Workshop slides (https://training.matthiasnoback.nl/)

1. CQRS
& EVENT
SOURCING
I - Event sourcing

2. TRADITIONAL PERSISTENCE
Store only the current state

3. TRADITIONAL PERSISTENCE
Store only the current state

4. TRADITIONAL PERSISTENCE
Snapshots

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

9. RECONSTITUTION
Append new events
event
event
event
event
command

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

26. assignments/03.md
Twitsup: Unfollowing

27. assignments/04.md
Twitsup: Mentions

28. assignments/05.md
Twitsup: Mentions

29. CQRS
& EVENT
SOURCING
III - Potential

30. CQRS & ES
Very useful for
Business insights
Scaling
Application integration
Microservices