Python Client for Google Cloud Pub / Sub
========================================

|beta| |pypi| |versions|

`Google Cloud Pub / Sub`_ is a fully-managed real-time messaging service that
allows you to send and receive messages between independent applications. You
can leverage Cloud Pub/Sub’s flexibility to decouple systems and components
hosted on Google Cloud Platform or elsewhere on the Internet. By building on
the same technology Google uses, Cloud Pub / Sub is designed to provide “at
least once” delivery at low latency with on-demand scalability to 1 million
messages per second (and beyond).

Publisher applications can send messages to a ``topic`` and other applications
can subscribe to that topic to receive the messages. By decoupling senders and
receivers, Google Cloud Pub/Sub allows developers to communicate between
independently written applications.

- `Product Documentation`_
- `Client Library Documentation`_

.. |beta| image:: https://img.shields.io/badge/support-beta-silver.svg
   :target: https://github.com/googleapis/google-cloud-python/blob/master/README.rst#beta-support
.. |pypi| image:: https://img.shields.io/pypi/v/google-cloud-pubsub.svg
   :target: https://pypi.org/project/google-cloud-pubsub/
.. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-pubsub.svg
   :target: https://pypi.org/project/google-cloud-pubsub/
.. _Google Cloud Pub / Sub: https://cloud.google.com/pubsub/
.. _Product Documentation: https://cloud.google.com/pubsub/docs
.. _Client Library Documentation: https://googleapis.github.io/google-cloud-python/latest/pubsub/

Quick Start
-----------

In order to use this library, you first need to go through the following steps:

1. `Select or create a Cloud Platform project.`_
2. `Enable billing for your project.`_
3. `Enable the Google Cloud Pub / Sub API.`_
4. `Setup Authentication.`_

.. _Select or create a Cloud Platform project.: https://console.cloud.google.com/project
.. _Enable billing for your project.: https://cloud.google.com/billing/docs/how-to/modify-project#enable_billing_for_a_project
.. _Enable the Google Cloud Pub / Sub API.:  https://cloud.google.com/pubsub
.. _Setup Authentication.: https://googleapis.github.io/google-cloud-python/latest/core/auth.html

Installation
~~~~~~~~~~~~

Install this library in a `virtualenv`_ using pip. `virtualenv`_ is a tool to
create isolated Python environments. The basic problem it addresses is one of
dependencies and versions, and indirectly permissions.

With `virtualenv`_, it's possible to install this library without needing system
install permissions, and without clashing with the installed system
dependencies.

.. _`virtualenv`: https://virtualenv.pypa.io/en/latest/


Supported Python Versions
^^^^^^^^^^^^^^^^^^^^^^^^^
Python >= 3.5

Deprecated Python Versions
^^^^^^^^^^^^^^^^^^^^^^^^^^
Python == 2.7. Python 2.7 support will be removed on January 1, 2020.


Mac/Linux
^^^^^^^^^

.. code-block:: console

    pip install virtualenv
    virtualenv <your-env>
    source <your-env>/bin/activate
    <your-env>/bin/pip install google-cloud-pubsub


Windows
^^^^^^^

.. code-block:: console

    pip install virtualenv
    virtualenv <your-env>
    <your-env>\Scripts\activate
    <your-env>\Scripts\pip.exe install google-cloud-pubsub


Example Usage
~~~~~~~~~~~~~

Publishing
^^^^^^^^^^

To publish data to Cloud Pub/Sub you must create a topic, and then publish
messages to it

.. code-block:: python

    import os
    from google.cloud import pubsub_v1

    publisher = pubsub_v1.PublisherClient()
    topic_name = 'projects/{project_id}/topics/{topic}'.format(
        project_id=os.getenv('GOOGLE_CLOUD_PROJECT'),
        topic='MY_TOPIC_NAME',  # Set this to something appropriate.
    )
    publisher.create_topic(topic_name)
    publisher.publish(topic_name, b'My first message!', spam='eggs')

To learn more, consult the `publishing documentation`_.

.. _publishing documentation: https://googleapis.github.io/google-cloud-python/latest/pubsub/publisher/index.html


Subscribing
^^^^^^^^^^^

To subscribe to data in Cloud Pub/Sub, you create a subscription based on
the topic, and subscribe to that, passing a callback function.

.. code-block:: python

    import os
    from google.cloud import pubsub_v1

    subscriber = pubsub_v1.SubscriberClient()
    topic_name = 'projects/{project_id}/topics/{topic}'.format(
        project_id=os.getenv('GOOGLE_CLOUD_PROJECT'),
        topic='MY_TOPIC_NAME',  # Set this to something appropriate.
    )
    subscription_name = 'projects/{project_id}/subscriptions/{sub}'.format(
        project_id=os.getenv('GOOGLE_CLOUD_PROJECT'),
        sub='MY_SUBSCRIPTION_NAME',  # Set this to something appropriate.
    )
    subscriber.create_subscription(
        name=subscription_name, topic=topic_name)

    def callback(message):
        print(message.data)
        message.ack()

    future = subscriber.subscribe(subscription_name, callback)

The future returned by the call to ``subscriber.subscribe`` can be used to
block the current thread until a given condition obtains:

.. code-block:: python

    try:
        future.result()
    except KeyboardInterrupt:
        future.cancel()

It is also possible to pull messages in a synchronous (blocking) fashion. To
learn more about subscribing, consult the `subscriber documentation`_.

.. _subscriber documentation: https://googleapis.github.io/google-cloud-python/latest/pubsub/subscriber/index.html


Authentication
^^^^^^^^^^^^^^

It is possible to specify the authentication method to use with the Pub/Sub
clients. This can be done by providing an explicit `Credentials`_ instance. Support
for various authentication methods is available from the `google-auth`_ library.

For example, to use JSON Web Tokens, provide a `google.auth.jwt.Credentials`_ instance:

.. code-block:: python

    import json
    from google.auth import jwt

    service_account_info = json.load(open("service-account-info.json"))
    audience = "https://pubsub.googleapis.com/google.pubsub.v1.Subscriber"

    credentials = jwt.Credentials.from_service_account_info(
        service_account_info, audience=audience
    )

    subscriber = pubsub_v1.SubscriberClient(credentials=credentials)

    # The same for the publisher, except that the "audience" claim needs to be adjusted
    publisher_audience = "https://pubsub.googleapis.com/google.pubsub.v1.Publisher"
    credentials_pub = credentials.with_claims(audience=publisher_audience) 
    publisher = pubsub_v1.PublisherClient(credentials=credentials_pub)

.. _Credentials: https://google-auth.readthedocs.io/en/latest/reference/google.auth.credentials.html#google.auth.credentials.Credentials
.. _google-auth: https://google-auth.readthedocs.io/en/latest/index.html
.. _google.auth.jwt.Credentials: https://google-auth.readthedocs.io/en/latest/reference/google.auth.jwt.html#google.auth.jwt.Credentials
