Event Sourcing

Table of Contents Introduction Core Concepts 2.1. What Is Event Sourcing? 2.2. What Is CQRS? Why Combine Event Sourcing and CQRS? Designing a Resilient Architecture 4.1. Event Store Selection 4.2. Command Side Design 4.3. Query Side Design 4.4. Event Publishing & Messaging Practical Implementation Example 5.1. Domain Model: Order Management 5.2. Command Handlers 5.3. Event Handlers & Projections 5.4. Sample Code (C# with EventStoreDB & MediatR) Operational Concerns 6.1. Event Versioning & Schema Evolution 6.2. Idempotency & Exactly‑Once Processing 6.3. Consistency Models 6.4. Testing Strategies 6.5. Monitoring & Observability Real‑World Case Studies Best‑Practice Checklist Conclusion Resources Introduction Modern distributed systems must cope with high traffic volumes, evolving business rules, and ever‑changing infrastructure. Traditional CRUD‑centric designs often become brittle under these pressures: they mix read and write concerns, hide domain intent, and make scaling unpredictable. ...

Introduction Command Query Responsibility Segregation (CQRS) is an architectural pattern that separates reads (queries) from writes (commands). Rather than using a single data model and layer to both modify and read state, CQRS encourages designing optimized models and pathways for each. This separation can improve scalability, performance, and clarity—especially in complex domains—while introducing new challenges around consistency, messaging, and operational complexity. This guide provides a practical, vendor-neutral overview of CQRS: what it is, when it helps, how to implement it (with and without event sourcing), and the pitfalls to avoid. Code examples are provided to illustrate implementation techniques. ...