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Event Sourcing

  • Ref:
    - https://www.youtube.com/watch?v=i2eVTk2Fb40
  • If we are using [[Event Driven Architecture]], we can (and should) store state changes of the system/database using events in some kind of Event Logs
  • The events in the Event Logs should be stored in the order that it was created.
  • By reading from the Event Logs, we can know the history of changes of the data, and can also recreate the data/rebuild the state of the system at any point => Event Sourcing
  • Features
    • Complete Rebuild: We can always delete the database, and then let the system process the event logs again to recreate everything
    • Temporal Query: Because we have the history of changes, we can always query to see what was the state before the current state.

Usage

  • Audit Log: When we want to keep track of what happen in the past.
  • Parallel Processing: 
    - We can have multiple system subscribing to the same data sources
    - When a change happens, all these services can process the changes in parallel
    - This is really helpful if we want to duplicate the data in multiple system easily
    - Not all services will process the event at the same time, so for some period of time, the data between services will be out of sync, but will gradually catchup/sync with each other => [[Consistency patterns#Eventual Consistency | Eventual Consistency]]