Service-Oriented Architectures (SOA) has been around for a number of years. SOA is a great principle to design software. They are self-contained components providing services to other components. As we agreed before, it is all about maintaining low coupling on the different modules of the system as if it was a puzzle so that we can replace the pieces without causing a big impact on the overall system.

In principle, SOA looks very similar to microservices architectures. So what is the difference?

Microservices are fine-grained SOA components. In other words, a single SOA component can be decomposed in a number of microservices that can work together in order to provide the same level of functionality:

Another difference between microservices and SOA is the technologies used for interconnecting and writing the services.

J2EE is a technology stack that was designed to write SOA architectures as it enforced enterprise standards. Java Naming and Directory Interface, Enterprise Java Beans, and Enterprise Service Bus (ESB) were the ecosystems where SOA applications were built and maintained. Although ESB is a standard, very few engineers who graduated after 2005 have heard about ESB, even fewer have used it, and nowadays the modern frameworks such as Ruby on Rails do not even consider such complex pieces of software.

On the other hand, microservices enforce the use of standards (such as HTTP) that are broadly known and broadly interoperable. We can choose the right language or tool to build a component (microservice) following one of the key benefits explained earlier in this chapter, in the Technology heterogeneity section.

Aside from the technology stack and the size of the services, there is an even bigger difference between SOA and microservices: the domain model. Earlier in this chapter, we have talked about decentralization. Decentralization of the governance, but, moreover, decentralization of the data. In a microservices-based software, every microservice should store its own data locally, isolating the domain models to a single service; whereas, on an SOA oriented-software, the data is usually stored in a few big databases and the services share the domain models.