| # 38. Distributed Event bus |
| |
| Date: 2020-05-25 |
| |
| ## Status |
| |
| Accepted (lazy consensus) & implemented |
| |
| ## Context |
| |
| Read [Event Bus ADR](0037-eventbus.md) for context. |
| |
| Given several James servers, we need them to share a common EventBus. |
| |
| This: |
| - Ensures a better load balancing for `group mailbox listners`. |
| - Is required for correctness of notifications (like IMAP IDLE). |
| |
| ## Decision |
| |
| Provide a distributed implementation of the EventBus leveraging RabbitMQ. |
| |
| Events are emitted to a single Exchange. |
| |
| Each group will have a corresponding queue, bound to the main exchange, with a default routing key. Each eventBus |
| will consume this queue and execute the relevant listener, ensuring at least once execution at the cluster level. |
| |
| Retries are managed via a dedicated exchange for each group: as we need to count retries, the message headers need to |
| be altered and we cannot rely on rabbitMQ build in retries. Each time the execution fails locally, a new event is emitted |
| via the dedicated exchange, and the original event is acknowledged. |
| |
| Each eventBus will have a dedicated exclusive queue, bound to the main exchange with the `registrationKeys` used by local |
| notification mailboxListeners (to only receive the corresponding subset of events). Errors are not retried for |
| notifications, failures are not persisted within `DeadLetter`, achieving at most once event delivery. |
| |
| ## Related ADRs |
| |
| The implementation of the the distributed EventBus suffers from the following flows: |
| |
| - [Removing a configured additional MailboxListener](0026-removing-configured-additional-mailboxListeners.md) |
| - [Distributed Mailbox Listeners Configuration](0035-distributed-listeners-configuration.md) also covers more in details |
| topology changes and supersedes ADR 0026. |
| |
| The following enhancement have furthermore been contributed: |
| |
| - [EventBus error handling upon dispatch](0027-eventBus-error-handling-upon-dispatch.md) |
| |
| ## References |
| |
| * [JIRA 1](https://issues.apache.org/jira/browse/MAILBOX-367) |
| * [JIRA 2](https://issues.apache.org/jira/browse/MAILBOX-368) |
| * [JIRA 3](https://issues.apache.org/jira/browse/MAILBOX-371) |
