What is a modular monolith
Before diving into the specifics of a modular monolith, it’s essential to understand the basics of a monolithic architecture. In simple terms, a monolithic application is built as a single, cohesive unit. All the features and functionalities reside in a single codebase, making deployment more straightforward and often allowing for faster iteration in the early stages of development.
However, as applications grow, the complexity of a monolith can increase exponentially. Without a clear internal structure, it can devolve into what’s known as a “big ball of mud,” where it becomes difficult to maintain, understand, or extend. This is where the concept of modularity comes in.
A modular monolith is an architectural approach where the application is still a single unit, but its internal structure is divided into distinct, well-defined modules. Each module has a clear responsibility and operates independently within the overall system, following principles like separation of concerns and high cohesion. This allows teams to work on different application parts without stepping on each other’s toes, reducing complexity and improving maintainability. The core principles of a modular monolith are:
- Single deployment unit: The application is deployed as a single unit, making it easier to manage and scale.
- Encapsulation: Modules are self-contained and have clear boundaries, reducing dependencies and coupling.
- Independent development: Teams can work on different modules without interfering with each other, improving productivity.
Why choose a modular monolith
Choosing between a modular monolith and a microservices architecture is not always clear-cut. Both approaches have pros and cons, and the decision should be based on the specific needs and constraints of the project. Here are some reasons why you might choose a modular monolith:
- Simplicity: A modular monolith is more straightforward to develop, deploy, and maintain than a microservices architecture, making it a good choice for smaller projects or teams.
- Performance: A monolithic application can be more performant than a distributed system, as it avoids network communication overhead between services.
- Ease of development: With a modular monolith, developers can work on the entire application without dealing with the complexities of distributed systems.
- Clear boundaries: Modules provide clear boundaries between different application parts, making it easier to reason about and maintain.
- Scalability: While a monolith may not scale as quickly as a microservices architecture, it can still be scaled horizontally by running multiple instances behind a load balancer.
The key to a successful modular monolith is to strike the right balance between modularity and complexity. By breaking down the application into well-defined modules and following best practices for software design, you can create a system that is both maintainable and scalable.
How to build a modular monolith
Building a modular monolith involves dividing your application into distinct modules, each with a clear responsibility and well-defined boundaries. To start splitting your monolith into modules, try to create logical boundaries between different parts of your application. For example, you might have modules for user management, authentication, product catalog, and order processing. Each module should encapsulate a specific set of features and should not be concerned with the internal workings of other modules. So, the starting point is to identify the modules and their responsibilities.
Once you have identified the modules, you can start defining the interfaces between them. This includes determining the APIs that each module exposes to other modules, as well as the data structures and protocols that they use to communicate.
When designing the interfaces between modules, it’s essential to follow the principles of high cohesion and low coupling. This means that each module should be highly cohesive, meaning that the components within a module should be closely related and work together to achieve a common goal. At the same time, modules should be loosely coupled, meaning that they should interact with each other through well-defined interfaces and should not depend on the internal implementation of other modules.
Finally, you can start implementing the modules themselves. Each module should be self-contained and should have a clear API that other modules can use to interact with it. You should also write automated tests for each module to ensure that it behaves as expected and does not introduce regressions.
Modular monolith best practices
Building a modular monolith is not just about dividing your application into modules; it’s also about following best practices to ensure that your architecture is maintainable, scalable, and resilient. When designing a modular monolith, you should consider the following best practices:
- Separation of concerns: Each module should have a clear responsibility and not be concerned with the internal workings of other modules.
- High cohesion: Modules should be highly cohesive, meaning that the components within a module should be closely related and work together to achieve a common goal.
- Low coupling: Modules should be loosely coupled, meaning that they should interact with each other through well-defined interfaces and should not depend on the internal implementation of other modules.
- Consistent module structure: Follow a consistent module structure across your application to make it easier to navigate and understand.
- Automated testing: Write automated tests for each module to ensure that changes do not introduce regressions.
By following these best practices, you can build a modular monolith that is easy to maintain, extend, and scale as your application grows.
Example implementation
For one of our customers, we needed to design a system that allowed the development team to easily expose new API endpoints and integrate them with third-party services.
We decided to build a modular monolith that would allow us to add new modules as needed and scale the system as the customer’s business grew. The modules we identified were based on the type of data they needed to expose and the context in which they were used. For example, we have 2 product modules, one with a customer context and one without. This allowed us to reuse the same codebase for different customers without duplicating the code.
Each module has its own API endpoints, data models, and business logic and is responsible for a specific set of features. The customer-aware product module includes price data specific to a customer, while the generic product module includes only the product catalog data. This enables us to reuse the same codebase for different implementations without having to duplicate the code.
Conclusion
A modular monolith is a pragmatic approach to building large-scale applications that combines the simplicity of a monolithic architecture with the benefits of modularity. By dividing your application into well-defined modules and following best practices for software design, you can create a system that is both maintainable and scalable. While a modular monolith may not be the right choice for every project, it can be a good fit for smaller teams or projects where simplicity and ease of development are more important than scalability and flexibility. Ultimately, the choice between a modular monolith and a microservices architecture should be based on the specific needs and constraints of your project, and both approaches have their own strengths and weaknesses.
