[Questions] In very rare cases topic routing does not work on RabbitMQ 4.2

[gomoripeti]

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• I run RabbitMQ 4.x, the only series currently covered by community support
• I promise to provide all relevant information (versions, logs from all nodes, rabbitmq-diagnostics output, detailed reproduction steps)

RabbitMQ version used

4.2.2

Erlang version used

28.3.x

Operating system (distribution) used

Linux

How is RabbitMQ deployed?

Debian package

rabbitmq-diagnostics status output

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Logs from node 1 (with sensitive values edited out)

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Logs from node 2 (if applicable, with sensitive values edited out)

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Logs from node 3 (if applicable, with sensitive values edited out)

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rabbitmq.conf

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Steps to deploy RabbitMQ cluster

_

Steps to reproduce the behavior in question

would be hard to reproduce such a rare race condition

advanced.config

 {rabbit,
     [{log,
          [{exchange,
               [{enabled,true},
                {level,error},
                {formatter,{rabbit_logger_text_fmt,#{single_line => true}}}]},
...

Application code

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Kubernetes deployment file

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What problem are you trying to solve?

We've noticed that after an upgrade to 4.2.2 or creating a new 4.2.2 cluster in very rare cases topic routing did not work at all. That is publishing to any topic exchange failed, logging:

operation basic.publish caused a channel exception precondition_failed: Cannot route message through exchange 'amq.topic' in vhost '/': exchange type topic not found

Internally this turned out because

> persistent_term:get(topic, undefined).
{fun rabbit_exchange_type_invalid:route/3,3}

Some relevant logs from broker startup

$ grep 'Starting Rabbit\|<0.214.0>\|topic' rabbit@hostname.log
2026-02-06 08:57:44.920727+00:00 [info] <0.211.0>  Starting RabbitMQ 4.2.2 on Erlang 28.1.1 [jit]
2026-02-06 08:57:46.089070+00:00 [warning] <0.214.0> Could not find exchange type topic.
2026-02-06 08:57:46.089790+00:00 [info] <0.211.0> Running boot step rabbit_exchange_type_topic defined by app rabbit
2026-02-06 08:57:46.091667+00:00 [info] <0.214.0> Logging to exchange 'amq.rabbitmq.log' in vhost '/' ready

Exchange logging is enabled on the server. Based on the Pid <0.214.0> I suspect there is a race condition between registering the topic exchange type in a boot step and logging via the log exchange.

As it happened on 4.2.2 for us I don't know the exact versions affected but could be related to #14804. It might make it possible to log via the log exchange earlier in the RabbitMQ startup process than before. I don't know if a newer change in exchange logging in 4.2.4 #15331 has any effect on the behaviour (it might have as it declares the log exchange in a boot step after core_initialized, which is hopefully after exchange types are registered.)