From 854a37994bc8e941bbbef6d905fb1bf75ffed88d Mon Sep 17 00:00:00 2001
From: Flatnotes <flatnotes@autocommit>
Date: Sun, 5 May 2024 18:09:43 +0000
Subject: [PATCH] Autocommit action=CREATE on file=oslo.messaging - How it
 works with rabbit.md detected

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 oslo.messaging - How it works with rabbit.md | 39 ++++++++++++++++++++
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+# 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.
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