Domain Driven Design In Practice: An Isomorphic TypeScript Project (Part I)

By Guy Perkal, Software ConsultantFeb 21, 2020 • Read time 3 min

Introduction

Recently me and fellow doggo Sudo have wrapped up a project at a large e-retailer in Germany. We were part of a development team that was struggling with a messy codebase, and had communication issues with the product team.

After talking to the team members and looking at the code, we noticed a common cause for a lot of the problems - developers did not have a clear picture of the full domain. Different parts of the code were written by different people, and they were sometimes rewriting parts of the domain logic that already existed elsewhere, either without realizing it or in order to ‘save time’ from extracting the business logic to be reusable. Of course, this short-term time saving ended up costing a lot of time in future code maintenance.

In addition, the product people didn’t have a shared language with the developers - for every new task or feature coming in from the product team, a lot of time had to be spent on “translating” what it is the product people wanted to terms the development team could understand and know how to implement in the existing codebase.

By applying the principles of Domain Driven Design (or DDD for short) we were able to help out with those issues. In the scope of this post I'll discuss how we applied it on our team, and the benefits it provided. In a follow up post we’ll go more in depth into what the actual implementation looked like.

What is Domain Driven Design?

For those unfamiliar, DDD is an approach to software projects where every decision you make has to align with the product domain model, which is constantly evolving when features are added, removed or modified.

This applies to product specification decisions, but perhaps not as obviously it also applies to every architectural or coding decision the R&D team makes. Since the model is shared between the R&D and product teams, maintaining its integrity fosters communication between them and keeps everyone on the same page by providing a common "language". There's a lot more to this topic, and you can read all about it in Eric J. Evans' book, where it was first introduced.

Sometimes it’s very tempting when I start a new project to dive in and immediately start writing code, but in order for a codebase to scale a lot of thought and effort need to be put into defining and maintaining the business’ underlying domain.

Step by step

The first step was, naturally, figuring out what our domain was. This was made easier by the fact that when we joined the project, the backend team was already working on a simplified, unified API to replace the existing one, which had the frontend calling many different backend micro-services. So our domain model could be based on that API structure, at least as a starting point. Now the main challenge was aligning the frontend with this model.

This existing frontend was split into fragments, with each fragment having its own output static file. However, all of these fragments shared one monolithic codebase, and were coupled together in messy ways - this made navigating the codebase difficult, and making code changes was a very slow and error prone process, especially as business requirements got more and more complex. It was decided a rewrite was in order, with each fragment having its own project.

Conclusion

For me, this project was a great example of the benefits you get from spending a lot of time thinking about a project’s foundations. Sometimes it’s very tempting when I start a new project to dive in and immediately start writing code, but in order for a codebase to scale a lot of thought and effort need to be put into defining and maintaining the business’ underlying domain.

Software engineering is all about writing code other people can build on, and that requires everyone to understand how the part they’re building is connected to the rest of the system. Approaching projects from a DDD perspective helps me with that. I previously mentioned the Toolkit library we wrote as part of this project. In the next article we’ll go more in depth into how that library was built, including code examples.