On his blog Frank De Jonge, member of the PHP League and creator of Flysystem, explains a cool pragmatic solution how to ensure queued processes for the same user complete in the right order.

The use of multiple workers allows for much higher throughput, but it also allows for race conditions during processing. When messages for the same user are sent to different workers, handling order can no longer be guaranteed. Therefore we have failed to fulfil our business rule.

If we were able to ensure every message from the same user were sent to the same worker, the worker could ensure those messages are handled in order while the system as a whole would still benefit from the degree of parallelism. But how do we make this happen?