How to handle projection errors by event sourcing and CQRS?

Question

I want to use event sourcing and CQRS, and so I need projections (I hope I use the proper term) to update my query databases. How can I handle database errors?

For example one of my query cache databases is not available, but I already updated the others. So the not-available database won't be in snyc with the others when it comes back to business. How will it know that it have to run for instance the last 10 domain events from the event storage? I guess I have to store information about the current state of the databases, but what if that database state storage fails? Any ideas, best practices how to solve this kind of problems?

Answer 1

In either case, you must tell your messaging bus that the processing failed and it should redeliver the event later, in the hope that the database will be back online then. This is essentially why we are using message bus systems with an "at least once"-delivery guarantee.

For transactional query databases, you should also rollback the transaction, of course. If your query database(s) do not support transactions, you must make sure on the application side that updates are idempotent - i.e., if your event arrives on the next delivery attempt, your projection code and/or database must be designed such that the repeated processing of the event does not harm the state of the database. This is sometimes trivial to achieve (e.g., when the event leads to a changed person's name in the projection), but often not-so-trivial (e.g., when the projection simply increments view counts). But this is what you pay for when you are using non-transactional databases.