSocketMessageBroker` Java config and the `<websocket:message-broker>` XML config
both assemble a concrete message flow. Below is a diagram of the part of the setup when using
the simple, in-memory broker:

image::images/message-flow-simple-broker.png[width=640]

The above setup that includes 3 message channels:

* `"clientInboundChannel"` for messages from WebSocket clients.
* `"clientOutboundChannel"` for messages to WebSocket clients.
* `"brokerChannel"` for messages to the broker from within the application.

The same three channels are also used with a dedicated broker except here a
"broker relay" takes the place of the simple broker:

image::images/message-flow-broker-relay.png[width=640]

Messages on the `"clientInboundChannel"` can flow to annotated
methods for application handling (e.g. a stock trade execution request) or can
be forwarded to the broker (e.g. client subscribing for stock quotes).
The STOMP destination is used for simple prefix-based routing. For example
the "/app" prefix could route messages to annotated methods while the "/topic"
and "/queue" prefixes could route messages to the broker.

When a message-handling annotated method has a return type, its return
value is sent as the payload of a Spring `Message` to the `"brokerChannel"`.
The broker in turn broadcasts the message to clients. Sending a message
to a destination can also be done from anywhere in the application with
the help of a messaging template. For example, an HTTP POST handling method
can broadcast a message to connected clients, or a service component may
periodically broadcast stock quotes.

Below is a simple example to illustrate the flow of messages:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/portfolio");
		}

		@Override
		public void configureMessageBroker(MessageBrokerRegistry registry) {
			registry.setApplicationDestinationPrefixes("/app");
			registry.enableSimpleBroker("/topic");
		}

	}

	@Controller
	public class GreetingController {

		@MessageMapping("/greeting") {
		public String handle(String greeting) {
			return "[" + getTimestamp() + ": " + greeting;
		}

	}

----

The following explains the message flow for the above example:

* WebSocket clients connect to the WebSocket endpoint at "/portfolio".
* Subscriptions to "/topic/greeting" pass through the "clientInboundChannel"
and are forwarded to the broker.
* Greetings sent to "/app/greeting" pass through the "clientInboundChannel"
and are forwarded to the `GreetingController`. The controller adds the current
time, and the return value is passed through the "brokerChannel" as a message
to "/topic/greeting" (destination is selected based on a convention but can be
overridden via `@SendTo`).
* The broker in turn broadcasts messages to subscribers, and they pass through
the `"clientOutboundChannel"`.

The next section provides more details on annotated methods including the
kinds of arguments and return values supported.



[[websocket-stomp-handle-annotations]]
=== Annotation Message Handling

The `@MessageMapping` annotation is supported on methods of `@Controller` classes.
It can be used for mapping methods to message destinations and can also be combined
with the type-level `@MessageMapping` for expressing shared mappings across all
annotated methods within a controller.

By default destination mappings are treated as Ant-style, slash-separated, path
patterns, e.g. "/foo*", "/foo/**". etc. They can also contain template variables,
e.g. "/foo/{id}" that can then be referenced via `@DestinationVariable`-annotated
method arguments.

[NOTE]
====
Applications can also use dot-separated destinations (vs slash).
See <<websocket-stomp-destination-separator>>.
====

The following method arguments are supported for `@MessageMapping` methods:

* `Message` method argument to get access to the complete message being processed.
* `@Payload`-annotated argument for access to the payload of a message, converted with
a `org.springframework.messaging.converter.MessageConverter`.
The presence of the annotation is not required since it is assumed by default.
Payload method arguments annotated with validation annotations (like `@Validated`) will
be subject to JSR-303 validation.
* `@Header`-annotated arguments for access to a specific header value along with
type conversion using an `org.springframework.core.convert.converter.Converter`
if necessary.
* `@Headers`-annotated method argument that must also be assignable to `java.util.Map`
for access to all headers in the message.
* `MessageHeaders` method argument for getting access to a map of all headers.
* `MessageHeaderAccessor`, `SimpMessageHeaderAccessor`, or `StompHeaderAccessor`
for access to headers via typed accessor methods.
* `@DestinationVariable`-annotated arguments for access to template
variables extracted from the message destination. Values will be converted to
the declared method argument type as necessary.
* `java.security.Principal` method arguments reflecting the user logged in at
the time of the WebSocket HTTP handshake.

The return value from an `@MessageMapping` method is converted with a
`org.springframework.messaging.converter.MessageConverter` and used as the body
of a new message that is then sent, by default, to the `"brokerChannel"` with
the same destination as the client message but using the prefix `"/topic"` by
default. An `@SendTo` message level annotation can be used to specify any
other destination instead.

An `@SubscribeMapping` annotation can also be used to map subscription requests
to `@Controller` methods. It is supported on the method level, but can also be
combined with a type level `@MessageMapping` annotation that expresses shared
mappings across all message handling methods within the same controller.

By default the return value from an `@SubscribeMapping` method is sent as a
message directly back to the connected client and does not pass through the
broker. This is useful for implementing request-reply message interactions; for
example, to fetch application data when the application UI is being initialized.
Or alternatively an `@SubscribeMapping` method can be annotated with `@SendTo`
in which case the resulting message is sent to the `"brokerChannel"` using
the specified target destination.


[[websocket-stomp-handle-send]]
=== Sending Messages

What if you want to send messages to connected clients from any part of the
application? Any application component can send messages to the `"brokerChannel"`.
The easiest way to do that is to have a `SimpMessagingTemplate` injected, and
use it to send messages. Typically it should be easy to have it injected by
type, for example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class GreetingController {

		private SimpMessagingTemplate template;

		@Autowired
		public GreetingController(SimpMessagingTemplate template) {
			this.template = template;
		}

		@RequestMapping(path="/greetings", method=POST)
		public void greet(String greeting) {
			String text = "[" + getTimestamp() + "]:" + greeting;
			this.template.convertAndSend("/topic/greetings", text);
		}

	}
----

But it can also be qualified by its name "brokerMessagingTemplate" if another
bean of the same type exists.


[[websocket-stomp-handle-simple-broker]]
=== Simple Broker

The built-in, simple message broker handles subscription requests from clients,
stores them in memory, and broadcasts messages to connected clients with matching
destinations. The broker supports path-like destinations, including subscriptions
to Ant-style destination patterns.

[NOTE]
====
Applications can also use dot-separated destinations (vs slash).
See <<websocket-stomp-destination-separator>>.
====




[[websocket-stomp-handle-broker-relay]]
=== Full-Featured Broker

The simple broker is great for getting started but supports only a subset of
STOMP commands (e.g. no acks, receipts, etc.), relies on a simple message
sending loop, and is not suitable for clustering. As an alternative, applications
can upgrade to using a full-featured message broker.

Check the STOMP documentation for your message broker of choice (e.g.
http://www.rabbitmq.com/stomp.html[RabbitMQ],
http://activemq.apache.org/stomp.html[ActiveMQ], etc.), install the broker,
and run it with STOMP support enabled. Then enable the STOMP broker relay in the
Spring configuration instead of the simple broker.

Below is example configuration that enables a full-featured broker:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/portfolio").withSockJS();
		}

		@Override
		public void configureMessageBroker(MessageBrokerRegistry registry) {
			registry.enableStompBrokerRelay("/topic", "/queue");
			registry.setApplicationDestinationPrefixes("/app");
		}

	}
----

XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker application-destination-prefix="/app">
			<websocket:stomp-endpoint path="/portfolio" />
				<websocket:sockjs/>
			</websocket:stomp-endpoint>
			<websocket:stomp-broker-relay prefix="/topic,/queue" />
		</websocket:message-broker>

	</beans>
----

The "STOMP broker relay" in the above configuration is a Spring
http://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/messaging/MessageHandler.html[MessageHandler]
that handles messages by forwarding them to an external message broker.
To do so it establishes TCP connections to the broker, forwards all
messages to it, and then forwards all messages received
from the broker to clients through their WebSocket sessions. Essentially
it acts as a "relay" that forwards messages in both directions.

[NOTE]
====
Please add a dependency on `org.projectreactor:reactor-net` for TCP connection management.
====

Furthermore, application components (e.g. HTTP request handling methods,
business services, etc.) can also send messages to the broker relay, as described
in <<websocket-stomp-handle-send>>, in order to broadcast messages to
subscribed WebSocket clients.

In effect, the broker relay enables robust and scalable message broadcasting.

[[websocket-stomp-handle-broker-relay-configure]]
=== Connections To Full-Featured Broker

A STOMP broker relay maintains a single "system" TCP connection to the broker.
This connection is used for messages originating from the server-side application
only, not for receiving messages. You can configure the STOMP credentials
for this connection, i.e. the STOMP frame `login` and `passcode` headers. This
is exposed in both the XML namespace and the Java config as the
++systemLogin++/++systemPasscode++ properties with default values ++guest++/++guest++.

The STOMP broker relay also creates a separate TCP connection for every connected
WebSocket client. You can configure the STOMP credentials to use for all TCP
connections created on behalf of clients. This is exposed in both the XML namespace
and the Java config as the ++clientLogin++/++clientPasscode++ properties with default
values ++guest++/++guest++.

[NOTE]
====
The STOMP broker relay always sets the `login` and `passcode` headers on every `CONNECT`
frame that it forwards to the broker on behalf of clients. Therefore WebSocket clients
need not set those headers; they will be ignored. As the following section explains,
instead WebSocket clients should rely on HTTP authentication to protect the WebSocket
endpoint and establish the client identity.
====

The STOMP broker relay also sends and receives heartbeats to and from the message
broker over the "system" TCP connection. You can configure the intervals for sending
and receiving heartbeats (10 seconds each by default). If connectivity to the broker
is lost, the broker relay will continue to try to reconnect, every 5 seconds,
until it succeeds.

[NOTE]
====
A Spring bean can implement `ApplicationListener<BrokerAvailabilityEvent>` in order
to receive notifications when the "system" connection to the broker is lost and
re-established. For example a Stock Quote service broadcasting stock quotes can
stop trying to send messages when there is no active "system" connection.
====

The STOMP broker relay can also be configured with a `virtualHost` property.
The value of this property will be set as the `host` header of every `CONNECT` frame
and may be useful for example in a cloud environment where the actual host to which
the TCP connection is established is different from the host providing the
cloud-based STOMP service.

[[websocket-stomp-destination-separator]]
=== Using Dot as Separator in `@MessageMapping` Destinations

Although slash-separated path patterns are familiar to web developers, in messaging
it is common to use a "." as the separator, for example in the names of topics, queues,
exchanges, etc. Applications can also switch to using "." (dot) instead of "/" (slash)
as the separator in `@MessageMapping` mappings by configuring a custom `AntPathMatcher`.

In Java config:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
  @Configuration
  @EnableWebSocketMessageBroker
  public class WebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {

    // ...

    @Override
    public void configureMessageBroker(MessageBrokerRegistry registry) {
      registry.enableStompBrokerRelay("/queue/", "/topic/");
      registry.setApplicationDestinationPrefixes("/app");
      registry.setPathMatcher(new AntPathMatcher("."));
    }

  }
----

In XML config:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
  <beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
      http://www.springframework.org/schema/beans
      http://www.springframework.org/schema/beans/spring-beans.xsd
      http://www.springframework.org/schema/websocket
      http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:message-broker application-destination-prefix="/app" path-matcher="pathMatcher">
      <websocket:stomp-endpoint path="/stomp" />
      <websocket:simple-broker prefix="/topic, /queue"/>
    </websocket:message-broker>

    <bean id="pathMatcher" class="org.springframework.util.AntPathMatcher">
      <constructor-arg index="0" value="." />
    </bean>

  </beans>
----

And below is a simple example to illustrate a controller with "." separator:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
  @Controller
  @MessageMapping("foo")
  public class FooController {
    
    @MessageMapping("bar.{baz}")
    public void handleBaz(@DestinationVariable String baz) {
    }

  }
----

If the application prefix is set to "/app" then the foo method is effectively mapped to "/app/foo.bar.{baz}".




[[websocket-stomp-authentication]]
=== Authentication

In a WebSocket-style application it is often useful to know who sent a message.
Therefore some form of authentication is needed to establish the user identity
and associate it with the current session.

Existing Web applications already use HTTP based authentication.
For example Spring Security can secure the HTTP URLs of the application as usual.
Since a WebSocket session begins with an HTTP handshake, that means URLs mapped
to STOMP/WebSocket are already automatically protected and require authentication.
Moreover the page that opens the WebSocket connection is itself likely protected
and so by the time of the actual handshake, the user should have been authenticated.

When a WebSocket handshake is made and a new WebSocket session is created,
Spring's WebSocket support automatically propagates the `java.security.Principal`
from the HTTP request to the WebSocket session. After that every message flowing
through the application on that WebSocket session is enriched with
the user information. It's present in the message as a header.
Controller methods can access the current user by adding a method argument of
type `javax.security.Principal`.

Note that even though the STOMP `CONNECT` frame has "login" and "passcode" headers
that can be used for authentication, Spring's STOMP WebSocket support ignores them
and currently expects users to have been authenticated already via HTTP.

In some cases it may be useful to assign an identity to a WebSocket session even
when the user has not been formally authenticated. For example, a mobile app might
assign some identity to anonymous users, perhaps based on geographical location.
The do that currently, an application can sub-class `DefaultHandshakeHandler`
and override the `determineUser` method. The custom handshake handler can then
be plugged in (see examples in <<websocket-server-deployment>>).



[[websocket-stomp-user-destination]]
=== User Destinations

An application can send messages targeting a specific user, and Spring's STOMP support
recognizes destinations prefixed with `"/user/"` for this purpose.
For example, a client might subscribe to the destination `"/user/queue/position-updates"`.
This destination will be handled by the `UserDestinationMessageHandler` and
transformed into a destination unique to the user session,
e.g. `"/queue/position-updates-user123"`. This provides the convenience of subscribing
to a generically named destination while at the same time ensuring no collisions
with other users subscribing to the same destination so that each user can receive
unique stock position updates.

On the sending side messages can be sent to a destination such as
`"/user/{username}/queue/position-updates"`, which in turn will be translated
by the `UserDestinationMessageHandler` into one or more destinations, one for each
session associated with the user. This allows any component within the application to
send messages targeting a specific user without necessarily knowing anything more
than their name and the generic destination. This is also supported through an
annotation as well as a messaging template.

For example, a message-handling method can send messages to the user associated with
the message being handled through the `@SendToUser` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Controller
public class PortfolioController {

    @MessageMapping("/trade")
    @SendToUser("/queue/position-updates")
    public TradeResult executeTrade(Trade trade, Principal principal) {
        // ...
        return tradeResult;
    }
}
----

If the user has more than one session, by default all of the sessions subscribed
to the given destination are targeted. However sometimes, it may be necessary to
target only the session that sent the message being handled. This can be done by
setting the `broadcast` attribute to false, for example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Controller
public class MyController {

    @MessageMapping("/action")
    public void handleAction() throws Exception{
        // raise MyBusinessException here
    }

    @MessageExceptionHandler
    @SendToUser(destinations="/queue/errors", broadcast=false)
    public ApplicationError handleException(MyBusinessException exception) {
        // ...
        return appError;
    }
}
----


[NOTE]
====
While user destinations generally imply an authenticated user, it isn't required
strictly. A WebSocket session that is not associated with an authenticated user
can subscribe to a user destination. In such cases the `@SendToUser` annotation
will behave exactly the same as with `broadcast=false`, i.e. targeting only the
session that sent the message being handled.
====

It is also possible to send a message to user destinations from any application
component by injecting the `SimpMessagingTemplate` created by the Java config or
XML namespace, for example (the bean name is `"brokerMessagingTemplate"` if required
for qualification with `@Qualifier`):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Service
public class TradeServiceImpl implements TradeService {

	private final SimpMessagingTemplate messagingTemplate;

	@Autowired
	public TradeServiceImpl(SimpMessagingTemplate messagingTemplate) {
		this.messagingTemplate = messagingTemplate;
	}

	// ...

	public void afterTradeExecuted(Trade trade) {
		this.messagingTemplate.convertAndSendToUser(
				trade.getUserName(), "/queue/position-updates", trade.getResult());
	}
}
----

[NOTE]
====
When using user destinations with an external message broker, check the broker
documentation on how to manage inactive queues, so that when the user session is
over, all unique user queues are removed. For example, RabbitMQ creates auto-delete
queues when destinations like `/exchange/amq.direct/position-updates` are used.
So in that case the client could subscribe to `/user/exchange/amq.direct/position-updates`.
Similarly, ActiveMQ has
http://activemq.apache.org/delete-inactive-destinations.html[configuration options]
for purging inactive destinations.
====

In a multi-application server scenario a user destination may remain unresolved because
the user is connected to a different server. In such cases you can configure a
destination to broadcast unresolved messages to so that other servers have a chance to try.
This can be done through the `userDestinationBroadcast` property of the
`MessageBrokerRegistry` in Java config and the `user-destination-broadcast` attribute
of the `message-broker` element in XML.




[[websocket-stomp-appplication-context-events]]
=== Listening To ApplicationContext Events and Intercepting Messages

Several `ApplicationContext` events (listed below) are published and can be
received by implementing Spring's `ApplicationListener` interface.

* `BrokerAvailabilityEvent` -- indicates when the broker becomes available/unavailable.
While the "simple" broker becomes available immediately on startup and remains so while
the application is running, the STOMP "broker relay" may lose its connection
to the full featured broker, for example if the broker is restarted. The broker relay
has reconnect logic and will re-establish the "system" connection to the broker
when it comes back, hence this event is published whenever the state changes from connected
to disconnected and vice versa. Components using the `SimpMessagingTemplate` should
subscribe to this event and avoid sending messages at times when the broker is not
available. In any case they should be prepared to handle `MessageDeliveryException`
when sending a message.
* `SessionConnectEvent` -- published when a new STOMP CONNECT is received
indicating the start of a new client session. The event contains the message representing the
connect including the session id, user information (if any), and any custom headers the client
may have sent. This is useful for tracking client sessions. Components subscribed
to this event can wrap the contained message using `SimpMessageHeaderAccessor` or
`StompMessageHeaderAccessor`.
* `SessionConnectedEvent` -- published shortly after a `SessionConnectEvent` when the
broker has sent a STOMP CONNECTED frame in response to the CONNECT. At this point the
STOMP session can be considered fully established.
* `SessionSubscribeEvent` -- published when a new STOMP SUBSCRIBE is received.
* `SessionUnsubscribeEvent` -- published when a new STOMP UNSUBSCRIBE is received.
* `SessionDisconnectEvent` -- published when a STOMP session ends. The DISCONNECT may
have been sent from the client, or it may also be automatically generated when the
WebSocket session is closed. In some cases this event may be published more than once
per session. Components should be idempotent with regard to multiple disconnect events.

[NOTE]
====
When using a full-featured broker, the STOMP "broker relay" automatically reconnects the
"system" connection in case the broker becomes temporarily unavailable. Client connections
however are not automatically reconnected. Assuming heartbeats are enabled, the client
will typically notice the broker is not responding within 10 seconds. Clients need to
implement their own reconnect logic.
====

Furthermore, an application can directly intercept every incoming and outgoing message by
registering a `ChannelInterceptor` on the respective message channel. For example
to intercept inbound messages:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
  @Configuration
  @EnableWebSocketMessageBroker
  public class WebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {

    @Override
    public void configureClientInboundChannel(ChannelRegistration registration) {
      registration.setInterceptors(new MyChannelInterceptor());
    }
  }
----

A custom `ChannelInterceptor` can extend the empty method base class
`ChannelInterceptorAdapter` and use `StompHeaderAccessor` or `SimpMessageHeaderAccessor`
to access information about the message.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
  public class MyChannelInterceptor extends ChannelInterceptorAdapter {

    @Override
    public Message<?> preSend(Message<?> message, MessageChannel channel) {
      StompHeaderAccessor accessor = StompHeaderAccessor.wrap(message);
      StompCommand command = accessor.getStompCommand();
      // ...
      return message;
    }
  }
----


[[websocket-stomp-client]]
=== STOMP Client

Spring provides STOMP/WebSocket client and STOMP/TCP client support with the
following built-in choices (other libraries can be adapted):

* `WebSocketStompClient` built on the Spring WebSocket API with
  support for standard JSR-356 WebSocket, Jetty 9, as well as SockJS
  for HTTP-based WebSocket emulation (see <<websocket-fallback-sockjs-client>>).
* `Reactor11TcpStompClient` built on `NettyTcpClient` from the reactor-net project.

To begin, create and configure the client:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
WebSocketClient transport = new StandardWebSocketClient();
WebSocketStompClient stompClient = new WebSocketStompClient(transport);
stompClient.setMessageConverter(new StringMessageConverter());
stompClient.setTaskScheduler(taskScheduler); // for heartbeats, receipts
----

Then connect to the WebSocket and provide a handler for the STOMP session:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
String url = "ws://127.0.0.1:8080/endpoint";
StompSessionHandler handler = ... ;
stompClient.connect(url, handler);
----

When the session is ready for use, the handler is notified:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
public class MySessionHandler extends StompSessionHandlerAdapter {

    @Override
    public void afterConnected(StompSession session, StompHeaders connectedHeaders) {
        // ...
    }
}
----

Send any Object as the payload and it will be serialized with a `MessageConverter`:
 
[source,java,indent=0]
[subs="verbatim,quotes"]
----
session.send("/topic/foo", "payload");
----

The subscribe methods take a `StompFrameHandler` for messages on the subscription
and return a `Subscription` handle for unsubscribing. For each received message,
the handler must help to select the target type and then handle the
deserialized payload:
 
[source,java,indent=0]
[subs="verbatim,quotes"]
----
session.subscribe("/topic/foo", new StompFrameHandler() {

    @Override
    public Type getPayloadType(StompHeaders headers) {
        return String.class;
    }

    @Override
    public void handleFrame(StompHeaders headers, Object payload) {
        // ...
    }

});
----

STOMP supports heartbeats. To use this feature simply configure the
`WebSocketStompClient` with a `TaskScheduler` and if desired customize the
default heartbeat intervals (10, 10 seconds respectively by default) for
write inactivity which causes a heartbeat to be sent and for read
inactivity which closes the connection.

The STOMP protocol also supports receipts where the client must add a "receipt"
header to which the server responds with a RECEIPT frame after the send or
subscribe are processed. To support this the `StompSession` offers
`setAutoReceipt(boolean)` that causes a "receipt" header to be
added on every subsequent send or subscribe.
Alternatively you can also manually add a "receipt" header to the `StompHeaders`.
Both send and subscribe return an instance of `Receiptable`
that can be used to register for receipt success and failure callbacks.
For this feature the client must be configured with a `TaskScheduler`
and the amount of time before a receipt expires (15 seconds by default).

Note that `StompSessionHandler` itself is a `StompFrameHandler` which allows
it to handle ERROR frames in addition to the `handleException` callback for
exceptions from the handling of messages, and `handleTransportError` for
transport-level errors including `ConnectionLostException`.



[[websocket-stomp-websocket-scope]]
=== WebSocket Scope

Each WebSocket session has a map of attributes. The map is attached as a header to
inbound client messages and may be accessed from a controller method, for example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Controller
public class MyController {

    @MessageMapping("/action")
    public void handle(SimpMessageHeaderAccessor headerAccessor) {
        Map<String, Object> attrs = headerAccessor.getSessionAttributes();
        // ...
    }
}
----

It is also possible to declare a Spring-managed bean in the `"websocket"` scope.
WebSocket-scoped beans can be injected into controllers and any channel interceptors
registered on the "clientInboundChannel". Those are typically singletons and live
longer than any individual WebSocket session. Therefore you will need to use a
scope proxy mode for WebSocket-scoped beans:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Component
@Scope(name = "websocket", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class MyBean {

    @PostConstruct
    public void init() {
        // Invoked after dependencies injected
    }

    // ...

    @PreDestroy
    public void destroy() {
        // Invoked when the WebSocket session ends
    }
}

@Controller
public class MyController {

    private final MyBean myBean;

    @Autowired
    public MyController(MyBean myBean) {
        this.myBean = myBean;
    }

    @MessageMapping("/action")
    public void handle() {
        // this.myBean from the current WebSocket session
    }
}
----

As with any custom scope, Spring initializes a new `MyBean` instance the first
time it is accessed from the controller and stores the instance in the WebSocket
session attributes. The same instance is returned subsequently until the session
ends. WebSocket-scoped beans will have all Spring lifecycle methods invoked as
shown in the examples above.



[[websocket-stomp-configuration-performance]]
=== Configuration and Performance

There is no silver bullet when it comes to performance. Many factors may
affect it including the size of messages, the volume, whether application
methods perform work that requires blocking, as well as external factors
such as network speed and others. The goal of this section is to provide
an overview of the available configuration options along with some thoughts
on how to reason about scaling.

In a messaging application messages are passed through channels for asynchronous
executions backed by thread pools. Configuring such an application requires
good knowledge of the channels and the flow of messages. Therefore it is
recommended to review <<websocket-stomp-message-flow>>.

The obvious place to start is to configure the thread pools backing the
`"clientInboundChannel"` and the `"clientOutboundChannel"`. By default both
are configured at twice the number of available processors.

If the handling of messages in annotated methods is mainly CPU bound then the
number of threads for the `"clientInboundChannel"` should remain close to the
number of processors. If the work they do is more IO bound and requires blocking
or waiting on a database or other external system then the thread pool size
will need to be increased.

[NOTE]
====
`ThreadPoolExecutor` has 3 important properties. Those are the core and
the max thread pool size as well as the capacity for the queue to store
tasks for which there are no available threads.

A common point of confusion is that configuring the core pool size (e.g. 10)
and max pool size (e.g. 20) results in a thread pool with 10 to 20 threads.
In fact if the capacity is left at its default value of Integer.MAX_VALUE
then the thread pool will never increase beyond the core pool size since
all additional tasks will be queued.

Please review the Javadoc of `ThreadPoolExecutor` to learn how these
properties work and understand the various queuing strategies.
====

On the `"clientOutboundChannel"` side it is all about sending messages to WebSocket
clients. If clients are on a fast network then the number of threads should
remain close to the number of available processors. If they are slow or on
low bandwidth they will take longer to consume messages and put a burden on the
thread pool. Therefore increasing the thread pool size will be necessary.

While the workload for the "clientInboundChannel" is possible to predict --
after all it is based on what the application does -- how to configure the
"clientOutboundChannel" is harder as it is based on factors beyond
the control of the application. For this reason there are two additional
properties related to the sending of messages. Those are the `"sendTimeLimit"`
and the `"sendBufferSizeLimit"`. Those are used to configure how long a
send is allowed to take and how much data can be buffered when sending
messages to a client.

The general idea is that at any given time only a single thread may be used
to send to a client. All additional messages meanwhile get buffered and you
can use these properties to decide how long sending a message is allowed to
take and how much data can be buffered in the mean time. Please review the
Javadoc and documentation of the XML schema for this configuration for
important additional details.

Here is example configuration:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
			registration.setSendTimeLimit(15 * 1000).setSendBufferSizeLimit(512 * 1024);
		}

		// ...

	}
----

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker>
			<websocket:transport send-timeout="15000" send-buffer-size="524288" />
			<!-- ... -->
		</websocket:message-broker>

	</beans>
----

The WebSocket transport configuration shown above can also be used to configure the
maximum allowed size for incoming STOMP messages. Although in theory a WebSocket
message can be almost unlimited in size, in practice WebSocket servers impose
limits -- for example, 8K on Tomcat and 64K on Jetty. For this reason STOMP clients
such as stomp.js split larger STOMP messages at 16K boundaries and send them as
multiple WebSocket messages thus requiring the server to buffer and re-assemble.

Spring's STOMP over WebSocket support does this so applications can configure the
maximum size for STOMP messages irrespective of WebSocket server specific message
sizes. Do keep in mind that the WebSocket message size will be automatically
adjusted if necessary to ensure they can carry 16K WebSocket messages at a
minimum.

Here is example configuration:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
			registration.setMessageSizeLimit(128 * 1024);
		}

		// ...

	}
----

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker>
			<websocket:transport message-size="131072" />
			<!-- ... -->
		</websocket:message-broker>

	</beans>
----

An important point about scaling is using multiple application instances.
Currently it is not possible to do that with the simple broker.
However when using a full-featured broker such as RabbitMQ, each application
instance connects to the broker and messages broadcast from one application
instance can be broadcast through the broker to WebSocket clients connected
through any other application instances.



[[websocket-stomp-stats]]
=== Runtime Monitoring

When using `@EnableWebSocketMessageBroker` or `<websocket:message-broker>` key
infrastructure components automatically gather stats and counters that provide
important insight into the internal state of the application. The configuration
also declares a bean of type `WebSocketMessageBrokerStats` that gathers all
available information in one place and by default logs it at `INFO` level once
every 30 minutes. This bean can be exported to JMX through Spring's
`MBeanExporter` for viewing at runtime, for example through JDK's `jconsole`.
Below is a summary of the available information.

Client WebSocket Sessions::
    Current::: indicates how many client sessions there are
    currently with the count further broken down by WebSocket vs HTTP
    streaming and polling SockJS sessions.
    Total::: indicates how many total sessions have been established.
    Abnormally Closed:::
        Connect Failures:::: these are sessions that got established but were
        closed after not having received any messages within 60 seconds. This is
        usually an indication of proxy or network issues.
        Send Limit Exceeded:::: sessions closed after exceeding the configured send
        timeout or the send buffer limits which can occur with slow clients
        (see previous section).
        Transport Errors:::: sessions closed after a transport error such as
        failure to read or write to a WebSocket connection or
        HTTP request/response.
    STOMP Frames::: the total number of CONNECT, CONNECTED, and DISCONNECT frames
    processed indicating how many clients connected on the STOMP level. Note that
    the DISCONNECT count may be lower when sessions get closed abnormally or when
    clients close without sending a DISCONNECT frame.
STOMP Broker Relay::
    TCP Connections::: indicates how many TCP connections on behalf of client
    WebSocket sessions are established to the broker. This should be equal to the
    number of client WebSocket sessions + 1 additional shared "system" connection
    for sending messages from within the application.
    STOMP Frames::: the total number of CONNECT, CONNECTED, and DISCONNECT frames
    forwarded to or received from the broker on behalf of clients. Note that a
    DISCONNECT frame is sent to the broker regardless of how the client WebSocket
    session was closed. Therefore a lower DISCONNECT frame count is an indication
    that the broker is pro-actively closing connections, may be because of a
    heartbeat that didn't arrive in time, an invalid input frame, or other.
Client Inbound Channel:: stats from thread pool backing the "clientInboundChannel"
    providing insight into the health of incoming message processing. Tasks queueing
    up here is an indication the application may be too slow to handle messages.
    If there I/O bound tasks (e.g. slow database query, HTTP request to 3rd party
    REST API, etc) consider increasing the thread pool size.
Client Outbound Channel:: stats from the thread pool backing the "clientOutboundChannel"
    providing insight into the health of broadcasting messages to clients. Tasks
    queueing up here is an indication clients are too slow to consume messages.
    One way to address this is to increase the thread pool size to accommodate the
    number of concurrent slow clients expected. Another option is to reduce the
    send timeout and send buffer size limits (see the previous section).
SockJS Task Scheduler:: stats from thread pool of the SockJS task scheduler which
    is used to send heartbeats. Note that when heartbeats are negotiated on the
    STOMP level the SockJS heartbeats are disabled.

[[websocket-stomp-testing]]
=== Testing Annotated Controller Methods

There are two main approaches to testing applications using Spring's STOMP over
WebSocket support. The first is to write server-side tests verifying the functionality
of controllers and their annotated message handling methods. The second is to write
full end-to-end tests that involve running a client and a server.

The two approaches are not mutually exclusive. On the contrary each has a place
in an overall test strategy. Server-side tests are more focused and easier to write
and maintain. End-to-end integration tests on the other hand are more complete and
test much more, but they're also more involved to write and maintain.

The simplest form of server-side tests is to write controller unit tests. However
this is not useful enough since much of what a controller does depends on its
annotations. Pure unit tests simply can't test that.

Ideally controllers under test should be invoked as they are at runtime, much like
the approach to testing controllers handling HTTP requests using the Spring MVC Test
framework. i.e. without running a Servlet container but relying on the Spring Framework
to invoke the annotated controllers. Just like with Spring MVC Test here there are two
two possible alternatives, either using a "context-based" or "standalone" setup:

1. Load the actual Spring configuration with the help of the
Spring TestContext framework, inject "clientInboundChannel" as a test field, and
use it to send messages to be handled by controller methods.

2. Manually set up the minimum Spring framework infrastructure required to invoke
controllers (namely the `SimpAnnotationMethodMessageHandler`) and pass messages for
controllers directly to it.

Both of these setup scenarios are demonstrated in the
https://github.com/rstoyanchev/spring-websocket-portfolio/tree/master/src/test/java/org/springframework/samples/portfolio/web[tests for the stock portfolio]
sample application.

The second approach is to create end-to-end integration tests. For that you will need
to run a WebSocket server in embedded mode and connect to it as a WebSocket client
sending WebSocket messages containing STOMP frames.
The https://github.com/rstoyanchev/spring-websocket-portfolio/tree/master/src/test/java/org/springframework/samples/portfolio/web[tests for the stock portfolio]
sample application also demonstrates this approach using Tomcat as the embedded
WebSocket server and a simple STOMP client for test purposes.
