Applied Domain-Driven Design in Java

What is Domain Driven Design (DDD) really about? How might Command Query Responsibility Segregation (CQRS) help us to build better software? Far from being esoteric concepts, they can be harnessed by everyday developers to create long-lasting, flexible solutions.

DDD and CQRS are gaining in popularity because they attack genuine challenges in software development. This course balances explaining important underlying theory with both design- and code-focused exercises, giving participants both understanding and new practical development skills. 

Rather than teaching a framework, the course teaches a factoring and some ways to implement it, highlighting the key pitfalls to avoid along the way. During the course, a small application will gradually be built up from scratch. The course also includes coverage of testing, how to apply the insights from the course to existing systems and how to scale out a system developed using CQRS. Read more about CQRS and DDD on the CQRS FAQ


Java Developers 


A good working knowledge of the Java language, including classes, interfaces and generics. Some basic experience with JUnit is beneficial, but not essential.

Carl Mäsak - instructor of the course

Carl has extensive experience in software development and software architecture, both in active use, blogging, and teaching. He uses a variety of programming languages daily. Carl has worked in the software industry for the last 15 years. His experience spans over a wide range of paradigms and platforms, and he has developed everything from custom web site design and bioinformatics toolkits to grammar engines and compilers. His specialty is to make systems coordinate and systematize large amounts of heterogeneous data - or, more casual, "bringing order out of chaos." He also likes to heal major systems that are in need of an architect.

Course outline:

  • The Challenge of "Database Driven" Software Development

    • Types of complexity
    • Scaling and consistency
    • Testing
    • Integration

    Introduction to Domain Driven Design

    • Domains and Domain Models
    • Ubiquitous Language
    • Exercise on building a domain model

    Bounded Contexts

    • Why boundaries matter
    • Context Maps
    • Decomposition in the domain
    • Composition in the UI
    • Exercises on boundary hunting

    Commands, Queries and Events

    • Normalization and denormalization
    • Write models and read models
    • Events as the write-side/read-side
    • link
    • Eventual consistency and its
    • consequences
    • Designing commands
    • Designing queries
    • Exercise on command and query design

    Message Busses

    • Messages as decoupling
    • Publish/Subscribe
    • Send vs. Publish
    • Building a simple message bus
    • Exercise on message bus implementation

    Write Side Architecture

    • Command handlers
    • State sourcing vs. event sourcing
    • Introduction to aggregates
    • Building up aggregates from past events
    • Transactional safety
    • Exercise on command handlers and aggregates
  • BDD Style Testing

    • Introduction to BDD
    • Commands, Events and BDD
    • Building a simple test framework
    • Exercise on BDD

    Event Stores

    • Existing options
    • Building an event store on top of an RDBMS
    • Implementing a repository using an event store
    • Advantages of freeing aggregate representation from persistence
    • Optimizations using snapshots
    • Exercise on building an event store

    Advanced Aggregate Design

    • Finding aggregate boundaries
    • Coping with relationships
    • Exercise on aggregate design

    Building Read Sides

    • Overall approach
    • Relational DB read sides
    • NoSQL read sides
    • Other options
    • Re-building and introducing new read sides
    • Exercise on building read sides


  • Sagas

    • What is a saga?
    • Different types of saga
    • Handling saga state
    • Building a simple saga framework
    • Exercise on sagas

    Evolving a System

    • Refining domain models
    • Handling changes to commands
    • Event versioning
    • Handling event upgrade
    • Exercise on event upgrading

    CQRS, DDD and Existing Codebases

    • Introducing bounded contexts to existing systems
    • Separating commands and queries
    • Migrating towards event sourcing

    Scaling out

    • Using the read/write separation to scale
    • Scaling write sides by bounded context and by aggregate
    • Scaling within an aggregate
    • Message distribution and reliability
    • Message queues and ESBs

    Other Opportunities

    • Parallel development and outsourcing
    • Recreating previous states for debugging

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