There are two ways to handle concurrency. You can either use our built in optimistic concurrency control support or make sure you’re always writing data to one Aggregate using a single writer thread. Both methods have their pros and cons and are perfectly viable solutions.

Optimistic Concurrency-Control (OCC)

To guarantee consistent writes to an Aggregate in a multi-threaded scenario you can use the field expectedVersionwhen posting event batches. When creating a new Aggregate the field should be set to 0 to guarantee that no previous events have been stored for that id.

When loading events for an Aggregate the response will contain the field aggregateVersion. That number is what you are supposed to use as the expectedVersion in your next Aggregate update. If the Aggregate was updated by some other process concurrently, ie. the server-side version check fails, you will get a HTTP 409 (Conflict) response back. You will then have to load the Aggregate once again, re-apply your change and try to save it again.

Single-writer

If you are reading and writing Aggregates from a single thread or process (eg. like in the Actor model) you don’t have to care about the version fields at all.