Understanding Event Sourcing

Event sourcing is a pattern where you store the state of your application as a sequence of events rather than just the current state. Instead of updating records in place, you append new events describing what happened.

Events as source of truth#

In traditional CRUD applications, you store the current state:

UPDATE students SET name = 'Alice Johnson' WHERE id = 123;

With event sourcing, you store what happened:

StudentRegistered(id: 123, name: 'Alice Johnson')
StudentNameChanged(id: 123, from: 'Alice Johnson', to: 'Alice Cooper')

The current state is derived by replaying these events in order.

Replaying events to reconstruct state#

Systems rebuild state by processing events sequentially:

state = EmptyState
for each event in events:
    state = apply(state, event)

This replay mechanism enables:

• Audit trail: Complete history of all changes
• Debugging: Replay events to understand how the system reached its current state
• Time travel: Query past states by replaying events up to a specific point

Benefits of event sourcing#

Complete audit trail. Every change is recorded as an immutable event, providing a full history for compliance, debugging, and analysis.

Debugging and troubleshooting. Replay events to reproduce bugs or understand how the system reached a problematic state.

Flexible projections. Build multiple read models from the same events, optimized for different queries.

Business insights. Analyze event history to understand user behavior, identify trends, and make data-driven decisions.