nova/doc/source/reference/rpc.rst

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

AMQP is the messaging technology chosen by the OpenStack cloud.  The AMQP
broker, either RabbitMQ or Qpid, 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 'euca-terminate 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 (an only if) a 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 Qpid or RabbitMQ);
  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 either Qpid or
RabbitMQ 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 or "transient": the message is transient. Which means it is stored in
    memory only, and is lost if the server dies or restarts.

  * 2 or "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.