Building large systems following DDD leads to decomposition of the domain into subdomains and creation of multiple bounded contexts in the solution architecture. While this is a good thing and reduces complexity and cognitive load of the single contexts, you will need different solutions for the inter-context application flow of your over-all system. From time to time you may find yourself puzzled with the different possibilities these approaches have to offer. Having used them for quite some time now, we stumbled upon some interesting integration questions implementing bounded contexts using Axon Framework and CQRS/ES patterns. Especially, the design of the overall architecture offers trade-offs in combining of onion, hexagonal and other styles. In this talk we demonstrate identified patterns, share some experience in integration and discuss the decisions made during the implementation of the multiple bounded context solutions using Axon Framework. In doing so, we provide examples and propose best practices we identified in our projects.

1. Beyond DDD 101 A practice report on building multi
bounded context CQRS/ES applications
Jan Galinski & Simon Zambrovski
Amsterdam, 23.09.2022
1

2. Who we are
Jan Galinski
Simon Zambrovski
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Consultants @ holisticon
Backend developers, software architects
DDD, CQRS/ES, Axon Framework, Spring, Kotlin, Java

3. What is Holisticon
• IT Consulting
• Hamburg, Hannover, Kiel
• Cool crew
• Best Employer 2022
in Hamburg
• We are hiring!
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BESTE BERATUNG – BESTE SOFTWARE

4. Motivation
• We love architecture
• DDD
• CQRS
• Event Sourcing
• Large-scale applications running on JVM
• We love Axon Framework
• We want to share our experience
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https://www.pexels.com/de-de/foto/verschiedene-puzzlespiele-1586951/

5. Foundation – Theory - DDD
• Set of principles for system implementation
• Requirement Engineering
• Domain Modeling
• Strategic Design
• Bounded Context identification
• Context Mapping
• Design
• Ubiquitous Language
• Design Patterns
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https://www.amazon.com/Domain-Driven-Design-Tackling-Complexity-Software/dp/0321125215

6. Foundation – Theory – DDD Patterns
• Modeling of Bounded Context relationships (for Microservices)
• Separate Ways
• Upstream/Downstream
• Open Host Service + Published Language
• Anti Corruption Layer
• Tactical Design
• Value Object, Entity, Aggregate, Aggregate Root
• Domain Service, Factory, Repository
• Domain Event
• Saga Pattern
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7. Foundation – Theory – CQRS/ES
• Command Query Responsibility Segregation Pattern
• Reduce complexity of Domain model
• Separate Command model from Query model (optimized)
• Scale-out independently
• Event Sourcing Persistence Strategy
• Store Domain Events (versus store State)
• Derive State from Domain Event stream
• CQRS/ES
• Use Domain Events to communicate between Command and Query models
• Allow late stream consumption (stream replay)
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8. Domain
Foundation - Onion Architecture
Driving
(primary)
Adapter
Driven
(secondary)
Adapter
Domain
Model
Ring
Port Port
Application Service Ring
Infrastructure Ring
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9. Axon Framework 101
• FOSS framework in Java for building DDD CQRS(/ES) systems
• Explicit message concept
• 3 buses for decoupling
• Command -> Addressed intent of a change, may fail, unicast
• Event -> Fact, broadcast
• Query -> Request for data, has result, multicast
• Domain Model Segregation (implements CQRS)
• Event Sourcing implementation (different Event Stores)
• Separation of business logic from configuration
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https://developer.axoniq.io/download

10. Possible Architecture using Axon Framework
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https://docs.axoniq.io/reference-guide/

11. System evolution in single context
• Build components using buses for communication
• Start with modulith
• Distribute along buses
• Scale out projections
• Build new projections later
• Change infrastructure
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12. Domain Core
Onion Architecture with Axon Framework
REST
Adapter
Query Model
Command Model
Command
Gateway
Query
Gateway
Projection
Repository
Aggregate
Repository
Query
Bus
Comma
nd Bus
Event
Bus
Pivotal
Event
Bus
Pivotal
Event
Bus
Domain
Event
Bus
Domain
Comma
nd Bus
Query
Bus
Saga
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https://www.pexels.com/de-de/foto/mann-frustration-kopfschmerzen-brille-9843268/

14. Domain Core
Onion Architecture with Axon Framework
REST
Adapter
Query Model
Command Model
Command
Gateway
Query
Gateway
Projection
Repository
Aggregate
Repository
Query
Bus
Comma
nd Bus
Event
Bus
Pivotal
Event
Bus
Pivotal
Event
Bus
Domain
Event
Bus
Domain
Comma
nd Bus
Query
Bus
Saga
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15. Simplified context building block
Query Model
Command Model
Query
Bus
Comma
nd Bus
Event
Bus
Event
Bus
Comma
nd Bus
Query
Bus
Saga
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16. Patterns
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Q
S
C
? Q
S
C

17. Q
Master Data Query
Q
S
D U
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„You know some things that I need to know, please tell me!“
Query

18. Master Data Query
Q
S
Q
D U
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Query
Benefits
• Simple (~ GET)
• No redundancy (GDPR)
• Encapsulate complex business
logic
Challenges
• Runtime dependency
• Trap: Building specialized
projections for other teams
• Outdated data, caching

19. Trigger a query
C
U D
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Q Q
„I made some changes … ask me if you want more details.“
Query
C‘

20. Trigger a query
C
U D
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Q Q
Query
Benefits
•„Cache invalidation“
Challenges
•Runtime dependency*
•Load (query avalanche)

21. Query Aggregator
C
Q
U D
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„Here is some data that could be useful to build your projection.“
Query
Q‘
C‘

22. Query Aggregator
C
Q
U D
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Query
Benefits
• Decoupling
• Build complex projections
from other contexts
Challenges
• Event Payload definition
• Payload interpretation

23. Reacting to Fact
C
S
C
U D
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Command
„I changed some things … deal with it.“
C‘ C‘

24. Reacting to Fact
C
S
C
U D
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Command
Benefits
• Simple
• Decoupled
• Event Driven
Challenges
• Downstream Command model
must follow
• Command-Event-Command
bridge

25. Downstream Saga
C
S
C
D U
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Command
„I need you to do something for me, and I need the result.“
C*
C‘ C‘

26. Downstream Saga
C
S
C
D U
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Command
Benefits
• Build for failure
• Delegate sub processes
Challenges
• Compensation
• Complex for more than
one context

27. External Orchestrator
C
S
C
D U
U
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Command
C*
C* C‘
C‘
„If you two cannot handle it, let me take control.“

28. External Orchestrator
C
S
C
D U
U
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Command
Benefits
• Build for failure
• Separate ways
• Business process
Challenges
• Compensation
• Complex for more than one
context

29. Key Takeaways
• Domain Driven Design
• Start with Strategic Design
• Do proper Context Mapping – Relationships!
• CQRS/ES
• Use Command / Event / Query abstractions („Events only“ is not enough)
• Use Axon Framework if working on JVM
• Practical
• Start with a modulith
• Build evolutionary
• Distribute on demand
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30. Thank you... Questions?
• Holisticon AG
• https://holisticon.de/
• Jan
• https://about.me/jangalinski
• Simon
• https://about.me/zambrovski
• Axon Framework
• https://developer.axoniq.io/
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