Event Sourcing

Event Sourcing ensures that all changes to application state are stored as a sequence of events. We don't just store the current state; we store "what happened".

When to Use

• Audit Logs: When you need to know exactly how you got to the current state (Banking, Legal).
• Temporal Queries: "What did the system look like last Tuesday?".
• Intent Capture: distinguishing between "Correction" vs "Update".

Quick Start (Conceptual)

Traditional (State Stored): Order { id: 1, status: 'Shipped' } -> Update to 'Delivered' -> Order { id: 1, status: 'Delivered' } (History lost)

Event Sourcing (Events Stored):

1. OrderCreated(id=1)
2. PaymentReceived(id=1)
3. OrderShipped(id=1)
4. OrderDelivered(id=1)

To get current state: Replay (1) + (2) + (3) + (4).

Core Concepts

Event Store

A database optimized for appending immutable events (e.g., EventStoreDB).

Snapshots

To avoid replay performance issues for long histories (10,000 events), save a "Snapshot" every 100 events. Replay = Snapshot + subsequent events.

Projections

Code that listens to events and builds a read-optimized view (The "R" in CQRS).

Best Practices

Do:

• Keep Events Immutable. You can never change the past. To fix an error, append a "CorrectionEvent".
• Version your Events carefully (Upcasting) to handle schema changes over years.

Don't:

• Don't put logic in the Event Store. It's just a log.
• Don't query the Event Stream for complex searches (Use a Projection/Read Model).

Troubleshooting

Error	Cause	Solution
Slow Replay	Too many events for an aggregate.	Implement Snapshots or check Aggregate boundaries.
Schema Evolution	Old events don't match new code.	Implement "Upcasters" to transform old events on the fly.

References

• EventStoreDB
• Martin Fowler - Event Sourcing