Evolutionary Microservices with DDD, CQRS and Axon Framework

The rise microservices trend fosters many project teams to deal with a bunch of technologies. Some of the projects starts implementation by putting much effort in the distribution infrastructure.

This talk proposes a different approach. It shows how to use concepts of Domain Driven Design (DDD) combined with Command Query Responsibility Segregation pattern and Event Sourcing (CQRS/ES) and focus on the domain logic first. In doing so, we used Axon Framework for JVM to decouple resulting components and can distribute them after the system construction.

This talk has been presented on Java User Group Hamburg on 28. Jan 2020.

Transcript

1. Evolutionary Microservices with DDD, CQRS and Axon Framework Location transparency in practice @holisticon
2. Why are we here? ▪ Everyone builds microservices ▪ Many start with distribution... ▪ ... and fail ▪ we show how you can do it better
3. Agenda ▪ Domain Driven Design (DDD) ▪ Implementation Architecture □ CQRS Pattern □ Event Sourcing ▪ Axon Framework ▪ Demo and Examples ▪ Outline
4. Who we are? jangalinski jangalinski zambrovski zambrovski
5. Why holisticon? ▪ A better company ▪ "New work" for 10 years ▪ Based in Hamburg and Hannover ▪ 50 engineers ▪ We want you and are still hiring
6. Domain Driven Design (DDD) ▪ What is it? ▪ Why should I know it? ▪ Basic principles
7. Domain Driven Design ▪ Set of principles, patterns and techniques ▪ Analysis □ Domains □ Domain Models ▪ Strategic Design □ Bounded Contexts □ Relationships □ Ubiquitious Language ▪ Tactical Design □ Value Objects, Entities, Aggregates □ Services, Repositories, Factories □ Domain Events ▪ Architecture Design □ Layered □ Hexagonal / Ports and Adapters
8. DDD: Requirements analysis ▪ Event Storming □ 100.000 stickies from Alberto Brandolini □ Collaborative method for strategic and tactical design □ Identifies responsibilities based on events ▪ Domain Storytelling □ Scenarios visualisation from WPS □ Collaborative method to capture domain knowledge □ Identify responsibilities based on activities belonging together □ Can be used as foundation for tactical design ▪ ... or both
9. Example Domain Storytelling: Bank
10. DDD: Strategic Design ▪ Identify Domains / Subdomains ▪ Find Bounded Contexts (BC) ▪ Identify relationships between BC □ Customer / Supplier □ Conformist □ ... ▪ Ubiquitous Language
11. Example Domain: Bank
12. DDD: Tactical design ▪ Explore the Bounded Context ▪ Create the Domain Model "Domain is about Euros and Customers, not about Decimals and Strings." "Make the incorrect inexpressible!" (Albert Starreveld)
13. Example Domain Model: Bank
14. DDD: Tactical Design (Entities) ▪ Value Object □ represents domain value ▪ Entity □ has own identity □ values are Value Objects ▪ Aggregate □ object graph from Entities □ guarantees consistency □ aggregate root provides access
15. Example Domain Entities: Bank
16. DDD: Tactical Design (Services) ▪ Services □ Domain Services □ Application Services ▪ Repositories ▪ Factories
17. DDD: Architecture Design ▪ Layered (Tiered) ▪ Hexagonal (Port & Adapters) ▪ ...
18. Implementation Architecture ▪ Command Query Responsibility Segregation (CQRS) ▪ Event Sourcing (ES)
19. Command Query Responsibility Segregation (CQRS) ▪ Architectural pattern ▪ Separate write from read API
20. CQRS: When should I use it? ▪ Asymmetric read/write load ▪ Multiple read representations ▪ Eventually consistency is acceptable ▪ Read models will be constructed later
21. CQRS: Basic Principles ▪ Command (Write) □ used to express intent of state change (=mutation) □ has side effects □ has generally no return value ▪ Command handler □ accept / reject commands ▪ Query (Read) □ used to express the query □ has no side effects □ provides typed return ▪ Query handler □ Answers query ▪ Read model (Projection) □ Projection optimized for read □ Serves one purpose
22. CQRS in Hexagonal Architecture
23. Event Sourcing (ES) ▪ Persistence Strategy for aggregates ▪ Store state changes (events), instead of last state ▪ Implicit audit log ▪ Uses Event Store □ Append only store □ May be used for replays □ May support snapshotting
24. CQRS/ES ▪ Use events to transfer changes from aggregate to read model ▪ Use event bus for de-coupling ▪ Store events instead of current state
25. Axon Framework ▪ What is Axon Framework? ▪ Basic Principes ▪ Managed Evolution ▪ Deployment Strategies
26. What is Axon Framework ▪ Java Framework to construct hexagonal architecture and CQRS(/ES) ▪ FOSS ▪ Created by AxonIQ (Netherlands) ▪ Native Spring(boot) support ▪ Can run on Java SE / CDI ▪ Promotes annotations for implementation ▪ Promotes late configuration / evolution
27. Axon Framework: Basic Principles ▪ Implementing application using three buses □ CommandBus □ EventBus □ QueryBus ▪ Implementing Aggregates □ @Aggregate □ @CommandHandler □ @EventSourcingHandler □ Provides aggregate repositories ▪ Implementing Projections (Read Models) □ @EventHandler □ @QueryHandler ▪ Configuration □ Event store □ Token store □ Implementation for buses
28. Example Axon Architecture: Bank
29. Axon Framework: Deployment strategies ▪ Local □ single JVM deployment ▪ Distributed with Axon Server □ Axon Server Community □ Axon Server Enterprise □ Axon Cloud ▪ Distributed without Axon Server □ Implement message distribution on your own
30. Demo and Examples
31. Outline ▪ DDD □ helps to understand the problem space □ identify the solution space □ design solution architecture ▪ CQRS/ES □ can be applied during implementation □ makes event stream to the ONLY truth ▪ Axon Framework □ Promotes own architecture for CQRS/ES implementation □ Uses buses to de-couple components □ Best-in class framework for JVM □ Promotes late evolution / scaling on demand
32. Challenges ▪ Same words have different meanings ▪ Strategic design matters ▪ Event design is crucial ▪ Event Store is append only
33. Thank you! Questions?