Event Sourcing is an architectural pattern that captures all changes to application state as a sequence of immutable events stored in an append-only log, using this event stream as the authoritative record from which current state can be derived. It establishes events as the primary source of truth, enabling complete system history, state reconstruction, temporal queries, and advanced auditing capabilities through the preserved event sequence.
Event Sourcing fundamentally transforms state management from storing current values to recording the complete history of changes that produced those values. It replaces traditional state-oriented persistence with an event-centric approach where each business action generates immutable events that are persisted sequentially and never modified. This historical record enables powerful capabilities including complete auditability, point-in-time reconstruction, alternative state projections, and complex event processing that traditional state-oriented architectures cannot easily support.
Modern event sourcing implementations have evolved beyond basic event logging to incorporate sophisticated patterns including event versioning, event upcasting, snapshotting, and projection management that address practical challenges in long-lived event-sourced systems. Leading organizations implement event sourcing selectively in domains that benefit from temporal querying, complete audit trails, or complex event analysis, while recognizing that it introduces additional complexity through eventual consistency, projection management, and event schema evolution. These implementations frequently combine event sourcing with CQRS to separate the event-based write model from optimized read projections that serve query needs efficiently. When effectively applied to suitable domains, event sourcing becomes a powerful architectural pattern that provides unprecedented visibility into system history while enabling advanced analytical capabilities through the preserved event record. As regulatory requirements, business intelligence needs, and system integration complexities increase, event sourcing has become an essential pattern for domains where understanding the complete history of state changes delivers substantial business value.
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