CQRS meets modern Java

Command Query Responsibility Segregation (CQRS) is a proven design pattern that enables the clear separation of data changes (commands) and data queries (queries). This separation allows these responsibilities to be scaled and optimized independently. The application code is also structured more understandably, which improves maintainability.

This talk will deepen our understanding of the practical implementation of the CQRS principles using modern Java language features such as records, sealed classes, switch expressions, and pattern matching. An example application will demonstrate how the theoretical foundations of CQRS can be applied in practice and how this affects application design while increasing data access efficiency.

Transcript

1. CQRS meets modern Java Simon Martinelli martinelli.ch
2. About Me • 30 years in software engineering • 25 years with Java • Self-employed since 2009 • Teacher at two universities in Switzerland • JUG Switzerland
3. Client/Server Communication Client Server
4. REST Controller DTO Mapper Entity Service Repository Mapping Waterfall
5. Example Project • https://github.com/simasch/ cqrs-meets-modern-java
6. Issue 1: Too Much • Over fetching • Loading data that is not needed • Not all data can be changed • Too much data is sent in the update request • Potential bugs if blindly mapped
7. Issue 2: Too Little • Under fetching • Results in multiple requests • Produces the N1 select problem on the client-side • Under storing • Updates/inserts not within the same transaction
8. CQRS 😱 Source: https://cqrs.wordpress.com/wp-content/uploads/2010/11/cqrs_documents.pdf
9. Start Simple Command Query DB Client
10. Example CreateOrder FindOrders DB Client Command Handler Query Handler
11. Modern Java: Records • Provide a compact syntax for declaring classes as transparent holders for immutable data Preview: 14 Final: 16 record CreateOrderCommand(long customerId) { }
12. Modern Java: Sealed Classes • Sealed classes and interfaces restrict which other classes or interfaces may extend or implement them Preview: 15 Final: 17 abstract sealed class Shape permits Circle, Rectangle, Square { }
13. Modern Java: Switch Expression • Exhaustive: the switch expression must cover all cases • Preview in Java 12 and finalized in Java 14 String result = switch (color) { case RED -> "Red"; case GREEN -> "Green"; case BLUE -> "Blue"; }; Preview: 12 Final: 14
14. Modern Java: Pattern Matching • instanceof operator • switch expression and statement double area = switch (shape) { case Circle c -> Math.PI * c.radius * c.radius; case Rectangle r -> r.length * r.width; default -> throw new IllegalArgumentException(); }; Preview: 14/17 Final: 17/21
15. Modern Java: Record Patterns • Use to test whether a value is an instance of a record class type and, if it is, recursively perform pattern matching on its component values record Pair(int left, int right) {} if (p instanceof Pair(int left, int right)) { ... } Preview: 20 Final: 21
16. DB jOOQ Generator Java Classes jOOQ DSL What is jOOQ?
17. Why jOOQ? • Commands • Usually, insert, update, or delete data in the database • Use jOOQ to generate minimal updates • Queries • Often return nested structures like Order à Item • jOOQ provides the MULTISET value constructor
19. Conclusion Separation of commands and queries • Improves understandability • Helps to avoid over- and under-fetching • Can make mapping Entity ↔ DTO obsolete • Enables an event-driven approach
20. Thank you! • Web https://martinelli.ch • EMail simon@martinelli.ch • Bluesky @martinelli.ch • LinkedIn https://linkedin.com/in/ simonmartinelli