Kafka driver deployment guide

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diff --git a/doc/source/admin/index.rst b/doc/source/admin/index.rst
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@@ -7,3 +7,4 @@ Deployment Guide
 
    drivers
    AMQP1.0
+   kafka
diff --git a/doc/source/admin/kafka.rst b/doc/source/admin/kafka.rst
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@@ -0,0 +1,245 @@
+-----------------------------
+Kafka Driver Deployment Guide
+-----------------------------
+
+============
+Introduction
+============
+
+The Kafka Driver is an experimental messaging transport backend
+in *oslo.messaging*. The driver maps the base oslo.messaging
+capabilities for notification message exchange onto V2.0 of the
+Apache Kafka distributed streaming platform. More detail regarding
+the Apache Kafka server is available from the `Apache Kafka website`__.
+
+__ https://kafka.apache.org/
+
+More detail regarding the driver's implementation is available from
+the `adding kafka driver specification`_ and the `update kafka driver
+specification`_ .
+
+.. _adding kafka driver specification: https://git.openstack.org/cgit/openstack/oslo-specs/tree/specs/liberty/adding-kafka-support.rst
+.. _update kafka driver specification: https://git.openstack.org/cgit/openstack/oslo-specs/tree/specs/queens/update-kafka-support.rst
+
+========
+Overview
+========
+
+The Kafka driver **only** supports use for sending and receiving
+*oslo.messaging* notifications. Specifically, the Kafka driver
+does not support *oslo.messaging* RPC transfers. Communications between
+the driver and Kafka server backend uses a `binary protocol over TCP`_
+that defines all APIs as request response message pairs. The Kafka
+driver integrates the confluent kafka python client for full
+protocol support and utilizes the Producer API to publish notification
+messages and the Consumer API for notification listener subscriptions.
+The driver is able to work with a single instance of a Kafka server or
+a clustered Kafka server deployment.
+
+.. _binary protocol over TCP: https://kafka.apache.org/protocol.html
+
+Hybrid Messaging Deployment
+---------------------------
+
+*Oslo.messaging* provides a mechanism to configure separate messaging
+backends for RPC and notification communications. This is supported
+through the definition of separate RPC and notification
+`transport urls`_ in the service configuration. When the Kafka driver
+is deployed for *oslo.messaging* notifications, a separate driver and
+messaging backend must be deployed for RPC communications. For these
+hybrid messaging configurations, either the `rabbit`_ or `amqp`_
+drivers can be deployed for *oslo.messaging* RPC.
+
+.. _transport urls: https://docs.openstack.org/oslo.messaging/latest/reference/transport.html
+.. _rabbit: https://docs.openstack.org/oslo.messaging/latest/admin/drivers.html#rabbit
+.. _amqp: https://docs.openstack.org/oslo.messaging/latest/admin/AMQP1.0.html
+
+Topics and vhost Support
+------------------------
+
+The Kafka topic is the feed name to which records are
+published. Topics in Kafka are multi-subscriber such that a topic can
+have zero, one or many consumers that subscribe to the data written to
+it. In *oslo.messaging*, a notification listener subscribes to a topic
+in a supplied target that is directly mapped by the driver to the
+Kafka topic. The Kafka server architecture does not natively support
+vhosts. In order to support the presence of a vhost in the transport
+url provided to the driver, the topic created on the Kafka server will
+be appended with the virtual host name. This creates a unique topic
+per virtual host but **note** there is otherwise no access
+control or isolation provided by the Kafka server.
+
+Listener Pools
+--------------
+
+The Kafka driver provides support for listener pools. This capability
+is realized by mapping the listener pool name to a Kafka server
+*consumer group* name. Each record published to a topic will be
+delivered to one consumer instance within each subscribing pool
+(e.g. *consumer group*). If a listener pool name is not assigned to the
+notification listener, a single default *consumer group* will be used by
+the Kafka driver and all listeners will be assigned to that
+group and the messages will effectively be load balanced across the
+competing listener instances.
+
+
+Synchronous Commit
+------------------
+
+A primary functional difference between a Kafka server and a
+classic broker queue is that the offset or position of the
+message read from the commit log is controlled by the listener
+(e.g. consumer). The driver will advance the offset it maintains
+linearly as it reads message records from the server. To ensure that
+duplicate messages are not generated during downtime or communication
+interruption, the driver will *synchronously* commit the consumed
+messages prior to the notification listener dispatch. Due to this, the
+driver does not support the re-queue operation and the driver can not
+replay messages from a Kafka partition.
+
+=============
+Prerequisites
+=============
+
+In order to run the driver the confluent-kafka python client must be
+installed. The Kafka driver integrates a `Python client based on librdkafka`_
+for full protocol support and utilizes the Producer API to publish
+notification messages and the Consumer API for notification listener
+subscriptions.
+
+.. _Python client based on librdkafka: https://github.com/confluentinc/confluent-kafka-python
+
+Source packages for the `confluent-kafka library`_ are available via PyPI.
+Since the Kafka driver is an optional extension to *oslo.messaging*
+these packages are not installed by default.  Use the 'kafka' extras
+tag when installing *oslo.messaging* in order to pull in these extra
+packages:
+
+::
+
+   python -m pip install oslo.messaging[kafka]
+
+
+.. _confluent-kafka library: https://pypi.org/project/confluent-kafka/
+
+=============
+Configuration
+=============
+
+Transport URL Enable
+--------------------
+
+In *oslo.messaging*, the transport_url parameters define the OpenStack service
+backends for RPC and Notify. The url is of the form::
+
+    transport://user:pass@host1:port[,hostN:portN]/virtual_host
+
+Where the transport value specifies the rpc or notification backend as
+one of ``amqp``, ``rabbit``, ``kafka``, etc. To specify and enable the
+Kafka driver for notifications, in the section
+``[oslo_messaging_notifications]`` of the service configuration file,
+specify the ``transport_url`` parameter::
+
+  [oslo_messaging_notifications]
+  transport_url = kafka://username:password@kafkahostname:9092
+
+Note, that if a ``transport_url`` parameter is not specified in the
+``[oslo_messaging_notifications]`` section, the ``[DEFAULT]``
+transport_url will be used for both RPC and Notify backends.
+
+Driver Options
+--------------
+
+It is recommended that the default configuration options provided by
+the Kafka driver be used. The configuration options can be modified
+in the ``oslo_messaging_kafka`` section of the service configuration file.
+
+Notification Listener Options
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+kafka_max_fetch_bytes
+    Initial maximum number of bytes per topic+partition to request
+    when fetching messages from the broker.
+
+kafka_consumer_timeout
+    Consumer polling interval timeout.
+
+consumer_group
+    The default client group identifier.
+
+enable_auto_commit
+    Indicator to perform asynchronous consumer message commits.
+
+max_poll_records
+    The maximum number of messages to return per consume/poll operation.
+
+Notifier Options
+^^^^^^^^^^^^^^^^
+
+producer_batch_timeout
+    Delay (ms) to wait for messages in the producer queue to
+    accumulate before constructing message sets to transmit to broker
+
+producer_batch_size
+    The maximum number of messages batched into one message set
+
+Security Options
+^^^^^^^^^^^^^^^^
+
+security_protocol
+    The protocol used to communicate with the Kafka brokers.
+
+sasl_mechanisms
+    SASL mechanism to use for authentication. Current driver support
+    is for PLAIN only.
+
+ssl_cafile
+    A file containing the trusted Certificate Authority's digital
+    certificate (in PEM format). This certificate is used to
+    authenticate the messaging backend.
+
+================
+DevStack Support
+================
+
+The plugin for the Kafka *oslo.messaging* driver is supported by
+DevStack. As the Kafka driver can only be deployed for notifications,
+the plugin supports the deployment of several message bus configurations.
+In local.conf ``[localrc]`` section, the `devstack-plugin-kafka`_
+plugin repository must be enabled. For example:
+
+::
+
+    [[local|localrc]]
+    enable_plugin kafka https://git.openstack.org/openstack/devstack-plugin-kafka
+
+
+Set the Kafka and Scala version and location variables if needed for
+the configuration
+
+::
+
+   KAFKA_VERSION=2.0.0
+   KAFKA_BASEURL=http://www.apache.org/dist/kafka
+   SCALA_VERSION=2.12
+   SCALA_BASEURL=http://www.scala-lang.org/riles/archive
+
+The **RPC_** and **NOTIFY_** variables will define the message bus
+configuration that will be used. The hybrid configurations will allow
+for the *rabbit* and *amqp* drivers to be used for the RPC transports
+while the *kafka* driver will be used for the Notify transport. The
+setting of the service variables will select which messaging
+intermediary is enabled for the configuration:
+
++------------+--------------------+--------------------+
+|            |         RPC        |        NOTIFY      |
+|            +-----------+--------+-----------+--------+
+|            |  SERVICE  |  PORT  |  SERVICE  |  PORT  |
++------------+-----------+--------+-----------+--------+
+|  Config 1  |  rabbit   |  5672  |  kafka    |  9092  |
++------------+-----------+--------+-----------+--------+
+|  Config 1  |  amqp     |  5672  |  kafka    |  9092  |
++------------+-----------+--------+-----------+--------+
+
+
+.. _devstack-plugin-kafka: https://github.com/openstack/devstack-plugin-kafka.git
diff --git a/oslo_messaging/_drivers/impl_kafka.py b/oslo_messaging/_drivers/impl_kafka.py
index 1871be1..ea3d102 100644
--- a/oslo_messaging/_drivers/impl_kafka.py
+++ b/oslo_messaging/_drivers/impl_kafka.py
@@ -378,7 +378,7 @@ class KafkaListener(base.PollStyleListener):
 class KafkaDriver(base.BaseDriver):
     """Kafka Driver
 
-    Note: Current implementation of this driver is experimental.
+    See :doc:`kafka` for details.
     """
 
     def __init__(self, conf, url, default_exchange=None,