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Category: Domain Drive Design

Agile and domain driven design (part 3): Microservices

In the last post we imaged we are a programmer on an agile digital service team who had gotten as far as writing some DDD code. It is a rich OO domain model that knows how to obey the business rules (aka enforce the invariants of the contract aggregate). The next question is where should this code run? What provides the surrounding workflow? Which screens, running where, drive this “thing”? Talking it through with the other developers in the team you decided to build and deploy a contracts microserviceRead the rest of this entry »

Agile and domain driven design (part 2): Event Storming

The last post set the scene for how an agile digital services team gets to the point where it is ready to cut some DDD code. Imagine that you just joined the team as a programmer as the programme is ramping up its private beta build out. To align ourselves to the demo code of the last blog series you are need to build out some stories about how customers and your internal users create and agree a contract to deliver products. You are an agile analyst programmer who wants to build a ubiquitous language with the users of the system. So you attend an event storming workshop with the users. Read the rest of this entry »

Agile and domain driven design (part 1): Digital Services

In the last mini-series of posts I sketched out how to use DDD to build an explicit rich domain library that models the business domain and enforces the invariants within a narrow scope of an aggregate of entities. Catching up with my friend we got into a discussion about how we get to the point were are ready to implement things. How big can the model be? How do we scale to many two pizza teams? How do user needs, business processes, and screens relate to the domain focused OO design? We ended up talking about microservices. So in this series of post I am going to sketch how a large scale agile digital transformation programme gets to the point of cutting DDD code. I will then get into how a large project comes to be a platform with a micro-services architectureRead the rest of this entry »

Domain Driven Design: Entities, Value Objects, Aggregates and Roots with JPA (Part 5)

This is the last article in the series which discusses a sample app that does DDD using JPA. I would hesitate to recommend using JPA for DDD unless you are very familiar with JPA. Why? Some of the issues you can hit using JPA are written up on the Scabl blog on Advanced Enterprise DDD. In my own code and this blog I explain how to dodge some of the bullets but you need to be quite aware of the pitfalls of JPA to dodge them all. So why did I write the code? Read the rest of this entry »

Domain Driven Design: Entities, Value Objects, Aggregates and Roots with JPA (Part 4)

Don’t abuse the `public` keyword in Java. The source code has very few public classes or methods. This is unusual for Java projects. Typically Java projects have package layouts that model the solution; “this package has all the entities, that package has all database related code, that package is all the services code”. That approach forces you to make almost everything public. In the long term on a big project brittle connections are made across business responsibility boundaries. There is no way the compiler can enforce boundaries that align to the business domain.  Read the rest of this entry »

Domain Driven Design: Entities, Value Objects, Aggregates and Roots with JPA (Part 3)

Where’s the application in the demo code? There isn’t one.

If you look at the sourcecode there is no front-end, no web servlets, no screens, and no Java main class, and so no way to run it as an application. All that you can do is run the test class. So it is a library project. It is a rich “back-end” that can talk to a database.  Read the rest of this entry »

Domain Driven Design: Entities, Value Objects, Aggregates and Roots with JPA (Part 2)

Detour: Why use  JPA in this demo?

For the purposes of this demo JPA is an officially supported part of the Java ecosystem and is a mature and well documented Java-to-relational mapping tool. Yes it has a number of quirks. If you fight it your probably going to loose (your mind). If you learn how to do the basics and don’t deviate from that it can be a used as a rapid application tool to support an agile TDD build on Java against a relational datbase. Read the rest of this entry »

Domain Driven Design: Entities, Value Objects, Aggregates and Roots with JPA (Part 1)

A friend with a relational database background was working on an OO domain modelling problem. I started talking about “aggregates” and “roots” and things like “make the contract entity an aggregate controlling the other entities” and that “external logic should speak to the object model via a few root entities”. So I wrote demo project is some Spring and JPA code in Java to demonstrate those concepts. This blog series will be some discussion around the design and implementation techniques.  Read the rest of this entry »