[james-project] 03/07: [ADR] Removing a configured additional MailboxListener

[bt...@apache.org]

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commit adc399bdc156aaa0d28ef9cda09034edee46192d
Author: Benoit Tellier <bt...@linagora.com>
AuthorDate: Fri Apr 3 12:10:11 2020 +0700

    [ADR] Removing a configured additional MailboxListener
---
 ...oving-configured-additional-mailboxListeners.md | 40 ++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/src/adr/0026-removing-configured-additional-mailboxListeners.md b/src/adr/0026-removing-configured-additional-mailboxListeners.md
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+# 26. Removing a configured additional MailboxListener
+
+Date: 2020-04-03
+
+## Status
+
+Accepted (lazy consensus)
+
+## Context
+
+James enables a user to register additional mailbox listeners.
+
+The distributed James server is handling mailbox event processing (mailboxListener execution) using a RabbitMQ work-queue
+per listener.
+
+The distributed James server then declares a queue upon start for each one of these user registered listeners, that it
+binds to the main event exchange. 
+
+If the user unconfigures the listener, the queue and the binding are still present but not consumed. This results in 
+unbounded queue growth eventually causing RabbitMQ resource exhaustion and failure.
+
+## Decision
+
+For rabbitMQ, configuration changes of additional mailbox listeners should be tracked via event sourcing. Event sourcing is 
+desirable as it allows:
+ - Detecting previously removed MailboxListener upon start
+ - Audit of unbind decisions
+ - Enables writing more complex business rules in the future
+
+We need, upon start, to sanitize bindings, and remove the ones corresponding to mailboxListeners that were removed not configured.
+
+The queue should not be deleted to prevent message loss.
+
+Given a James topology with a non uniform configuration, the effective RabbitMQ routing will be the one of the latest 
+started James server.
+
+## Alternatives
+
+We could also consider adding a webadmin endpoint to sanitize eventBus bindings, allowing more predictability than the
+above solution but it would require admin intervention.


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