CQRS: An Introduction for Beginners
- 1. Jonathan OliverCommand QueryResponsibility Segregation
- 2. Who Cares?“This time, it’s going to be different. Trust me.”“Doctor, it hurts when I do that.”Why?
- 3. A Question of PerspectiveWhy are we doing this?It’s a “Best Practice”.The way it’s always been done.
- 4. A Few ProblemsUIServices / CachingBottlenecksCachingScalabilityTranslation codeImpedance mismatchRace conditionsStale dataBusiness LogicDALDatabase
- 5. Stale Data?Get dataGet dataStale dataUpdate data
- 6. QueriesIf it’s stale anyway…List of Customers…………………….…………………….…………………….…………………….…………………….(from 10 minutes ago)
- 7. A Persistent View ModelRead only.Stored in structure used by view.QueryPersistent View ModelSELECT * FROM Table WHERE ID = …
- 8. That’s Blasphemy!It’s the simplest thing that could possibly work.Testability?Maintainability?Security?Data already shown over the web.It’s already in your cache on the web tier.Role-based SELECTs.
- 9. Commands and User InputWhat is the user telling us?Do we even know?How?Task-based UIs vs. CRUD-based UIs.Capture user intent explicitly.
- 10. Validation and Business RulesPre-validation from view model.Validation: Is this command complete?String lengthsRequired fieldsRegex, etc.Business Rules: Can I do what the user wants?Where is the source of truth?
- 11. Good CommandsValidated on client.Revalidated on server.Reply can be asynchronous:Amazon
- 12. CQRSImage courtesy Udi Dahan
- 13. Service LayerTransaction Script.Active Record / Table Module.Domain Model.No Getters/SettersTransactional.Separate database from queries.The “source of truth”.
- 14. Questions?