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name: Event Sourcing description: Implementing event sourcing pattern for maintaining application state as a sequence of immutable events.

Event Sourcing

Overview

Event Sourcing stores state changes as an immutable sequence of events. Current state is derived by replaying events, enabling auditability, time travel, and rebuildable read models.

Table of Contents

1. What is Event Sourcing
2. Event Sourcing vs CRUD
3. Event Store Design
4. Event Types and Schema
5. Versioning and Upcasting
6. Snapshots
7. Projections and Read Models
8. Event Replay
9. Consistency Guarantees
10. Event Sourcing with CQRS
11. Aggregate Design
12. Event Store Implementations
13. Schema Evolution
14. Testing
15. When to Use

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What is Event Sourcing

Instead of storing only the latest state, store each state change as an event:

CopyOrderCreated -> PaymentAuthorized -> OrderShipped

The current state is a fold of all events for an aggregate.

Event Sourcing vs CRUD

• CRUD: Overwrites state, limited audit history.
• Event Sourcing: Immutable history, full traceability, replayable.

Trade-off: higher complexity and operational overhead.

Event Store Design

Key characteristics:

• Append-only log
• Strong ordering per aggregate
• Optimistic concurrency via version checks

Minimum fields:

Copy{
  "eventId": "uuid",
  "aggregateId": "order-123",
  "type": "PaymentAuthorized",
  "version": 3,
  "timestamp": "2026-01-01T12:00:00Z",
  "data": {}
}

Event Types and Schema

Guidelines:

• Use explicit domain events.
• Validate event payloads.
• Avoid generic "update" events.

Versioning and Upcasting

Events evolve. Use upcasters to map old versions to new schemas:

• Add defaults for new fields
• Convert deprecated fields
• Preserve backward compatibility

Snapshots

Snapshots reduce replay time:

• Snapshot every N events or time interval
• Store snapshot version and payload
• Replay events after the snapshot

Projections and Read Models

Build query-optimized views by projecting events:

• Materialized views per feature
• Denormalized read models
• Rebuildable from event log

Event Replay

Rebuild state by replaying events:

• For new projections
• For recovery from corruption
• For audits and simulations

Use replay safeguards:

• Throttle rebuilds
• Run in isolated environments

Consistency Guarantees

• Strong ordering per aggregate
• Eventual consistency across aggregates
• Read models may be stale until projections catch up

Event Sourcing with CQRS

Command side writes events; query side reads projections:

• Separate write model from read model
• Scale reads independently
• Clear separation of responsibilities

Aggregate Design

Aggregates enforce invariants:

• Small, cohesive boundaries
• Single source of truth for rules
• Avoid cross-aggregate transactions

Event Store Implementations

• EventStoreDB: Purpose-built event store.
• PostgreSQL: Append-only table + indexes.

For relational DBs, enforce:

• Append-only writes
• Unique (aggregateId, version)

Schema Evolution

Guidelines:

• Keep events immutable.
• Deprecate old event types gradually.
• Maintain versioned schemas and upcasters.

Testing

• Unit test aggregates with given-when-then.
• Integration test projections and rebuilds.
• Verify idempotency of projection handlers.

When to Use

Use when:

• Auditability and replay are required.
• Complex workflows need traceability.

Avoid when:

• CRUD is sufficient.
• Team lacks ops maturity for event-driven systems.

Related Skills

• 09-microservices/cqrs-pattern
• 09-microservices/saga-pattern
• 09-microservices/event-driven

Best Practices

Event Design

• Use explicit domain events: Name events based on business language
• Make events immutable: Never modify events after creation
• Include event metadata: Add timestamps, correlation IDs, and causation IDs
• Keep events small: Prefer focused events over large payloads
• Validate event schemas: Ensure events conform to defined schemas

Event Store Design

• Use append-only storage: Never update or delete events
• Enforce ordering per aggregate: Ensure events are ordered by version
• Implement optimistic concurrency: Use version checks to prevent conflicts
• Choose appropriate storage: EventStoreDB, PostgreSQL, or other options
• Plan for scalability: Design for high write throughput

Snapshots

• Snapshot periodically: After N events or time interval
• Store snapshot version: Include event version for consistency
• Use snapshots for replay: Start replay from last snapshot
• Keep snapshots consistent: Ensure snapshots match event history
• Monitor snapshot performance: Track snapshot creation time

Projections

• Design for queries: Build projections optimized for read patterns
• Make projections rebuildable: Allow complete rebuild from event log
• Use idempotent handlers: Handle duplicate events safely
• Handle event ordering: Process events in correct order
• Monitor projection lag: Track how far behind projections are

Event Replay

• Test replay procedures: Verify replay works correctly
• Use throttling: Limit replay impact on production
• Plan replay time: Estimate how long replay will take
• Use isolated environments: Test replay in staging first
• Monitor replay progress: Track replay completion

CQRS Integration

• Separate read and write models: Use different schemas for each
• Scale independently: Scale read and write sides separately
• Handle eventual consistency: Communicate staleness to users
• Use read-your-writes: When strong consistency is needed
• Design for async updates: Accept temporary inconsistency

Schema Evolution

• Never modify existing events: Create new event types instead
• Use upcasters: Convert old event versions to new schemas
• Deprecate gradually: Mark old event types before removal
• Maintain backward compatibility: Support multiple event versions
• Document schema changes: Keep clear records of all changes

Performance

• Batch event writes: Write multiple events together when possible
• Use appropriate indexes: Index for read patterns, not write patterns
• Monitor event throughput: Track events per second
• Optimize replay: Use snapshots to reduce replay time
• Choose appropriate storage: Match storage to workload

Monitoring

• Track event counts: Monitor events per aggregate and type
• Monitor projection lag: Alert on large delays
• Track replay performance: Measure replay time and resource usage
• Monitor event store health: Track storage metrics
• Set up alerts: Notify on anomalies or failures

Checklist

Event Design

• Define event schemas
• Use explicit domain event names
• Include event metadata (timestamps, correlation IDs)
• Validate event payloads
• Document event types

Event Store Setup

• Choose appropriate event store
• Configure append-only writes
• Implement optimistic concurrency
• Set up indexes for queries
• Configure replication for high availability

Aggregate Design

• Define aggregate boundaries
• Implement business invariants
• Design event sequence
• Implement version checking
• Test aggregate behavior

Snapshot Strategy

• Define snapshot frequency
• Implement snapshot creation
• Store snapshot versions
• Test replay from snapshots
• Monitor snapshot performance

Projection Design

• Identify read patterns
• Design projection schemas
• Implement projection handlers
• Make projections idempotent
• Test projection rebuilds

CQRS Setup

• Separate read and write models
• Configure separate databases if needed
• Implement command handlers
• Implement query handlers
• Handle eventual consistency

Event Replay

• Design replay procedures
• Implement replay throttling
• Test replay in staging
• Plan replay windows
• Monitor replay progress

Schema Evolution

• Define event versioning strategy
• Implement upcasters
• Plan deprecation process
• Maintain backward compatibility
• Document schema changes

Monitoring

• Set up event metrics
• Monitor projection lag
• Track event throughput
• Configure alerts
• Create dashboards

Testing

• Test aggregate behavior
• Test projection handlers
• Test event replay
• Test schema evolution
• Performance test event store

Documentation

• Document event schemas
• Document aggregate design
• Document projection logic
• Create replay runbooks
• Maintain API documentation