Postgres projector and checkpoint store - transaction

[peteglassey]

I have made a custom projector and checkpoint store using eventuous extension points which uses Marten DB over postgres as its store. I works well.

I have a question about how could we commit the read-model change AND the checkpoint in the same PG txn? Is there an easy way, or will it require some more custom code?

I looked at the eventuous postgres implementation (checkpoint store and projector), and couldnt see how the two are enlisted in the same transaction, so i assume they arent. I assume that the philosophy is everything is an upsert, so it would be less of an issue if we update the checkpoint, but not update/create a read-model due to failure in between - and we get a duplicate event on restart.

[alexeyzimarev]

Right now it's not supported. I thought about it. With the current design, it's virtually impossible, but there must be another way to do it. Let me think about it.
But be aware that it 100% impossible to do partitioned projection with transactional checkpointing. That's why I always advise to design idempotent projections. However, knowing how Marten works as a doc db, I understand that it's "read-detect-write" pattern compared to pure SQL with hand-crafted updates.