--- id: kb-2026-00253 title: Event Sourcing schema_type: TechArticle category: computer-science language: en confidence: medium last_verified: '2026-06-24' created_date: '2026-05-22' generation_method: ai_structured ai_models: - gpt-5-codex derived_from_human_seed: true conflict_of_interest: none_declared is_live_document: false data_period: static atomic_facts: - id: fact-event-sourcing-001 statement: >- Microsoft Azure Architecture Center describes Event Sourcing as storing the full series of actions taken on an object in an append-only store instead of storing only current state in a relational database. source_title: Event Sourcing pattern source_url: https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing confidence: medium - id: fact-event-sourcing-002 statement: >- Microsoft Azure Architecture Center says an Event Sourcing event store acts as the system of record and can be used to materialize domain objects. source_title: Event Sourcing pattern source_url: https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing confidence: medium - id: fact-event-sourcing-003 statement: >- Microsoft Azure Architecture Center says applications derive the current state of an entity by replaying all events in its stream, a process it calls rehydration. source_title: Event Sourcing pattern source_url: https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing confidence: medium - id: fact-event-sourcing-004 statement: >- AWS Prescriptive Guidance describes an event store as an immutable, append-only, chronologically ordered repository or data store. source_title: Event sourcing pattern - AWS Prescriptive Guidance source_url: https://docs.aws.amazon.com/prescriptive-guidance/latest/cloud-design-patterns/event-sourcing.html confidence: medium - id: fact-event-sourcing-005 statement: >- Microsoft Azure Architecture Center describes Command Query Responsibility Segregation as separating read and write operations for a data store into separate data models. source_title: Command Query Responsibility Segregation pattern source_url: https://learn.microsoft.com/en-us/azure/architecture/patterns/cqrs confidence: medium - id: fact-event-sourcing-006 statement: >- Martin Fowler describes Event Sourcing as capturing all changes to application state as a sequence of events and using the event log to reconstruct past states. source_title: Event Sourcing source_url: https://martinfowler.com/eaaDev/EventSourcing.html confidence: medium completeness: 0.88 known_gaps: - >- This compact repair covers the core storage model, replay semantics, CQRS relationship, and auditability tradeoff; it does not compare event-store products, schema-evolution strategies, or broker-specific delivery behavior. disputed_statements: [] primary_sources: - title: Event Sourcing pattern type: documentation year: 2026 url: https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing institution: Microsoft - title: Event sourcing pattern - AWS Prescriptive Guidance type: documentation year: 2026 url: https://docs.aws.amazon.com/prescriptive-guidance/latest/cloud-design-patterns/event-sourcing.html institution: Amazon Web Services - title: Command Query Responsibility Segregation pattern type: documentation year: 2026 url: https://learn.microsoft.com/en-us/azure/architecture/patterns/cqrs institution: Microsoft - title: Event Sourcing type: reference_article year: 2005 url: https://martinfowler.com/eaaDev/EventSourcing.html institution: MartinFowler.com secondary_sources: [] updated: '2026-06-24' --- ## TL;DR Event Sourcing stores state changes as ordered events instead of treating the latest row or document as the only durable record. Current state is rebuilt from the event history, while derived read models can be optimized for queries. ## Core Explanation In an event-sourced system, each meaningful change is recorded as an event. The event store is append-only and becomes the system of record for the entity's history. Application code can rebuild an entity by replaying its event stream, and teams can derive materialized views when replaying every event would be too expensive for reads. The pattern is useful when auditability, historical reconstruction, or alternate projections justify the extra complexity. It also changes operational design: teams must think about event schema evolution, replay safety, concurrency, snapshots, projection lag, and the difference between a durable event store and a message broker. CQRS is a common companion pattern but not a requirement. CQRS separates the write model from the read model, so event-sourced writes can feed denormalized views that are better suited for queries. ## Source-Mapped Facts - Microsoft Azure Architecture Center describes Event Sourcing as storing the full series of actions taken on an object in an append-only store instead of storing only current state in a relational database. ([source](https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing)) - Microsoft Azure Architecture Center says an Event Sourcing event store acts as the system of record and can be used to materialize domain objects. ([source](https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing)) - Microsoft Azure Architecture Center says applications derive the current state of an entity by replaying all events in its stream, a process it calls rehydration. ([source](https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing)) - AWS Prescriptive Guidance describes an event store as an immutable, append-only, chronologically ordered repository or data store. ([source](https://docs.aws.amazon.com/prescriptive-guidance/latest/cloud-design-patterns/event-sourcing.html)) - Microsoft Azure Architecture Center describes Command Query Responsibility Segregation as separating read and write operations for a data store into separate data models. ([source](https://learn.microsoft.com/en-us/azure/architecture/patterns/cqrs)) - Martin Fowler describes Event Sourcing as capturing all changes to application state as a sequence of events and using the event log to reconstruct past states. ([source](https://martinfowler.com/eaaDev/EventSourcing.html)) ## Further Reading - [Event Sourcing pattern](https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing) - [Event sourcing pattern - AWS Prescriptive Guidance](https://docs.aws.amazon.com/prescriptive-guidance/latest/cloud-design-patterns/event-sourcing.html) - [Command Query Responsibility Segregation pattern](https://learn.microsoft.com/en-us/azure/architecture/patterns/cqrs) - [Event Sourcing](https://martinfowler.com/eaaDev/EventSourcing.html) ## Related Articles - [Command Pattern](command-pattern.md) - [Webhooks and Event-Driven APIs](webhooks-and-event-driven-apis.md) - [AsyncAPI and Event API Schemas](asyncapi-and-event-api-schemas.md)