# Messaging in Openstack

## oslo_messaging

In PCI infra, oslo_messaging is configured using:

- rabbitmq driver for RPC server/agent communication
- kafka and log driver for notifications (send events to third party app)


### RPC implementation in rabbitmq

[RPC in openstack](https://docs.openstack.org/oslo.messaging/stein/reference/rpcclient.html) is implemented using oslo_messaging library.

!!! note "tldr"

    - rpc call()
        - blocking call to invoke a method on a topic with 1 reply expected
    - rpc cast()
        - invoke a method on a topic in 'best effort' mode without reply. If fanout=true message, is broadcasted to all topic consumers


In rabbitmq, a message is published to a queue using an exchange/routing_key.
Consumers are directly connected to a queue to read messages from.

A oslo.messaging 'topic' is almost equivalent to a rabbitmq queue.

With a rpc call, message will be sent to rabbitmq through exchange=target.exchange  queue={target.topic}.{target.server}
Response will be sent back to caller using exchange=target.exchange queue={message.reply_queue}

With a rpc cast fanout=false, it's the same but there is no reply mechanism

With a rpc cast fanout=true, message will be sent to rabbitmq through exchange=target.exchange  queue={target.topic}_fanout

For rpc call and rpc cast (fanout=false), we are using quorum queues (1 publisher / 1 consumer).
For rpc cast (fanout=true), stream queues are used because it's purpose is to broadcast messages (1 publisher / N consumers).

On startup, every server/agent declare queues they will consume from. If queue does not exist on rabbit cluster, it is created.
It's the same for publishing part with the exchange.