feat: rejection events

[ansgarlichter]

Closes #400.

If you do not see the feature request as a valid enhancement, just close the PR.

[schiwekM]

Thanks again for the changes. We will add docs to the readme to document the events and then the PR is good to go.

[ansgarlichter]

Thanks again for the changes. We will add docs to the readme to document the events and then the PR is good to go.

I just had another point, when I saw your review: would it be beneficial to emit those events inside cds.spawn so that errors inside those event handlers do not crash the attachment's request? As an example: if logging to audit log fails in the event handler, the user should still get the response properly in the plugins POV.

[schiwekM]

That should be the case ootb. Emit should write the events to the outbox, the transaction continues. And the outbox picks up the event, starts a new transaction and calls the handler

[ansgarlichter]

That should be the case ootb. Emit should write the events to the outbox, the transaction continues. And the outbox picks up the event, starts a new transaction and calls the handler

Just for clarification - in the docs for the outbox (see https://pages.github.tools.sap/cap/docs/node.js/queue#persistent-queue):

Using the persistent queue, the to-be-emitted message is stored in a database table within the current transaction, therefore transactional consistency is guaranteed.
Once the transaction succeeds, the messages are read from the database table and dispatched. If processing was successful, the respective message is deleted from the database table. If processing failed, the system retries the message after exponentially increasing delays.

If I understand this correctly, the event won't be written to the outbox but the request's transaction is rolled back via req.reject the transaction is not committed. Therefore, the event is never transmitted. So we would need cds.spawn or immediate events if the emit method offers a param for this?

[schiwekM]

Ahh, so you want to even emit the event in cases where the transaction is rolled back? Yes in that case we need to wrap it inside cds.spawn

[ansgarlichter]

Ahh, so you want to even emit the event in cases where the transaction is rolled back? Yes in that case we need to wrap it inside cds.spawn

Yes, if you look in the code, the request gets rejected if for example the attachment is too large. I will adapt it accordingly.