eventsourced

Event sourcing is a software design pattern in which all changes to an application's state are captured as a sequence of events. Instead of persisting only the current state, an eventsourced system records every state-changing fact as an immutable event, and the state is derived by replaying these events from an initial baseline.

Key concepts: an event store is an append-only log of events. When a command is processed, one or more new events are produced and saved. Aggregates validate and apply events to evolve their state. Over time, the current state can be reconstructed by replaying the event stream; snapshots may be used to accelerate replay and reduce load.

Relationship to CQRS: Event sourcing is commonly used with CQRS, where the write model appends events and one or more read models or projections respond to those events, enabling optimized query paths.

Benefits: provides a complete audit trail, supports time travel debugging and reconstruction of past states, enables reliable integration through event notification, and can improve scalability by using append-only storage.

Challenges: increases architectural and operational complexity, requires robust event versioning and schema evolution, longer-term storage needs, and careful handling of eventual consistency between writes and reads; tooling and infrastructure for replay, snapshots, and event migration are important. Deleting data can be handled with tombstone events.

When to use: well suited for domains with complex business logic and need for history, auditing, or integration with other systems; may be overkill for simple CRUD apps.

Related terms: event store, event stream, event-driven architecture; common implementations include dedicated event stores and using log-based systems; in practice, many organizations pair event sourcing with CQRS.