SOA has multiple definitions that vary with the vendors that provide platforms to host SOA services. One of the commonly accepted SOA definitions was coined by Don Box of Microsoft. His definition is essentially a set of design guidelines which a service-oriented system should adhere to.

Although this definition was originally explained in relation to Microsoft Indigo (now WCF), the tenets still hold true for other SOA platforms as well. An understanding of this principle is that all the services should be available in the network. This tenet dictates that no modules, routines, or procedures can be considered as participants in SOA. Let's take a look at the original tenets in a little detail. The first tenet says that a service should implement a domain functionality and should be discoverable by the other services making up the system. The discovery of service is generally done by registering each service in a directory. The clients of the services can discover each service at runtime. The second tenet explains that the services should be independent of the other services that make up the system. Since the services are independent of each other, they may also enjoy independence of platform and programming language. The third tenet advices that each service should expose an interface through which the rest of the services can communicate with it. The knowledge of this contract should be sufficient to operate with the service. The fourth tenet dictates that the services define the boundaries in which they would work. An example of such a boundary can be a range of integers within which a service that performs arithmetic operations would operate. Such policies should be mentioned in the form of policy expressions and should be machine readable. In WCF, the policies are implemented by the Web Services Policy (WS-Policy) framework.

Although none of the original tenets dictate the size of individual services built using SOA architecture, to obtain independence from other services in the system, an individual service in SOA needs to be coarse-grained. To minimize interaction between services, each service should implement functionalities that work together.

In essence, both the Microservices architecture and the SOA architecture try to solve the problems of monolithic design by modularizing the components. In fact, a system already designed using the SOA architecture is a step in the right direction to realize Microservices architecture.