CQRS & Event Sourcing
4 likes740 views
This document discusses Command Query Responsibility Segregation (CQRS) and event sourcing patterns. It explains that with event sourcing, all state changes are stored as a sequence of events. This allows reconstructing past application states and handling errors. CQRS separates read and write operations so that read models optimized for queries can be updated from the event stream to improve read performance. Projections are used to update separate read models from the event stream to support multiple query views of the data.
CQRS & Event Sourcing
- 1. CQRS & EVENT SOURCING I - Event sourcing
- 2. TRADITIONAL PERSISTENCE Store only the current state
- 3. TRADITIONAL PERSISTENCE Store only the current state
- 5. EVENT SOURCING Store the events that have led to every state 1. Created 2. Updated foo 3. Updated bar 4. Deleted
- 6. EVENT SOURCING Benefits Natural audit log Ability to reconstruct a previous state Ability to respond to bugs in state
- 7. COMMAND-QUERY SEPARATION Command function/method Produces state change Has observable side-effects Either succeeds or fails Has a "void" return type Throws exceptions
- 8. COMMAND Produces a state change and emits an event Store only the events Reproduce the state using the events
- 10. EVENTS Type (class) Object identifier Timestamp Any relevant data (Playhead)
- 11. assignments/01.md Event sourcing the `Meetup` aggregate
- 12. CQRS & EVENT SOURCING II - CQRS
- 13. EVENT SOURCING Slow on the "read" side We need to: 1. Fetch and recreate all event objects 2. Reconstitute the object based on its events
- 14. CQRS Helps with scaling on the read side
- 15. COMMAND-QUERY SEPARATION Query method Has no observable side-effects Either succeeds or fails Has a specific return type Throws exceptions
- 16. COMMAND/QUERY SEPARATION PRINCIPLE (CQS) Every method should be either a command method, or a query method command(): void query(): type
- 17. COMMAND/QUERY RESPONSIBILITY SEGREGATION (CQRS) Domain objects should have either command methods, or query methods query(): typecommand(): void
- 18. WE WANT TO PERSIST EVENTS But we need the current state too
- 19. IF ONLY WE COULD Tail the event log... event event event
- 20. PROJECTIONS Using events to update read models event
- 21. READ MODELS Are query objects query(): type query(): type query(): type
- 22. READ MODELS Are use-case specific
- 23. READ MODELS Are easy to query SELECT * FROM ... WHERE ...
- 24. READ MODELS Can use polyglot persistence
- 25. assignments/02.md Apply CQRS to the `Meetup` aggregate
- 29. CQRS & EVENT SOURCING III - Potential
- 30. CQRS & ES Very useful for Business insights Scaling Application integration Microservices