@include domain

Stability: 1 - Experimental

Domains provide a way to handle multiple different IO operations as a

single group. If any of the event emitters or callbacks registered to a

domain emit an `error` event, or throw an error, then the domain object

will be notified, rather than losing the context of the error in the

`process.on('uncaughtException')` handler, or causing the program to

exit with an error code.

This feature is new in Node version 0.8. It is a first pass, and is

expected to change significantly in future versions. Please use it and

provide feedback.

Due to their experimental nature, the Domains features are disabled unless

the `domain` module is loaded at least once. No domains are created or

registered by default. This is by design, to prevent adverse effects on

current programs. It is expected to be enabled by default in future

Node.js versions.

Any time an Error object is routed through a domain, a few extra fields

are added to it.

* `error.domain` The domain that first handled the error.

* `error.domain_emitter` The event emitter that emitted an 'error' event

with the error object.

* `error.domain_bound` The callback function which was bound to the

domain, and passed an error as its first argument.

* `error.domain_thrown` A boolean indicating whether the error was

thrown, emitted, or passed to a bound callback function.

If domains are in use, then all new EventEmitter objects (including

Stream objects, requests, responses, etc.) will be implicitly bound to

the active domain at the time of their creation.

Additionally, callbacks passed to lowlevel event loop requests (such as

to fs.open, or other callback-taking methods) will automatically be

bound to the active domain. If they throw, then the domain will catch

the error.

In order to prevent excessive memory usage, Domain objects themselves

are not implicitly added as children of the active domain. If they

were, then it would be too easy to prevent request and response objects

from being properly garbage collected.

If you *want* to nest Domain objects as children of a parent Domain,

then you must explicitly add them, and then dispose of them later.

Implicit binding routes thrown errors and `'error'` events to the

Domain's `error` event, but does not register the EventEmitter on the

Domain, so `domain.dispose()` will not shut down the EventEmitter.

Implicit binding only takes care of thrown errors and `'error'` events.

* return: {Domain}

Returns a new Domain object.

The Domain class encapsulates the functionality of routing errors and

uncaught exceptions to the active Domain object.

Domain is a child class of EventEmitter. To handle the errors that it

catches, listen to its `error` event.

* {Array}

An array of timers and event emitters that have been explicitly added

to the domain.

* `emitter` {EventEmitter | Timer} emitter or timer to be added to the domain

Explicitly adds an emitter to the domain. If any event handlers called by

the emitter throw an error, or if the emitter emits an `error` event, it

will be routed to the domain's `error` event, just like with implicit

binding.

This also works with timers that are returned from `setInterval` and

`setTimeout`. If their callback function throws, it will be caught by

the domain 'error' handler.

If the Timer or EventEmitter was already bound to a domain, it is removed

from that one, and bound to this one instead.

* `emitter` {EventEmitter | Timer} emitter or timer to be removed from the domain

The opposite of `domain.add(emitter)`. Removes domain handling from the

specified emitter.

* `cb` {Function} The callback function

* return: {Function} The bound function

The returned function will be a wrapper around the supplied callback

function. When the returned function is called, any errors that are

thrown will be routed to the domain's `error` event.

var d = domain.create();

function readSomeFile(filename, cb) {

fs.readFile(filename, d.bind(function(er, data) {

// if this throws, it will also be passed to the domain

return cb(er, JSON.parse(data));

}));

}

d.on('error', function(er) {

// an error occurred somewhere.

// if we throw it now, it will crash the program

// with the normal line number and stack message.

});

* `cb` {Function} The callback function

* return: {Function} The intercepted function

This method is almost identical to `domain.bind(cb)`. However, in

addition to catching thrown errors, it will also intercept `Error`

objects sent as the first argument to the function.

In this way, the common `if (er) return cb(er);` pattern can be replaced

with a single error handler in a single place.

var d = domain.create();

function readSomeFile(filename, cb) {

fs.readFile(filename, d.intercept(function(er, data) {

// if this throws, it will also be passed to the domain

// additionally, we know that 'er' will always be null,

// so the error-handling logic can be moved to the 'error'

// event on the domain instead of being repeated throughout

// the program.

return cb(er, JSON.parse(data));

}));

}

d.on('error', function(er) {

// an error occurred somewhere.

// if we throw it now, it will crash the program

// with the normal line number and stack message.

});

The dispose method destroys a domain, and makes a best effort attempt to

clean up any and all IO that is associated with the domain. Streams are

aborted, ended, closed, and/or destroyed. Timers are cleared.

Explicitly bound callbacks are no longer called. Any error events that

are raised as a result of this are ignored.

The intention of calling `dispose` is generally to prevent cascading

errors when a critical part of the Domain context is found to be in an

error state.

Note that IO might still be performed. However, to the highest degree

possible, once a domain is disposed, further errors from the emitters in

that set will be ignored. So, even if some remaining actions are still

in flight, Node.js will not communicate further about them.