Question 1

What's the difference between Event Sourcing and CQRS?

Accepted Answer

Event Sourcing is about how you store state: you persist every change as an immutable event (e.g., MoneyDeposited, MoneyWithdrawn). CQRS (Command Query Responsibility Segregation) is about how you model access: you separate writes (commands) from reads (queries), often using different models. They’re commonly used together: Event Sourcing provides the write-side history, and CQRS read models are built by replaying events into query-optimized views.

Question 2

When should I use Event Sourcing?

Accepted Answer

Use it when you need a complete audit trail, the ability to reconstruct past state (“what did the account balance look like last Tuesday?”), or when multiple downstream systems need to react to the same business events. It’s a good fit for domains like finance, orders, inventory, and workflows. Avoid it if your domain is simple, you don’t need historical reconstruction, or your team can’t invest in the extra operational and design complexity (event versioning, replay, idempotency, and read-model maintenance).

Question 3

How much does Event Sourcing cost?

Accepted Answer

Costs come from the infrastructure you choose for the event store and event distribution, plus operational overhead. Key cost drivers include: event volume (writes per second), retention period (how long you keep events), storage type (SSD vs. object storage), replication and backups, and consumer processing (compute for projections/read models). If you use managed streaming (e.g., Kafka-as-a-service) and managed databases, expect costs to scale with throughput, storage, and availability requirements; if you self-manage, you may reduce direct service spend but increase engineering and operations cost.

Event Sourcing

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