Transitioning to Microservices: Domain-Driven Design in Practice

One of the most common challenges I see teams face when transitioning from a monolithic architecture to microservices is not technical—it's philosophical. The shift isn't just about splitting a codebase; it's about reshaping how we think about our systems, their responsibilities, and the boundaries that define them. For me, that transition was made smoother by applying Domain-Driven Design (DDD) principles.

In monolithic applications, there's a tendency to let things blur together. Features grow organically, dependencies creep in, and before long you're dealing with a tangled mess where any change feels risky. I’ve lived through that. I’ve seen how teams can become paralysed by the fear of breaking something they don't fully understand.

That’s why I advocate for starting any microservice journey with DDD. The first step isn't to slice the code—it’s to understand the domain. When I work with teams on this transition, we begin by identifying bounded contexts. These are the natural seams in the business logic where responsibilities are clearly defined. Once those boundaries are understood, you can begin structuring your monolith around them.

In many cases, I recommend the strangler pattern: rather than rewriting everything, we extract bounded contexts one at a time. The goal is to surround the monolith with new services and slowly choke it out as you build confidence and stability in your new architecture.

To make this work in practice, I often use CQRS (Command Query Responsibility Segregation) to define the interface between domains. This allows us to treat domain boundaries as clear contracts, making it easier to extract them into services when the time comes. A command handler that used to live inside the monolith becomes a service endpoint or can be published to a message bus for asynchronous processing. Queries, on the other hand, are typically served via dedicated query endpoints. No breaking changes—just redirection.

This approach has served me well in projects at SpaSpace and LMS, where we dealt with complex business domains and a rapidly evolving set of requirements. By modelling the system as a set of interacting domains, each with their own logic, data, and responsibilities, we could scale our teams, our deployments, and our codebase independently.

Transitioning to microservices isn’t about chasing trends. It’s about making your system easier to understand, evolve, and operate. If you start with a solid understanding of your domain and apply DDD principles consistently, you’ll find that microservices emerge naturally—not as a rewrite, but as an evolution.