Event sourcing: Write event before or after updating the model

Question

I'm reasoning about event sourcing and often I arrive at a chicken and egg problem. Would be grateful for some hints on how to reason around this.

If I execute all I/O-bound processing async (ie writing to the event log) then how do I handle, or sometimes even detect, failures?

I'm using Akka Actors so processing is sequential for each event/message. I do not have any database at this time, instead I would persist all the events in an event log and then keep an aggregated state of all the events in a model stored in memory. Queries are all against this model, you can consider it to be a cache.

Example

Creating a new user:

1. Validate that the user does not exist in model
2. Persist event to journal
3. Update model (in memory)

If step 3 breaks I still have persisted my event so I can replay it at a later date. If step 2 breaks I can handle that as well gracefully.

This is fine, but since step 2 is I/O-bound I figured that I should do I/O in a separate actor to free up the first actor for queries:

Updating a user while allowing queries (A0 = Front end/GUI actor, A1 = Processor Actor, A2 = IO-actor, E = event bus).

1. (A0->E->A1) Event is published to update user 'U1'. Validate that the user 'U1' exists in model
2. (A1->A2) Persist event to journal (separate actor)
3. (A0->E->A1->A0) Query for user 'U1' profile
4. (A2->A1) Event is now persisted continue to update model
5. (A0->E->A1->A0) Query for user 'U1' profile (now returns fresh data)

This is appealing since queries can be processed while I/O-is churning along at it's own pace.

But now I can cause myself all kinds of problems where I could have two incompatible commands (delete and then update) be persisted to the event log and crash on me when replayed up at a later date, since I do the validation before persisting the event and then update the model.

My aim is to have a simple reasoning around my model (since Actor processes messages sequentially single threaded) but not be waiting for I/O-bound updates when Querying. I get the feeling I'm modeling a database which in itself is might be a problem.

If things are unclear please write a comment.

Answer 1

Asychronous I/O can coexist with transactional updates. If you send an "ACK" or "NACK" after the command, then you can understand whether it has happened or not. In a distributed or truly asynchronous model, it is likely the "NACK" will come from both explicit failures and time-outs.