Event Sourcing Implementation Workflow
Implement event sourcing for a domain with an event store, projections, and snapshotting for performance.
Objective
Use event sourcing to maintain a complete history of state changes.
Environment
- A domain with complex state transitions
- An event store
- A read model projection strategy
Steps
1. Define events
Identify all domain events that represent state changes.
Expected state: A complete set of domain events is defined.
2. Build event store
Create an append-only event store with optimistic concurrency control.
Expected state: The event store accepts and persists domain events.
3. Implement projections
Build read model projections that transform events into queryable state.
Expected state: Read models are populated from the event stream.
4. Add snapshotting
Implement periodic snapshots to avoid replaying the full event history on each load.
Expected state: Snapshots reduce the number of events that must be replayed.
5. Test the model
Verify that the aggregate state can be fully reconstructed from events.
Expected state: The domain state is correctly reconstructed from the event history.
Decision points
Use snapshotting?
- Yes, for long event streams
- No, keep it simple
Use snapshotting when event streams exceed 1000 events; otherwise, full replay is fast enough.
Expected outcome
Event sourcing is implemented with an event store, projections, and snapshotting.
Verification
Reconstruct aggregate state from events and confirm it matches the expected state; verify projections are consistent.
Cleanup
Clear the event store if it is only for testing; never clear a production event store.
Event sourcing preserves history
By storing events instead of state, you gain full audit history and the ability to rebuild any past state.
Implementation Notes
This workflow provides a structured approach to ensure consistent results across team members and environments.
Common Pitfalls
Skipping verification steps or rushing through the process can lead to incomplete results. Always follow each step in order and verify before proceeding.
Quick Reference
# Execute the workflow steps as documented above
echo "Workflow completed successfully"