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AMQP and Nova
=============

AMQP is the messaging technology chosen by the OpenStack cloud. The AMQP
broker, default to Rabbitmq, sits between any two Nova components and
allows them to communicate in a loosely coupled fashion. More precisely, Nova
components (the compute fabric of OpenStack) use Remote Procedure Calls (RPC
hereinafter) to communicate to one another; however such a paradigm is built
atop the publish/subscribe paradigm so that the following benefits can be
achieved:

* Decoupling between client and servant (such as the client does not need to
  know where the servant's reference is).

* Full a-synchronism between client and servant (such as the client does not
  need the servant to run at the same time of the remote call).

* Random balancing of remote calls (such as if more servants are up and
  running, one-way calls are transparently dispatched to the first available
  servant).

Nova uses direct, fanout, and topic-based exchanges. The architecture looks
like the one depicted in the figure below:

.. image:: /_static/images/rpc-arch.png
   :width: 60%

Nova implements RPC (both request+response, and one-way, respectively nicknamed
``rpc.call`` and ``rpc.cast``) over AMQP by providing an adapter class which
take cares of marshaling and unmarshaling of messages into function calls. Each
Nova service (for example Compute, Scheduler, etc.) create two queues at the
initialization time, one which accepts messages with routing keys
``NODE-TYPE.NODE-ID`` (for example ``compute.hostname``) and another, which
accepts messages with routing keys as generic ``NODE-TYPE`` (for example
``compute``). The former is used specifically when Nova-API needs to redirect
commands to a specific node like ``openstack server delete $instance``. In this
case, only the compute node whose host's hypervisor is running the virtual
machine can kill the instance. The API acts as a consumer when RPC calls are
request/response, otherwise it acts as a publisher only.

Nova RPC Mappings
-----------------

The figure below shows the internals of a message broker node (referred to as a
RabbitMQ node in the diagrams) when a single instance is deployed and shared in
an OpenStack cloud. Every Nova component connects to the message broker and,
depending on its personality (for example a compute node or a network node),
may use the queue either as an Invoker (such as API or Scheduler) or a Worker
(such as Compute or Network). Invokers and Workers do not actually exist in the
Nova object model, but we are going to use them as an abstraction for sake of
clarity. An Invoker is a component that sends messages in the queuing system
via two operations: 1) ``rpc.call`` and ii) ``rpc.cast``; a Worker is a
component that receives messages from the queuing system and reply accordingly
to ``rpc.call`` operations.

Figure 2 shows the following internal elements:

Topic Publisher
  A Topic Publisher comes to life when an ``rpc.call`` or an ``rpc.cast``
  operation is executed; this object is instantiated and used to push a message
  to the queuing system. Every publisher connects always to the same
  topic-based exchange; its life-cycle is limited to the message delivery.

Direct Consumer
  A Direct Consumer comes to life if (and only if) an ``rpc.call`` operation is
  executed; this object is instantiated and used to receive a response message
  from the queuing system. Every consumer connects to a unique direct-based
  exchange via a unique exclusive queue; its life-cycle is limited to the
  message delivery; the exchange and queue identifiers are determined by a UUID
  generator, and are marshaled in the message sent by the Topic Publisher (only
  ``rpc.call`` operations).

Topic Consumer
  A Topic Consumer comes to life as soon as a Worker is instantiated and exists
  throughout its life-cycle; this object is used to receive messages from the
  queue and it invokes the appropriate action as defined by the Worker role. A
  Topic Consumer connects to the same topic-based exchange either via a shared
  queue or via a unique exclusive queue. Every Worker has two topic consumers,
  one that is addressed only during ``rpc.cast`` operations (and it connects to
  a shared queue whose exchange key is ``topic``) and the other that is
  addressed only during ``rpc.call`` operations (and it connects to a unique
  queue whose exchange key is ``topic.host``).

Direct Publisher
  A Direct Publisher comes to life only during ``rpc.call`` operations and it
  is instantiated to return the message required by the request/response
  operation. The object connects to a direct-based exchange whose identity is
  dictated by the incoming message.

Topic Exchange
  The Exchange is a routing table that exists in the context of a virtual host
  (the multi-tenancy mechanism provided by RabbitMQ etc); its type (such as
  topic vs. direct) determines the routing policy; a message broker node will
  have only one topic-based exchange for every topic in Nova.

Direct Exchange
  This is a routing table that is created during ``rpc.call`` operations; there
  are many instances of this kind of exchange throughout the life-cycle of a
  message broker node, one for each ``rpc.call`` invoked.

Queue Element
  A Queue is a message bucket. Messages are kept in the queue until a Consumer
  (either Topic or Direct Consumer) connects to the queue and fetch it. Queues
  can be shared or can be exclusive. Queues whose routing key is ``topic`` are
  shared amongst Workers of the same personality.

.. image:: /_static/images/rpc-rabt.png
   :width: 60%

RPC Calls
---------

The diagram below shows the message flow during an ``rpc.call`` operation:

1. A Topic Publisher is instantiated to send the message request to the queuing
   system; immediately before the publishing operation, a Direct Consumer is
   instantiated to wait for the response message.

2. Once the message is dispatched by the exchange, it is fetched by the Topic
   Consumer dictated by the routing key (such as 'topic.host') and passed to
   the Worker in charge of the task.

3. Once the task is completed, a Direct Publisher is allocated to send the
   response message to the queuing system.

4. Once the message is dispatched by the exchange, it is fetched by the Direct
   Consumer dictated by the routing key (such as ``msg_id``) and passed to the
   Invoker.

.. image:: /_static/images/rpc-flow-1.png
   :width: 60%

RPC Casts
---------

The diagram below shows the message flow during an ``rpc.cast`` operation:

1. A Topic Publisher is instantiated to send the message request to the queuing
   system.

2. Once the message is dispatched by the exchange, it is fetched by the Topic
   Consumer dictated by the routing key (such as 'topic') and passed to the
   Worker in charge of the task.

.. image:: /_static/images/rpc-flow-2.png
   :width: 60%

AMQP Broker Load
----------------

At any given time the load of a message broker node running RabbitMQ etc
is function of the following parameters:

Throughput of API calls
  The number of API calls (more precisely ``rpc.call`` ops) being served by the
  OpenStack cloud dictates the number of direct-based exchanges, related queues
  and direct consumers connected to them.

Number of Workers
  There is one queue shared amongst workers with the same personality; however
  there are as many exclusive queues as the number of workers; the number of
  workers dictates also the number of routing keys within the topic-based
  exchange, which is shared amongst all workers.

The figure below shows the status of a RabbitMQ node after Nova components'
bootstrap in a test environment. Exchanges and queues being created by Nova
components are:

* Exchanges

  1. nova (topic exchange)

* Queues

  1. ``compute.phantom`` (``phantom`` is hostname)
  2. ``compute``
  3. ``network.phantom`` (``phantom`` is hostname)
  4. ``network``
  5. ``scheduler.phantom`` (``phantom`` is hostname)
  6. ``scheduler``

.. image:: /_static/images/rpc-state.png
   :width: 60%

RabbitMQ Gotchas
----------------

Nova uses Kombu to connect to the RabbitMQ environment. Kombu is a Python
library that in turn uses AMQPLib, a library that implements the standard AMQP
0.8 at the time of writing. When using Kombu, Invokers and Workers need the
following parameters in order to instantiate a Connection object that connects
to the RabbitMQ server (please note that most of the following material can be
also found in the Kombu documentation; it has been summarized and revised here
for sake of clarity):

``hostname``
  The hostname to the AMQP server.

``userid``
  A valid username used to authenticate to the server.

``password``
  The password used to authenticate to the server.

``virtual_host``
  The name of the virtual host to work with. This virtual host must exist on
  the server, and the user must have access to it. Default is "/".

``port``
  The port of the AMQP server. Default is ``5672`` (amqp).

The following parameters are default:

``insist``
  Insist on connecting to a server. In a configuration with multiple
  load-sharing servers, the Insist option tells the server that the client is
  insisting on a connection to the specified server. Default is False.

``connect_timeout``
  The timeout in seconds before the client gives up connecting to the server.
  The default is no timeout.

``ssl``
  Use SSL to connect to the server. The default is False.

More precisely Consumers need the following parameters:

``connection``
  The above mentioned Connection object.

``queue``
  Name of the queue.

``exchange``
  Name of the exchange the queue binds to.

``routing_key``
  The interpretation of the routing key depends on the value of the
  ``exchange_type`` attribute.

  Direct exchange
    If the routing key property of the message and the ``routing_key`` attribute of
    the queue are identical, then the message is forwarded to the queue.

  Fanout exchange
    Messages are forwarded to the queues bound the exchange, even if the
    binding does not have a key.

  Topic exchange
    If the routing key property of the message matches the routing key of the
    key according to a primitive pattern matching scheme, then the message is
    forwarded to the queue. The message routing key then consists of words
    separated by dots (``.``, like domain names), and two special characters
    are available; star (``*``) and hash (``#``). The star matches any word,
    and the hash matches zero or more words. For example ``.stock.#`` matches
    the routing keys ``usd.stock`` and ``eur.stock.db`` but not
    ``stock.nasdaq``.

``durable``
  This flag determines the durability of both exchanges and queues; durable
  exchanges and queues remain active when a RabbitMQ server restarts.
  Non-durable exchanges/queues (transient exchanges/queues) are purged when a
  server restarts. It is worth noting that AMQP specifies that durable queues
  cannot bind to transient exchanges. Default is True.

``auto_delete``
  If set, the exchange is deleted when all queues have finished using it.
  Default is False.

``exclusive``
  Exclusive queues (such as non-shared) may only be consumed from by the
  current connection. When exclusive is on, this also implies ``auto_delete``.
  Default is False.

``exchange_type``
  AMQP defines several default exchange types (routing algorithms) that covers
  most of the common messaging use cases.

``auto_ack``
  Acknowledgment is handled automatically once messages are received.  By
  default ``auto_ack`` is set to False, and the receiver is required to manually
  handle acknowledgment.

``no_ack``
  It disable acknowledgment on the server-side. This is different from
  ``auto_ack`` in that acknowledgment is turned off altogether. This
  functionality increases performance but at the cost of reliability. Messages
  can get lost if a client dies before it can deliver them to the application.

``auto_declare``
  If this is True and the exchange name is set, the exchange will be
  automatically declared at instantiation. Auto declare is on by default.

Publishers specify most the parameters of Consumers (such as they do not
specify a queue name), but they can also specify the following:

``delivery_mode``
  The default delivery mode used for messages. The value is an integer. The
  following delivery modes are supported by RabbitMQ:

  ``1`` (transient)
    The message is transient. Which means it is stored in memory only, and is
    lost if the server dies or restarts.

  ``2`` (persistent)
    The message is persistent. Which means the message is stored both
    in-memory, and on disk, and therefore preserved if the server dies or
    restarts.

The default value is ``2`` (persistent). During a send operation, Publishers
can override the delivery mode of messages so that, for example, transient
messages can be sent over a durable queue.
