Event Sourcing

Event Sourcing is a data-management pattern that captures every change to application state as an immutable, ordered sequence of events instead of overwriting a single current record. Rather than storing “the balance is $100,” an event-sourced system stores the deposits and withdrawals that add up to $100, and derives the balance by replaying them. The log becomes the source of truth, and current state is a projection you can rebuild at any time.

• Append-only event log - Every state change is recorded as a new immutable event; nothing is updated or deleted in place.
• Replay and projection - Current state, and any number of alternate read models, are rebuilt by replaying the event stream from the beginning.
• Complete audit trail - Because the history is the storage model, you get a full, tamper-evident record of how the system reached its present state for free.
• Temporal queries - You can reconstruct the state of the system at any past point in time, which is invaluable for debugging and compliance.
• Pairs with CQRS - It commonly sits alongside Command Query Responsibility Segregation, separating the write-side event stream from optimized read-side projections.

In real API operations, event sourcing shows up behind event-driven and streaming architectures — the same systems I profile around Kafka, REST proxies, and webhook fan-out — where an append-only stream feeds downstream services, analytics, and integrations. It gives an API a durable, replayable backbone for state changes, which is exactly what agentic and audit-heavy workloads need when they have to explain not just what the current value is, but how it got there. For teams designing APIs today, treating meaningful business changes as first-class events is what makes those APIs observable, recoverable, and honest about their own history.