Question 1

What's the difference between Event Sourcing and CQRS?

Accepted Answer

Event Sourcing is about how you store state: you save every change as an immutable event (e.g., OrderPlaced, PaymentCaptured). CQRS (Command Query Responsibility Segregation) is about how you structure reads and writes: commands change state, queries read state, often using different models. They’re often used together: Event Sourcing provides the write-side history, and CQRS uses that history to build fast read-side projections.

Question 2

When should I use Event Sourcing?

Accepted Answer

Use Event Sourcing when you need a complete audit trail, the ability to reconstruct state at any point in time, or you expect to rebuild read models as requirements change. It’s a good fit for domains like payments, orders, inventory, and workflows where “what happened” matters as much as “what is.” Avoid it for simple CRUD apps where you only need the latest state and don’t need replay/audit, because it adds operational and design complexity.

Question 3

How much does Event Sourcing cost?

Accepted Answer

Costs depend on (1) event volume and retention (storage grows with every change), (2) throughput and latency requirements (streaming/queue costs and database IOPS), (3) projection infrastructure (compute to build and rebuild read models), and (4) operational overhead (schema evolution, monitoring, and replays). You can reduce cost by compressing/compacting events where appropriate, using tiered storage/archival for old events, and limiting projections to what you actually query.

Event Sourcing

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