If the queue is durable, the manual mention the following: If the queue is non-durable, the messages will be lost after a restart, as they are not persisted on disk When the mirror node comes up, the new situation Now, let’s say we shutdown the mirror node. Imagine the following queue, with 3 messages: Master server received during the mirror’s downtime have been processed. Mirror node is “synchronized” again, when all the messages that the This is not recommended, as this impacts availability.Īutomatic synchronization occurs as soon as the mirror node is back up. No new messages will be accepted or processed, until the sync has been completed. However, during this kind of sync, the queue will be unavailable. You can force a sync via the rabbitmqctl command or the management UI. Their own particularities, and you must be aware of those! Synchronization will happen in two cases, and both of them have This begs the question: How and when will the mirror node sync again? That it has missed messages from the master node and will put itself in the When the mirror node comes back up again, it will detect When a mirror node goes down, it will no longer receive replicated messagesįrom the master node. What happens when a mirror node goes down? PROTIP: When consistency is preferred above availability, it’s a good idea to set “ha-promote-on-failure: when-synced” as well. This means, when we stop RabbitMQ via systemctl, a synced mirror mode will be promoted, and during an unexpected downtime, a non-synced mirror might be promoted. When-synced: this means that only a mirror that is in sync with the master will be promotedīy default, the following settings apply: These two settings can have one of the following values:Īlways: this means any mirror node can be become a master Ha-promote-on-shutdown: this setting matters during an expected/announced downtime of the master node (this means the rabbitmq server is brought down via “systemctl stop rabbitmq-server” or another way to perform a controlled shutdown). Ha-promote-on-failure: this setting matters during an unexpected/unannounced downtime of the master node. How this happens depends on the following queue policy settings: When the master node goes down, another node will be promoted to master. What happens when the master node goes down? Once a message has been consumed, there is no need for the mirrors to keep their version of this message around. When this message is consumed, the master node will also replicate this information to the mirror nodes. This means that when a message is received on the master node, this message is replicated to the mirror server. Mirrored queues provide redundancy for messages, however, as only the master node is in charge of processing the messages and distributing them to consumers, mirrored queues do not distribute the workload to different nodes. Imagine you get a few hundreds requests per minute of those messages, and you’ll see there is quite some additional bandwidth overhead when using mirrored queues.Īs noted earlier, mirrored queues are master/mirror by design. This 10MB message will be replicated to 2 nodes, so an additional 20MB of network bandwidth is consumed. For example: Let’s say you receive messages that are 10MB in size, and you have 2 mirror nodes. Please be aware that this replication consumes network bandwidth. So, every time a message is received on a mirrored queue, the master will replicate that message to the configured mirror nodes. Consumers are transparently redirected to the master, no matter which node they connect to. Only one node (= the master) is in charge of handling the messages. Should you notice any, feel free to correct me □□♂️Ī very important thing to remember about mirrored queues, is that they work in a master/mirror mode. Although the root cause of my issue was quickly identified, I thought it would be a good idea to describe how mirrored queues work, as they are sometimes misunderstood.ĭisclaimer: I wrote this article based on my current experience with RabbitMQ. Recently, I experienced an issue with synchronization of mirrored queues in RabbitMQ. RabbitMQ mirrored queues master/slave graphic illustration
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