What Is a Modular Monolith?

In System Design, there are two main ways to structure big projects: the “all-in-one” approach called monolithic, and the “building block” approach called modular. But what if we could have the benefits of both? That’s where the modular monolith comes in.

What is Monolithic Architecture?

A monolithic architecture is a traditional approach to designing software where an entire application is built as a single, indivisible unit. In this architecture, all the different components of the application, such as the user interface, business logic, and data access layer, are tightly integrated and deployed together. 

• This means that any changes or updates to the application require modifying and redeploying the entire monolith.
• Monolithic architectures are often characterized by their simplicity and ease of development, especially for small to medium-sized applications. 
• However, they can become complex and difficult to maintain as the size and complexity of the application grow.

What are Modular Monoliths?

A modular monolith is an architectural approach that combines aspects of both monolithic and modular design paradigms.

• In a traditional monolithic architecture, an entire application is built as a single, indivisible unit, with all components tightly coupled together. This can lead to challenges in terms of scalability, maintainability, and flexibility as the application grows.
• a modular architecture breaks down an application into smaller, more manageable modules or components, each responsible for a specific set of functionality. These modules are designed to be loosely coupled, allowing for easier development, testing, and maintenance.

Instead of breaking the application into separate microservices or distributed components, which can introduce complexity in terms of deployment and communication overhead, a modular monolith organizes the codebase into well-defined modules within a single application. These modules are typically organized around business capabilities or domain-driven design principles.

Characteristics of Modular Monoliths

• Modularity: Modular monoliths are structured into smaller, independent modules, each responsible for specific functionality or business domain. These modules are organized around clear boundaries and responsibilities, promoting separation of concerns and maintainability.
• Tight Integration: Despite the modular structure, all modules are tightly integrated within a single codebase and runtime environment. This means there’s no need for separate deployments or communication mechanisms between modules.
• Shared Codebase and Data: All modules share the same codebase, libraries, and data storage. This simplifies development and deployment processes compared to distributed systems while retaining some benefits of modularity, such as code organization and separation of concerns.
• Scalability and Maintainability: Modular monoliths provide scalability and maintainability benefits over traditional monolithic architectures. By breaking down the application into modules, developers can manage complexity more effectively and scale individual components independently.
• Flexibility: Despite being a single codebase, modular monoliths offer flexibility in terms of development and deployment. Developers can easily add, remove, or modify modules to adapt to changing requirements without impacting the entire system.
• Ease of Deployment: Deploying a modular monolith is simpler compared to distributed systems, as there’s only one deployment artifact. This reduces the complexity of deployment and operations, making it easier to manage the application lifecycle.

By combining modularity with the simplicity of a monolithic architecture, modular monoliths strike a balance between flexibility, scalability, and maintainability, making them suitable for applications with moderate complexity and scalability requirements.

Principles of Modular Monoliths

The principles of modular monoliths revolve around promoting modularity, maintainability, and scalability within a single, cohesive codebase. Here are the key principles:

• Modularity: Break down the application into smaller, independent modules, each responsible for specific functionality or business domain. Modules should have clear boundaries and well-defined interfaces, promoting separation of concerns and ease of maintenance.
• High Cohesion, Low Coupling: Modules should exhibit high cohesion, meaning that each module should have a clear, focused purpose and should be responsible for a specific aspect of the application. At the same time, modules should have low coupling, meaning that they should be loosely connected and should not depend heavily on each other.
• Clear Boundaries: Define clear boundaries between modules to minimize dependencies and facilitate independent development and testing. Avoid intermixing modules or introducing tight coupling between them, as this can lead to complexity and difficulty in making changes.
• Single Responsibility Principle (SRP): Follow the Single Responsibility Principle, which states that each module should have only one reason to change. This helps maintain clarity and simplicity within modules and ensures that changes to one module do not impact unrelated parts of the system.

Benefits of Modular Monoliths

Modular monoliths offer several benefits that combine the advantages of monolithic and modular architectures. Some of these benefits include:

• Simplicity: Modular monoliths provide the simplicity of a single, unified codebase and deployment artifact, making them easier to develop, deploy, and manage compared to distributed systems.
• Modularity: By breaking down the application into smaller, independent modules, modular monoliths promote modularity and separation of concerns, making it easier to understand, develop, and maintain the codebase.
• Scalability: While not as scalable as fully distributed architectures, modular monoliths still offer scalability benefits by allowing individual modules to be scaled independently based on demand. This enables horizontal scaling for specific components without the overhead of managing separate services.
• Maintainability: The modular structure of monoliths facilitates code organization, refactoring, and maintenance, making it easier to add new features, fix bugs, and make changes to the application over time.
• Ease of Deployment: Deploying a modular monolith is simpler compared to distributed systems, as there’s only one deployment artifact to manage. This reduces the complexity of deployment and operations, making it easier to maintain and update the application.

Modular Architecture Patterns

Modular architecture patterns are design patterns that promote modularity and separation of concerns within software systems. These patterns help organize code into smaller, reusable components or modules, making it easier to understand, develop, and maintain complex applications. Here are some commonly used modular architecture patterns:

• Layered Architecture: Layered architecture divides the application into layers, with each layer responsible for a specific aspect of functionality. Common layers include presentation/UI layer, business logic layer, and data access layer. This pattern promotes separation of concerns and facilitates scalability and maintainability.
• Modular Monolith: A modular monolith breaks down the application into smaller, independent modules, each responsible for specific functionality or business domain. Despite being a single codebase, modules are loosely coupled and can be developed, deployed, and maintained independently. This pattern combines the simplicity of a monolithic architecture with the modularity of a modular architecture.
• Microservices Architecture: Microservices architecture decomposes the application into a set of small, independent services, each responsible for a specific business function. Each service is deployed independently and communicates with other services via lightweight protocols such as HTTP or messaging. This pattern promotes scalability, resilience, and flexibility but adds complexity in terms of deployment and operations.
• Component-Based Architecture: Component-based architecture organizes the application into reusable, self-contained components, each encapsulating a set of related functionality. Components can be assembled and composed to build larger applications, promoting code reuse and maintainability.

Frameworks and Libraries for Modular Monoliths

Several frameworks and libraries can be utilized to implement modular monoliths, simplifying development and maintenance. Here’s a list of some popular options:

• Spring Boot (Java): Spring Boot provides a powerful framework for building Java applications, including modular monoliths. It offers features like dependency injection, aspect-oriented programming, and Spring Boot Starters, which streamline the development process and promote modularity.
• ASP.NET Core (C#): ASP.NET Core is a cross-platform framework for building modular web applications using C#. It offers features like middleware pipeline, dependency injection, and modular routing, making it suitable for implementing modular monoliths.
• Ruby on Rails (Ruby): Ruby on Rails is a web application framework that encourages convention over configuration and follows the Model-View-Controller (MVC) pattern. It provides tools for organizing code into modules and promoting modularity.
• Django (Python): Django is a high-level web framework for Python that follows the Model-View-Template (MVT) pattern. It offers features like reusable apps, middleware, and class-based views, which facilitate the development of modular monoliths.
• Laravel (PHP): Laravel is a PHP web framework that provides tools for building modular monoliths. It offers features like routing, middleware, and Eloquent ORM, which simplify the development process and promote code organization.

Challenges with Modular Monoliths

While modular monoliths offer several benefits, they also come with their own set of challenges. Some common challenges associated with modular monoliths include:

• Complexity Management: As the application grows in size and complexity, managing the interactions between modules and ensuring consistency across the codebase can become challenging. Developers need to carefully design module boundaries and dependencies to avoid introducing unnecessary complexity.
• Dependency Management: Managing dependencies between modules can be challenging, especially when modules have interdependencies or when changes in one module impact others. Developers need to carefully manage dependencies and versioning to avoid introducing conflicts or breaking changes.
• Deployment Complexity: Deploying modular monoliths can be more complex compared to traditional monolithic architectures, as it involves deploying multiple modules together. Coordinating deployments and ensuring consistency across modules can be challenging, particularly in distributed environments.
• Testing Overhead: Testing modular monoliths can be more challenging compared to traditional monolithic architectures, as it involves testing interactions between modules as well as individual modules themselves. Developing comprehensive test suites and ensuring adequate test coverage across modules can be time-consuming.

Real-world Examples of companies with Modular Monolith Implementations

While modular monoliths may not be as widely publicized as other architectural approaches like microservices, several companies have successfully implemented modular monoliths to build scalable and maintainable software systems. Here are a few real-world examples:

• Shopify: Shopify, an e-commerce platform, initially started as a monolithic Rails application. Over time, as the company grew, they modularized their monolith into smaller, manageable components while maintaining a single codebase. This approach allowed Shopify to scale their engineering teams and development processes while still benefiting from the simplicity and cohesion of a monolithic architecture.
• GitHub: GitHub, a web-based hosting service for version control using Git, is built on a modular monolith architecture. Their application is organized into modular components, each responsible for specific functionality such as repositories, issues, and pull requests. This modular approach has allowed GitHub to evolve and scale their platform while maintaining consistency and reliability.
• Basecamp: Basecamp, a project management and team collaboration software, is another example of a company that utilizes a modular monolith architecture. Their application is structured into modular components, each representing different features and functionalities such as tasks, messages, and calendars. This modular approach has allowed Basecamp to iterate quickly and deliver new features while maintaining a cohesive user experience.
• Zendesk: Zendesk, a customer service software company, has a modular monolith architecture for its main product offering. Their application is organized into modular components, each responsible for specific aspects of customer support such as tickets, knowledge base, and chat. This modular architecture has allowed Zendesk to scale its platform and deliver new features efficiently.