Modular Monolith: Balancing Flexibility and Simplicity
Trade-offs are inevitable — whether in software architecture or life, every choice comes with costs and benefits. The key is to navigate them wisely, aligning decisions with long-term goals and your current situation in hand.
In modern software architecture people often see microservices and monoliths are two opposite paradigms, monoliths are criticised for not been scalable and manageable for large scale system and on the other side microservices are criticised for not been easy to manage and for added complexity and cost, but there’s no wrong or right answers but the situation and scale which decides what to pick.
In those cases a modular monolith becomes a middle ground which can balance maintainability and simplicity.
What’s a Modular Monolith?
A modular monolith is a single, deployable application that is internally structured into well-defined, loosely coupled modules. Unlike traditional monoliths, which often suffer from tight coupling and tangled dependencies, a modular monolith enforces strong modularity while maintaining the simplicity of a single deployment unit.
Key Characteristics:
- Single Deployment Unit: Unlike microservices, the entire application is deployed as a single unit.
- Encapsulated Modules: Each module has its own boundaries and interfaces.
- Explicit Contracts: Modules communicate through well-defined interfaces / boundaries (APIs, events, or service facades).
- Independent Development: Teams / Team members can work on different modules without affecting the entire codebase.
Benefits of a Modular Monolith
1. Simpler Deployment & Operations
Managing and deploying a modular monolith is significantly easier than coordinating multiple microservices. There is no need for complex distributed systems, service discovery, or inter-service communication.
2. Easier to Maintain & Refactor
Since all code resides in a single repository and process, refactoring is simpler compared to microservices, where updates require coordination across multiple services.
3. Better Performance
Unlike microservices that rely on network calls for inter-service communication, a modular monolith communicates internally, reducing latency and improving efficiency.
4. Smooth Transition to Microservices
A well-structured modular monolith can evolve into a microservices architecture by gradually extracting modules into independent services when needed.
5. Good for startup and small teams
When you’ve a small team, it helps in focusing and developing at a single service at the scale of startup, it makes more sense to not overcomplicate things and save cost using this approach.
Examples of When Modular Monoliths Can Be Applied:
- Early Stages of Development: When a project is in its initial phases, teams are often still understanding the domain and requirements. Implementing a modular monolith allows for rapid development and easier modifications without the complexities of managing multiple microservices.
- Small to Medium-Sized Applications: For applications that don’t require extensive scalability or have limited complexity, a modular monolith provides a straightforward approach, reducing the overhead associated with microservices, such as inter-service communication and distributed system challenges.
- Limited Development Resources: Organizations with smaller development teams or limited DevOps capabilities may benefit from the reduced operational complexity of a modular monolith, as it simplifies deployment and maintenance compared to a microservices architecture.
- Evolving Domain Knowledge: When the business domain is not yet fully understood, starting with a modular monolith allows teams to iteratively refine module boundaries. This approach facilitates a smoother transition to microservices in the future if needed, as modules can be extracted with minimal effort.
- Need for High Performance and Stability: In scenarios where performance and stability are critical, a modular monolith can minimize the latency and reliability issues that might arise from network communication between microservices.
- Cost Constraints: Adopting a microservices architecture often requires additional infrastructure for service discovery, load balancing, and monitoring. A modular monolith can be more cost-effective, especially in environments where resources are limited.
Real-World Examples:
- Shopify: Initially built as a monolithic application, Shopify evolved into a modular monolith to maintain there codebases which helped them in long run.
- Spotify: Although Spotify is heavily built on the micro-service architecture model, we continue to operate numerous large scale codebases. These often contain some of Spotify’s most important engineering properties, such as our Desktop app, Web Player, iOS, and Android apps. In the web community, there is also a growing use of monorepos for better dependency management with tools such as Yarn Workspaces.
- Amazon: Amazon managed its various services and vast functionalities — ranging from product listings to payment processing — through the efficient use of modular architecture. This approach enabled Amazon to effectively integrate new features and services without impacting existing ones, handling high traffic and global operations seamlessly.
When to move to Microservices
- Scalability becomes bottlenack — once you know that some parts of your system needs to scale independently if it better with microservices as you can scale them without touching the other parts.
- You and your team starts growing — once the team size grows which also means your business is also growing, it can become harder to manage work though monoliths so maybe the time to extract out microservices using DDD.
- Need for Polyglot Tech Stack— if you need to develop using different tech stack in different domain needs that’s also a chance to explore for microservices and doing it with monoliths won’t be an option anymore.
- Frequent and Independent Releases Needed — When different features need to be updated at different rates.
- Long Builds and Deployments — If CI/CD pipelines take too long due to the size of the monolith, Microservices allow smaller, faster deployments.
When NOT to Move to Microservices
Choose microservices for the benefits, not because your monolithic codebase is a mess — martin fowler
- If the monolith is manageable and not causing significant problems.
- If the team lacks experience with distributed systems (added complexity).
- When moving to microservices will not provide a clear business value.
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