User request to issue or trade credit
Backend & Runtime / Knowledge library
Event Sourcing
Stores an append-only sequence of domain events as the authoritative state history
Why learn Event Sourcing?
It supports audit, temporal reconstruction, and alternate projections
The problem it solves
It preserves how state changed instead of only its latest value
Where it came from
Event Sourcing is a software design pattern that stores an append-only sequence of domain events as the authoritative state history. Its guarantees and tradeoffs depend on the concrete implementation and operating context.
Build your understanding
Work through these exercises in order. Try the task before opening a reference.
- Define Event Sourcing and distinguish it from a neighboring concept.
- Choose a realistic task where Event Sourcing is appropriate.
- Identify a failure mode caused by misusing Event Sourcing.
- Interpret a small design or result involving Event Sourcing.
- Explain one tradeoff or limitation of Event Sourcing.
Documentation and sources
- Patterns of Enterprise Application Architecture (martinfowler.com)
Connect the ideas
Explore next
Command PatternCommand Query Responsibility Segregation (CQRS)Domain-Driven DesignEvent-Driven ArchitectureImmutabilitySaga Pattern
Real World Example
Auditable Carbon Credit Ledger
Environmental compliance firms managing carbon credit issuance and trading.
The business problem
Traditional CRUD databases overwrite current state, losing the historical context required for regulatory audits and preventing the reconstruction of credit balances at specific past timestamps.
How this topic solves it
Event Sourcing stores an append-only sequence of domain events as the authoritative state history, allowing the system to derive the current balance by replaying the event stream from the beginning.
- Define immutable event types such as CreditIssued, CreditTransferred, and CreditRetired.
- Append each new event to an immutable event store instead of updating a current balance row.
- Implement a projection engine that consumes the event stream to build a read-optimized view of current balances.
- Use the event stream to reconstruct the state of any account at any point in time for audit verification.
Explains how events are captured and projected into state.
Append-only log of domain events
Logic to derive current state
Current balance view for queries
- Command → Persist Event → Event Store
- Event Store → Stream Events → Projection
- Projection → Update State → Read Model
Expected business outcome
The system gains a complete, immutable audit trail of every credit movement, ensuring regulatory compliance and enabling point-in-time state reconstruction.
Limitations and checks
The primary limitation is the complexity of evolving the event schema over time; to check for this, monitor the cost of migration scripts required to handle legacy event formats when business rules change.