CQRS / ES: Meeting, 2016-10-07
Participants: Alexander, Bastian, Dominique, Robert
The big merge
The packages Neos.EventStore, and Neos.EventStore.DatabaseStorageAdapter (Neos.EventStore.InMemoryStorageAdapteris obsolete) were merged into Neos.Cqrs for now to ease development:
NOTE: Please use https://github.com/neos/Neos.Cqrs/issues exclusively for issues and don’t push new commits to the other repositories for now.
Event serialization
Currently the event serialization is broken:
Apart from a a fatal error due to a recent PR the current serializers don’t support nested objects.
The main issue with that: Events will be processed without error but won’t contain the complete payload
@bwaidelich: We should serialize/unserialize events everytime when publishing so that this result is always the same during event handling and replay
Projection replay
No code yet due to short & busy week.
Some implementation ideas though:
- central authority for projections (aka
ProjectionManager) that must be used for all interactions with projections (handle events, replay, flush, …) - The ProjectionManager can track usage like last user/date/id of replays etc
@robert shared his concerns that this data might get out of sync if it’s not stored via the same mechanism as the projection’s state.
Suggestion:
We go for a simple approach for now that works along those lines:
The ProjectionManager …
- …takes care of fetching relevant events from the
EventStore - …passes those to the respective handlers of the projector
- …stores the last processed event number (aka sequence, version) after it has been handled successfully
- …will only load events with a
minimumVersiongreater than the last stored pointer - …has a way to reset this pointer
If we come across issues with that approach we could add a new interface that lets the projector take care of the version tracking instead.
@dfeyer: We also might need to implement some locking mechanism in order to prevent race conditions.