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Hybrid messaging example

This section provides an overview of hybrid messaging deployment concepts and describes the necessary steps for a working OpenStack-Ansible (OSA) deployment where RPC and Notify communications are separated and integrated with different messaging server backends (e.g. rabbitmq and qdrouterd).

oslo.messaging library

The oslo.messaging library is part of the OpenStack Oslo project that provides intra-service messaging capabilities. The library supports two communication patterns (RPC and Notify) and provides an abstraction that hides the details of the messaging bus operation from the OpenStack services.


Notify communications are an asynchronous exchange from notifier to listener. The messages transferred typically correspond to information updates or event occurrences that are published by an OpenStack service. The listener need not be present when the notification is sent as notify communications are temporally decoupled. This decoupling between notifier and listener requires that the messaging backend deployed for notifications provide message persistence such as a broker queue or log store. It is noteworthy that the message transfer is unidirectional from notifier to listener and there is no message flow back to the notifier.


The RPC is intended as a synchronous exchange between a client and server that is temporally bracketed. The information transferred typically corresponds to a request-response pattern for service command invocation. If the server is not present at the time the command is invoked, the call should fail. The temporal coupling requires that the messaging backend deployed support the bi-directional transfer of the request from caller to server and the associated reply sent from the server back to the caller. This requirement can be satisfied by a broker queue or a direct messaging backend server.

Messaging transport

The oslo.messaging library supports a messaging transport plugin capability such that RPC and Notify communications can be separated and different messaging backend servers can be deployed.

The oslo.messaging drivers provide the transport integration for the selected protocol and backend server. The following table summarizes the supported oslo.messaging drivers and the communication services they support.

| Oslo.Messaging | Transport |  Backend  | RPC | Notify | Messaging |
|     Driver     | Protocol  |  Server   |     |        |   Type    |
|     rabbit     | AMQP V0.9 | rabbitmq  | yes |   yes  |   queue   |
|      amqp      | AMQP V1.0 | qdrouterd | yes |        |   direct  |
|     kafka      |  kafka    |  kafka    |     |   yes  |   queue   |
| (experimental) |  binary   |           |     |        |  (stream) |

Standard deployment of rabbitmq server

A single rabbitmq server backend (e.g. server or cluster) is the default deployment for OSA. This broker messaging backend provides the queue services for both RPC and Notification communications through its integration with the oslo.messaging rabbit driver. The oslo-messaging.yml file provides the default configuration to associate the oslo.messaging RPC and Notify services to the rabbitmq server backend.

oslomsg_rpc_transport: "{{ (groups[qdrouterd_host_group] | length > 0) | ternary('amqp', 'rabbit') }}"
oslomsg_rpc_port: "{{ (groups[qdrouterd_host_group] | length > 0) | ternary(qdrouterd_port, rabbitmq_port) }}"
oslomsg_rpc_servers: "{{ (groups[qdrouterd_host_group] | length > 0) | ternary(qdrouterd_servers, rabbitmq_servers) }}"
oslomsg_rpc_use_ssl: "{{ (groups[qdrouterd_host_group] | length > 0) | ternary(qdrouterd_use_ssl, rabbitmq_use_ssl) }}"
oslomsg_rpc_host_group: "{{ (groups[qdrouterd_host_group] | length > 0) | ternary(qdrouterd_host_group, rabbitmq_host_group) }}"
oslomsg_rpc_policies: "{{ rabbitmq_policies }}"

# Notify
oslomsg_notify_transport: "{{ (groups[rabbitmq_host_group] | length > 0) | ternary('rabbit', 'none') }}"
oslomsg_notify_port: "{{ rabbitmq_port }}"
oslomsg_notify_servers: "{{ rabbitmq_servers }}"
oslomsg_notify_use_ssl: "{{ rabbitmq_use_ssl }}"
oslomsg_notify_host_group: "{{ rabbitmq_host_group }}"
oslomsg_notify_policies: "{{ rabbitmq_policies }}"

Hybrid messaging deployment with qdrouterd server

In OSA, the deployment of disparate messaging backends is completely transparent to the OpenStack services. By managing the inventories for the messaging servers, a hybrid messaging configuration will be created. The instantiation of the qdrouterd server role in the qdrouterd_host_group will automatically setup the oslomsg_rpc* variables to use this messaging backend. No additional configuration is required. The result is that RPC services will communicate via the amqp (V1.0 protocol) driver and the Notify services will communicate via the rabbit driver. The separation and use of different messaging backends can provide increased scale and resiliency.