For C7 Remote implement outbox pattern for external task completion

[zambrovski]

Scenario

• Library version: latest
• Description of your use case: In order to improve the confidence in TX implementation of the used client, we want make sure the adapter can use an outbox-pattern approach to complete Camunda jobs. More details here: https://medium.com/miragon/leveling-up-mastering-the-distributed-transaction-problem-in-zeebe-d4bbbca295d6

Current Behaviour

Direct "TX after-commit" completion is used.

Wanted Behaviour

Outbox pattern implementation based on Spring Data JPA (as one of the options)...

Possible Workarounds

Manually implementation in every project.

[i-am-not-giving-my-name-to-a-machine]

Hi, @zambrovski, I think it would be sufficient for many users if the outbox were passive.

If my understanding of the article and desired feature is correct, the library would actively track whether it has sent external task completions to Camunda or not. However, it could be sufficient (for many, but not all) if the outbox were passive in the way that

• when an external task completes, the adapter saves the completion payload in the still running transaction (if any)
• now imagine calling Camunda fails (for whatever reason)
• when the external task is fetched again (and it will be fetched again, and is therefore locked, which gives the client permission to complete the task), the adapter simply looks for a previous completion and, if one exists, reuses the completion payload (variable map) and sends it to Camunda again, which would then complete the task in Camunda and the process could continue.

This would best be decoupled from the business code via an interface. The business code could optionally implement the interface, configure the adapter to use the implementation and take care of saving and loading completion payloads in its own database and structure according to its own rules and guidelines.
If such an implementation is configured, the adapter would also guarantee idempotency by reusing previous results, and it would sort of piggyback external task executions to complete tasks based on earlier executions.