[jira] [Created] (JAMES-4159) EventBus: improve slow listener handling

Benoit Tellier created JAMES-4159:
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             Summary: EventBus: improve slow listener handling
                 Key: JAMES-4159
                 URL: https://issues.apache.org/jira/browse/JAMES-4159
             Project: James Server
          Issue Type: Improvement
            Reporter: Benoit Tellier


h3. Context

At LINAGORA we developped some custom listeners responsible of doing AI related tasks, either querying or feeding AI models. Mailbox listeners are a good fit because they are not blocking / delaying mail reception.

The EventBus works the following way: it publishes a message onto RabbitMQ, and initially is consummed once by a James node that would execute all listeners. Using QOS we are consuming up to 10 messages simultaneously per node. Failed messages would then be re-published to individual per-listener retry queue.

This "bundling" is done so that we limit the chatter of deserialization + wiring of the event bus, where each listener would have needed those operations, with separated execution. The amount of CPU time dedicated to those operations was significant ( > 40%).

However this bundling means we are not efficient handling "slow" listeners which at scale can sinificantly distrurb and delay event treatment.

h3. Proposal

Be able to configure a timeout on the initial eventbus run, for each listener.

That way, slow listeners would be individually retried.

In `listeners.xml`:

{code:xml}
<listeners>
  <executeGroupListeners>true</executeGroupListeners>

  <!-- New block -->
  <qos>20</qos>
  <initialExecutionTimeout>30s</initialExecutionTimeout>
  <executionTimeout>120s</executionTimeout>
  <!-- -->

  <listener>
    <class>org.apache.james.mailbox.cassandra.MailboxOperationLoggingListener</class>
  </listener>
</listeners>
{code}




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