Domain-Driven Design and the Monolith-First Approach to Microservices

In the world of software architecture, the relationship between Domain-Driven Design (DDD) and Microservices has become increasingly important. This post explores why starting with a monolith might be the best approach, even when you know you'll eventually need microservices.

The Case for Monolith-First

Martin Fowler and other industry experts have observed a consistent pattern: successful microservice implementations often evolve from monoliths that grew too large and were strategically decomposed. Conversely, systems built as microservices from scratch frequently encounter significant challenges.

Almost all the successful microservice stories have started with a monolith that got too big and was broken up. Almost all the cases where I've heard of a system that was built as a microservice system from scratch, it has ended up in serious trouble.

Why Start with a Monolith?

There are several compelling reasons to begin with a monolithic architecture:

1. YAGNI (You Aren't Gonna Need It): When starting a new application, it's often unclear how useful it will be to users. A monolith allows faster development and feedback cycles.
2. Boundary Discovery: Microservices require stable boundaries between services (Bounded Contexts in DDD terms). These boundaries are extremely difficult to get right at the beginning.
3. Lower Initial Complexity: Microservices come with a significant operational overhead that can slow down early development.
4. Easier Refactoring: Changes to functionality are much easier to implement in a monolith than across service boundaries.

The Role of Domain-Driven Design

DDD principles are crucial whether you're building a monolith or microservices. They help you:

• Identify bounded contexts that could become future microservices
• Maintain clean separation of concerns within the monolith
• Design clear interfaces between different parts of the system
• Focus on the business domain rather than technical concerns

Strategies for Evolution

There are several proven approaches to evolving from a monolith to microservices:

1. Careful Initial Design: Build the monolith with modularity in mind, paying attention to API boundaries and data storage patterns.
2. Gradual Extraction: Start by peeling off microservices at the edges while maintaining a stable core monolith.
3. Sacrificial Architecture: Accept that the initial monolith might be replaced entirely once the domain is better understood.
4. Coarse-Grained First: Begin with a few large services and gradually refine them into smaller ones as boundaries become clear.

Industry Perspective: TMForum's Composable IT

The telecommunications industry, through TMForum, emphasizes the importance of composable IT and ecosystems. Their approach advocates for:

• Plug-and-play software architectures
• Open APIs and reusable platforms
• AI-ready composable software
• Strong partner ecosystem integration

Conclusion

While microservices offer significant benefits for complex systems, starting with a well-designed monolith is often the most pragmatic approach. This allows teams to:

• Validate business ideas quickly
• Discover natural service boundaries
• Build team experience with the domain
• Avoid premature architectural complexity

Remember, a monolithic architecture is not a compromise—it's a valid architectural choice that can serve as a stepping stone to a more distributed system when the need truly arises.