Single responsibility principle

Another principle that is applied to microservices and comes from the OOP world is specifically the letter S in SOLID. What could be thought before the class level should now be through the application level, so that microservices can be really micro in terms of what really matters at the domain level.

The microservice domain cannot be large; on the contrary, it must be limited. The limits that were cited in DDD return to be applied now and with more intensity. Precisely, the limits in the microservice domain is what will make this application sensitive to changes and open to the perception of possible errors.

It is a very common difficulty in maintaining pure microservices in their domains. The natural tendency, either by habit or ignorance, is to try to group all the business rules or similar codes in the same microservice, without even understanding whether they are part of the same domain.

To illustrate, think of the application on which we are working, our news portal. News and recommendations virtually work together all the time. Recommendations always put together some news that has some of the related labels. At first, it makes sense, because as the recommendations are always related to the news, apparently this does not cause a problem and, moreover, could reduce problems such as network latency. The main concept could be represented by the following diagram:

However, creating a microservice containing news and recommendations will generate unnecessary and very expensive engagement for future changes. In a simple demonstration exercise, we can think of a number of business changes with which this engagement would generate problems.

A new business requirement arose. Those responsible for thinking about the product had an idea to use recommendations to compose a personal home page according to the news that stands out most in our portal. So, the recommendations aren't just connected to the news anymore, but to the user too. With this new requirement, some problems relating to the coupling done previously will emerge at the level of deployment, scalability, and maintenance features code.

Based on the new requirement mentioned previously, it is clear that news and recommendations work together, but they are totally different domains from each other. Identifying areas to apply the principle of sole responsibility for each microservice is crucial to application architecture. The following diagram shows the main idea of the microservice distribution:

Working together and in concert, Home requests information either from News or Recommendation. There are some forms of data orchestration that we will come to later in the book, but now it is important to understand that Home can consult separate services and compose the received data conveniently.

The separation of News and Recommendation results in an application where the deploy becomes a simpler, more consistent code base and the fully defined and specific business domain for a purpose.