Tapahtumalähtö

Tapahtumalähtö, often translated as event sourcing, is an architectural pattern that structures an application around a sequence of immutable events. Instead of storing the current state of an entity directly, event sourcing records every change as a distinct event. These events are appended to an ordered log, often referred to as the event store.

When an application needs to retrieve the current state of an entity, it does so by replaying the sequence of relevant events from the event store. This process reconstructs the state as it would be at any given point in time. This "rehydration" of state can be optimized using snapshots, which store the state at a specific event sequence number, allowing for faster retrieval of recent states.

The benefits of event sourcing include a complete audit trail of all changes, enabling easy debugging and historical analysis. It also supports temporal queries, allowing users to view data as it existed in the past. Furthermore, event sourcing aligns well with distributed systems and microservices architectures, as events can be published and consumed asynchronously. It also facilitates the implementation of features like undo/redo functionality and complex state transitions. However, it can introduce complexity in querying and managing large event streams, and requires careful consideration for data schema evolution.