This document discusses CQRS and event sourcing patterns and the Axon framework. CQRS separates read and write operations into different models to address complexity issues. Event sourcing records all state changes as a sequence of immutable events. The Axon framework provides tools for building event-driven, DDD and CQRS applications including command handling, aggregates, and event routing. It was designed to focus on business logic and integrates with Spring Boot. The document concludes with references and an announcement of a hands-on demo project on GitHub.

1. CQRS and Event Sourcing
with Axon Framework

2. Apresentação
Tech Lead at MovilePay
João Rafael Campos
joao.campos@movile.com
/jrafaelcam
/rafaelcam
@rafaelcam
Senior Software Engineer at MovilePay
Diego Brener
diego.brener@movile.com
/diegobrener
/diegosilva13
@dbsilva

3. Agenda
● Pattern Applications
● CQRS
● Event Sourcing
● Axon Framework
● Demo

4. Pattern
Applications

5. Characteristics
➔Layered Architecture
➔Same domain for reading and writing
➔Same database

6. Day 2 Problems
➔Different reading and writing needs
➔Very large business classes
➔Complex queries
➔Queries between services (Microservices)
➔Lack of traceability of operations

7. "Solutions"
➔Cache (web, method invocation, queries, 2nd level)
➔Tracking application operations
➔Insert only tables
➔Data aggregations (data lakes)

8. CQRS

9. CQRS
➔Command Query Responsibility Segregation
➔It is a fancy name for an architecture that uses different data models to
represent read and write operations

10. TWO MODELS
Command Model
➔Focused on executing tasks
➔Only contains data necessary for task
execution and decision making
Query Model / Projections
➔Focused on delivering information.

11. Synchronization of Models
➔Changes in the Command Model should be visible in the Query Model.
◆ Shared data source
◆ Stored procedures
◆ Event Driven Architecture

12. CQRS is not...
➔Event sourcing
➔Eventual consistency
➔It is not noSql and/or 2 databases

13. Event Sourcing

14. DDD Concepts
➔Aggregates (A group of associated objects which are considered as one unit
with regard to data changes)
➔Emphasis on the design of a model
➔Use of ubiquitous language

15. What is Event Sourcing?
Martin Fowler
➔Event Sourcing ensures that all changes to application
state are stored as a sequence of events
➔Not just can we query these events, we can also use the
event log to reconstruct past states
➔Construct different current states
"We can query an application's state to find out the current state of the world,
and this answers many questions. However there are times when we don't just
want to see where we are, we also want to know how we got there"

16. Event Sourcing

17. Event
➔Are immutable
➔Statement of facts (past tense)
➔Never fails
➔May affect a single Aggregate

18. Command
➔A request (imperative sentence)
➔May fail and the operation is not performed

19. Event Sourcing
➔Send the command
➔Process the command
➔If the command is valid, the event will be sent
If the command is not valid, an error is returned
➔Event is stored in the Event Store and sent to the Event
Bus
➔Event is consumed by a handler and the status of the
view is changed

20. Additional Benefits
Easy eventual business consistency
> Corrective events
Robust to data corruption
(bugs, manual changes..)
> Rebuild state ignoring wrong events

21. Additional Benefits
"Rebuild state at a point in time"
History
(for free)

22. Main Benefits
Scalable
> Append only
> Low-latency writes
> Allows asynchronous processing
> Query are optimised for use cases

23. Drawbacks
Delayed reads
Multiple query implementations
No ACID Transactions
Additional complexity (!)

24. Axon Framework

25. Axon Framework
➔Provides a clean, elegant Java API for writing DDD, CQRS
and Event Sourcing applications
➔Open Source
➔It provides basic building blocks for writing aggregates,
commands
➔Integrates with Spring Boot and other frameworks
➔Focus on business logic

26. Command Handling & Aggregate Design

27. Command Handler
➔Accepts incoming commands
➔Interacts with the current state of the aggregate and
publishes events
➔Command model only contains data necessary for task
execution and decision making

28. Axon Command Handler

29. Axon Event Sourcing Handler

30. Command Message Routing

31. Dispatching Commands

32. Event Handling

33. Event Handler
Handles published events to:
➔Update Projections
➔Trigger (external) activities
➔Manage complex transactions (Sagas)

34. Event Handling Component

35. References
➔CQRS. https://martinfowler.com/bliki/CQRS.html
➔Event Sourcing. https://martinfowler.com/eaaDev/EventSourcing.html
➔Fast lane axon framework. https://lp.axoniq.io/fast-lane-axon-framework-online-training
➔Bootiful CQRS and Event Sourcing with Axon Framework by Allard Buijze @ Spring I/O 2018.
https://www.youtube.com/watch?v=jc0RGv4YFH0&t=806s
➔AxonIq. https://axoniq.io/
➔Domain Driven Design. https://martinfowler.com/bliki/DomainDrivenDesign.html

36. Hands-on!
GitHub https://github.com/MovilePay/axon-order-
app

37. That's it for now!
?? Questions ??