The single responsibility principle is one of the principles defined as part of the SOLID design pattern. It states that a unit should only have one responsibility.

This implies that a unit, either a class, a function, or a service, should have only one responsibility. At no point should two units share one responsibility or one unit have more than one responsibility. A unit with more than one responsibility indicates tight coupling.

As shown in the preceding diagram, Customer, Product, and Order are different functions of an e-commerce application. Rather than building all of them into one application, it is better to have three different services, each responsible for exactly one business function, so that changes to one responsibility will not impair others. In the preceding scenario, Customer, Product, and Order will be treated as three independent microservices.

Microservices are self-contained, independently deployable, and autonomous services that take full responsibility of a business capability and its execution. They bundle all dependencies, including library dependencies, and execution environments such as web servers and containers or virtual machines that abstract physical resources.

One of the major differences between microservices and SOA is in their level of autonomy. While most SOA implementations provide service-level abstraction, microservices go further and abstract the realization and execution environment.

In traditional application developments, we build a WAR or an EAR, then deploy it into a JEE application server, such as with JBoss, WebLogic, WebSphere, and so on. We may deploy multiple applications into the same JEE container. In the microservices approach, each microservice will be built as a fat Jar, embedding all dependencies and run as a standalone Java process.

Microservices may also get their own containers for execution, as shown in the preceding diagram. Containers are portable, independently manageable, lightweight runtime environments. Container technologies, such as Docker, are an ideal choice for microservices deployment.