Microservices.io is brought to you by Chris Richardson. Experienced software architect, author of POJOs in Action, the creator of the original CloudFoundry.com, and the author of Microservices patterns.
Chris helps clients around the world adopt the microservice architecture through consulting engagements, and training classes and workshops.
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You are developing a large, complex application and want to use the microservice architecture. The microservice architecture structures an application as a set of loosely coupled services. The goal of the microservice architecture is to accelerate software development by enabling continuous delivery/deployment.
The microservice architecture does this in two ways:
These benefits are not automatically guaranteed. Instead, they can only be achieved by the careful functional decomposition of the application into services.
A service must be small enough to be developed by a small team and to be easily tested. A useful guideline from object-oriented design (OOD) is the Single Responsibility Principle (SRP). The SRP defines a responsibility of a class as a reason to change, and states that a class should only have one reason to change. It make sense to apply the SRP to service design as well and design services that are cohesive and implement a small set of strongly related functions.
The application also be decomposed in a way so that most new and changed requirements only affect a single service. That is because changes that affect multiple services requires coordination across multiple teams, which slows down development. Another useful principle from OOD is the Common Closure Principle (CCP), which states that classes that change for the same reason should be in the same package. Perhaps, for instance, two classes implement different aspects of the same business rule. The goal is that when that business rule changes developers, only need to change code in a small number - ideally only one - of packages. This kind of thinking makes sense when designing services since it will help ensure that each change should impact only one service.
How to decompose an application into services?
Define services corresponding to Domain-Driven Design (DDD) subdomains. DDD refers to the application’s problem space - the business - as the domain. A domain is consists of multiple subdomains. Each subdomain corresponds to a different part of the business.
Subdomains can be classified as follows:
The subdomains of an online store include:
The corresponding microservice architecture would have services corresponding to each of these subdomains.
This pattern has the following benefits:
There are the following issues to address:
How to identify the subdomains? Identifying subdomains and hence services requires an understanding of the business. Like business capabilities, subdomains are identified by analyzing the business and its organizational structure and identifying the different areas of expertise. Subdomains are best identified using an iterative process. Good starting points for identifying subdomains are:
Application architecture patterns
Cross cutting concerns