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Event Sourcing

There are some parallels between the ideas we’ve discussed here and event sourcing, a technique that was developed in the domain-driven design (DDD) community [42, 43, 44]. We will discuss event sourcing briefly, because it incorporates some useful and relevant ideas for streaming systems.

Similarly to change data capture, event sourcing involves storing all changes to the application state as a log of change events. The biggest difference is that event sourcing applies the idea at a different level of abstraction:

  • • In change data capture, the application uses the database in a mutable way, updating and deleting records at will. The log of changes is extracted from the database at a low level (e.g., by parsing the replication log), which ensures that the order of writes extracted from the database matches the order in which they were actually written, avoiding the race condition in Figure 11-4. The application writing to the database does not need to be aware that CDC is occurring.
  • • In event sourcing, the application logic is explicitly built on the basis of immutable events that are written to an event log. In this case, the event store is append- only, and updates or deletes are discouraged or prohibited. Events are designed to reflect things that happened at the application level, rather than low-level state changes.

Event sourcing is a powerful technique for data modeling: from an application point of view it is more meaningful to record the user’s actions as immutable events, rather than recording the effect of those actions on a mutable database. Event sourcing makes it easier to evolve applications over time, helps with debugging by making it easier to understand after the fact why something happened, and guards against application bugs (see “Advantages of immutable events” on page 460).

For example, storing the event “student cancelled their course enrollment” clearly expresses the intent of a single action in a neutral fashion, whereas the side effects “one entry was deleted from the enrollments table, and one cancellation reason was added to the student feedback table” embed a lot of assumptions about the way the data is later going to be used. If a new application feature is introduced—for example, “the place is offered to the next person on the waiting list”—the event sourcing approach allows that new side effect to easily be chained off the existing event.

Event sourcing is similar to the chronicle data model [45], and there are also similarities between an event log and the fact table that you find in a star schema (see “Stars and Snowflakes: Schemas for Analytics” on page 93).

Specialized databases such as Event Store [46] have been developed to support applications using event sourcing, but in general the approach is independent of any particular tool. A conventional database or a log-based message broker can also be used to build applications in this style.

 
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