What is Event Sourcing

Definition

Event sourcing is a design pattern in which application state is stored not as a current snapshot but as an immutable, ordered log of every event that changed it, so the present state can always be rebuilt by replaying events from the beginning.

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Preserves a complete, immutable history of every change for audit and analysis
Allows state to be rebuilt or corrected by replaying the event log
Enables temporal queries that reconstruct state at any past point in time
Decouples state changes from their consumers, naturally fitting event-driven systems

Real World Example

A banking ledger uses event sourcing to record each deposit and withdrawal as an immutable event, so the current balance is derived by replaying events and a full, auditable history is preserved by design.

FAQs

How does event sourcing store state?

It stores an ordered, immutable sequence of events; current state is computed by replaying those events rather than overwriting a single record.

What is the main benefit of event sourcing?

A complete, auditable history that lets you reconstruct any past state and reason precisely about how the system reached its current state.

What is a trade-off of event sourcing?

It adds complexity, since reading current state requires replaying or maintaining projections, and the event log grows continuously.