Within chapter 4 an aggregator is implemented. The aggregator handler is written so that it is idempotent (i.e. it doesn’t matter if the handler is called once or N times with same parameters).

The way this is achieved is that view data tracks the global position of an event. In another words if event 5 is delivered multiple times the view data is updated only once.

However, the code cannot survive out-of-order delivery of events. In another words if message store delivers first event 5 followed by event 3 the videosWatched parameter should be updated twice (i.e. once for event 3 and once for event 5). In this case, however, the aggregator will increment the counter only once resulting in wrong state.

Perhaps message store can guarantee in-order delivery of events, but I suppose this would be a good thing to mention.

Otherwise there are probably multiple ways to fix this. One that comes to mind is to track a list of events (global position indices) processed by aggregator in the database and increment the counter if aggregator receives an event it has not yet processed.