Event Sourcing: Storing State as a Sequence of Events

Instead of persisting current state, store every change as an immutable event and derive state by replaying the log.

January 25, 2026

#event-sourcing#event-driven#cqrs

In event sourcing, the database is an append-only event log. Current state is a left-fold over all events for an aggregate. This gives you a complete audit trail, the ability to replay history for debugging, and a natural integration bus — other services subscribe to the same event stream. The trade-off is query complexity: you must build and maintain read-model projections.

json

{ "type": "OrderPlaced",   "orderId": "ord-1", "total": 49.99 }
{ "type": "ItemAdded",     "orderId": "ord-1", "sku": "ABC", "qty": 2 }
{ "type": "OrderShipped",  "orderId": "ord-1", "trackingId": "TRK-99" }

Event store entries for an Order

Dependency Injection and IoC Containers
Invert control of dependency creation to keep classes decoupled from their collaborators and trivially testable.
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The Observer Pattern and Event-Driven Design
Decouple publishers from subscribers by letting objects register interest in events without the producer knowing who is listening.
→
The Strategy Pattern: Encapsulating Algorithms
Define a family of algorithms, encapsulate each one, and make them interchangeable — open for extension, closed for modification.
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Dependency Injection and IoC Containers

The Observer Pattern and Event-Driven Design

The Strategy Pattern: Encapsulating Algorithms