What's the best way to choose granularity and modularity for your microservices?

1 Why granularity and modularity matter

Granularity and modularity are two related concepts that affect the design and performance of your microservices. Granularity refers to how fine-grained or coarse-grained your microservices are, that is, how much functionality and data they encapsulate. Modularity refers to how well-defined and cohesive your microservices are, that is, how clearly they separate their concerns and responsibilities. Both granularity and modularity have trade-offs and implications for your system's scalability, reliability, maintainability, and complexity. For example, too fine-grained microservices may increase the network overhead, latency, and failure rate, while too coarse-grained microservices may reduce the flexibility, reusability, and testability of your system. Similarly, too low modularity may lead to tight coupling, code duplication, and dependency hell, while too high modularity may introduce unnecessary abstraction, fragmentation, and communication challenges.

2 How to determine the optimal granularity

There is no definitive answer or formula to determine the optimal granularity for your microservices, as it depends on various factors such as your domain, requirements, constraints, and goals. However, some general guidelines and heuristics can help you make an informed decision. One of them is the single responsibility principle (SRP), which states that each microservice should have one and only one reason to change. This means that each microservice should focus on a specific business capability or function, and avoid mixing unrelated or orthogonal concerns. Another guideline is the bounded context principle, which is based on the domain-driven design (DDD) approach. This principle suggests that each microservice should correspond to a well-defined and consistent subdomain of your system, and have its own data model and language. A third guideline is the common closure principle (CCP), which implies that each microservice should change for the same reasons and at the same time as other microservices that share its functionality or data.

4 How to improve the modularity

If you find that your microservices are not modular enough, there are some techniques and practices that can help you improve their cohesion, reduce their coupling, and simplify their complexity. One of them is the interface segregation principle (ISP), which states that each microservice should expose multiple and specific interfaces for different types of clients or consumers, rather than a single and generic interface. This way, you can avoid forcing the clients to depend on functionality or data that they do not need or use, and increase the flexibility and adaptability of your microservice. Another technique is the dependency inversion principle (DIP), which states that each microservice should depend on abstractions rather than concretions. This means that you should use contracts, protocols, or standards to define the communication and collaboration between your microservices, rather than hard-coded or proprietary formats or methods. This way, you can decouple your microservices from their implementation details, and make them more resilient and interoperable. A third technique is the separation of concerns principle (SoC), which states that each microservice should address a distinct and orthogonal aspect of your system, and avoid mixing or overlapping with other aspects. This way, you can simplify and clarify the functionality and data of your microservice, and avoid duplication or inconsistency.

5 How to balance the trade-offs

Choosing the best granularity and modularity for your microservices involves balancing the trade-offs and implications of different design decisions. It is not about achieving perfection or absoluteness, but rather being pragmatic and suitable. To do so, you should consider your domain and business logic, requirements and constraints, goals and benefits, as well as costs and risks. Understanding the nature of your system, analyzing stakeholder needs, evaluating desired outcomes and advantages, and estimating potential challenges and drawbacks will help you make an informed and rational choice of granularity and modularity for your microservices. This will allow you to achieve a better balance between the trade-offs.