Running a system of more than two applications about eventsourcing

[johnbywater]

Hi @ehuebner!

Thanks for your question.

What happens if you express this with a list of pipeline expressions:

system = System(pipes=[
    [AppA, AppC],
    [AppB, AppC],
    [AppC, AppD],
    [AppA, AppD],
])

Does that work to accomplish what you desire?

Please note, the indeterminacy in AppC and AppD that is introduced by the fact that they are each processing event notifications from more than one log. Sometimes this indeterminacy doesn't matter, but with careful consideration it's often possible to arrange things such that this kind of indeterminacy isn't introduced by design. Please feel free to email me if you would like to discuss your project in detail.

Of course, please don't hesitate to post here any further questions you may have about the library. There's also a Slack workspace for this library, with a friendly community, which you are welcome to join (there's an invitation link on the README page and in the docs).

[ehuebner]

I have already tried it and it did not work.
My Event of AppB uses Transcodings for some custom event attributes. Recording the events & snapshotting works fine, but the policy not. Now I tried producing an event with setting the custom event attributes of B to None the policy of AppC is now triggered. But this works only without using Transcodings.

Do you have any idea that explains that behaviour?

[johnbywater]

Do you need also to add the transcodings to the applications that are processing the events that are stored using custom transcodings?

Do you get any exceptions that might be useful for me to see?

[ehuebner]

I only implemented the transcoder in AppB and not in AppC. But now that you say it, it makes total sense to me to have the transcoder & transcoding implemented on both sides.

There was no exception thrown, that is why I also thought the way you suggested in your answer is the wrong way. It is just silently not working when the application that implements the policy does not know the transcodings.

[johnbywater]

@ehuebner I'm glad you got it working! Well done.

I'm very interested to understand how this error was silent. I've been investigating, and can see exceptions being raised in test output, when I arrange things to be broken in this way. Please can you replicate what you are doing so I can understand your experience?