m1a2st commented on PR #19527:
URL: https://github.com/apache/kafka/pull/19527#issuecomment-3863984342
I should add more details for this scenario, although I do not have a
concrete solution yet.
In the test, we register two different login classes on the external
listener and the internal listener.
1. External listener
This case does not have the issue. The external listener is used by
client connections (admin, producer, and consumer), so it always registers the
login plugin in `ConnectionMode.SERVER`.
2. Internal listener
This is where the issue occurs. During broker startup, multiple
components are initialized. Some of these components act as clients, while
others act as servers. However, the login plugin is a singleton, meaning only
one instance exists.
If a component running in `ConnectionMode.SERVER` initializes the login
plugin first, that instance is stored in memory. When other components start up
later, they reuse the same login plugin instance from memory. As a result, the
metric tags depend on which component registers the plugin first.
Since we cannot guarantee the component startup order, the resulting
behavior is nondeterministic. Modifying the component startup order also seems
difficult in practice.
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