The SOA is a software design paradigm where services are the central focus. For the purposes of discussion and clarity, let's define a service as a discrete unit of functionality that can be accessed remotely and acted upon independently. The characteristics of a service in terms of a SOA are:
- It represents a specific business function or purpose (hopefully)
- It is self-contained
- It can and should function as a black box
- It may also be comprised of other associated services
- There is a hard and dedicated contract for each service (usually)
Some folks like to consider a microservice nothing more than a more formalized and refined version of an SOA. Perhaps in some ways, that could be the case. Many people believe that the SOA just never really formalized, and microservices are the missing formality. And although I am sure an argument could be made for that being true, microservices are usually designed differently, with a response-actor paradigm, and they usually use smaller or siloed databases (when permissible), and smaller and faster messaging protocols versus things like a giant Enterprise Service Bus (ESB).
Let's take a moment and talk about the microservice architecture itself.
