Durability and related matters

About This Guide

This guide covers queue, exchange and message durability, as well as other topics related to durability, for example, durability in clustered environments.

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What version of March Hare does this guide cover??

This guide covers March Hare 3.0.

Entity durability and message persistence

Exchange durability

AMQP separates the concept of entity durability (queues, exchanges) from message persistence. Exchanges can be durable or transient. Durable exchanges survive broker restart, transient exchanges do not (they have to be redeclared when the broker comes back online), however, not all scenarios and use cases mandate exchanges to be durable.

To create a durable exchange, declare it with the :durable => true argument:

ch = conn.create_channel
exch = ch.direct("my_direct_exchange", :durable => true)

Queue durability

Queues can be durable or transient. Durable queues survive broker restart, transient queues do not (they have to be redeclared when the broker comes back online), however, not all scenarios and use cases mandate queues to be durable.

To create a durable queue, declare it with the :durable => true argument:

ch = conn.create_channel
q = ch.queue("my_queue", :durable => true)

Durability of a queue does not make messages that are routed to that queue durable. If a broker is taken down and then brought back up, durable queues will be re-declared during broker startup, however, only persistent messages will be recovered.

Binding durability

Bindings of durable queues to durable exchanges are automatically durable and are restored after a broker restart. The AMQP 0.9.1 specification states that the binding of durable queues to transient exchanges must be allowed. In this case, since the exchange would not survive a broker restart, neither would any bindings to such and exchange.

Message persistence

Messages may be published as persistent and this, in conjunction with queue durability, is what makes an AMQP broker persist them to disk. If the server is restarted, the system ensures that received persistent messages in durable queues are not lost. Simply publishing a message to a durable exchange or the fact that a queue to which a message is routed is durable does not make that message persistent. Message persistence depends on the persistence mode of the message itself.

Note that publishing persistent messages affects performance (just like with data stores, durability comes at a certain cost to performance).

Pass the :persistent => true argument to the MarchHare::Exchange#publish method to publish your message as persistent:

exch.publish("My message", :persistent => true)

Clustering and High Availability

To achieve the degree of durability that critical applications need, it is necessary but not enough to use durable queues, exchanges and persistent messages. You need to use a cluster of brokers because otherwise, a single hardware problem may bring a broker down completely.

RabbitMQ offers a number of high availability features for both scenarios with more (LAN) and less (WAN) reliable network connections.

See the RabbitMQ clustering and high availability guides for in-depth discussion of this topic.

Highly Available (Mirrored) Queues

Whilst the use of clustering provides for greater durability of critical systems, in order to achieve the highest level of resilience for queues and messages, high availability configuration should be used. This is because although exchanges and bindings survive the loss of individual nodes by using clustering, messages do not. Without mirroring, queue contents reside on exactly one node, thus the loss of a node will cause message loss.

See the RabbitMQ high availability guide for more information about mirrored queues.

The documentation is organized as a number of guides, covering various topics.

We recommend that you read the following guides first, if possible, in this order:

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