Currently doing some exams and I'm struggling through some concepts. These have all been 'mentioned' in my notes really but I didn't really understand how they all linked together. As far as my understanding is:
SOA – a solution to make service consumers/providers communicate. (as far as I understand this is the umbrella term for everything else)
WSDL – A language that describes the provider service.
SOAP – A XML protocol 'wrapper' used by the services to send messages. Works in conjunction with WSDL as to provide parameters?
REST – A design pattern that is similar to SOAP in function but avoids the XML? (really not sure about this one)
JSON – An alternative to XML that uses javascript? (not sure about this one either)
Looking around in the internet there doesn't seem to be a clear definition of what all of these are and how they interlink.
Best Answer
Imagine you are developing a web-application and you decide to decouple the functionality from the presentation of the application, because it affords greater freedom.
You create an API and let others implement their own front-ends over it as well. What you just did here is implement an SOA methodology, i.e. using web-services.
So, you design an interchange mechanism between the back-end (web-service) that does the processing and generation of something useful, and the front-end (which consumes the data), which could be anything. (A web, mobile, or desktop application, or another web-service). The only limitation here is that the front-end and back-end must "speak" the same "language".
That's where SOAP and REST come in. They are standard ways you'd pick communicate with the web-service.
SOAP:
SOAP internally uses XML to send data back and forth. SOAP messages have rigid structure and the response XML then needs to be parsed. WSDL is a specification of what requests can be made, with which parameters, and what they will return. It is a complete specification of your API.
REST:
REST is a design concept.
It isn't as rigid as SOAP. RESTful web-services use standard URIs and methods to make calls to the webservice. When you request a URI, it returns the representation of an object, that you can then perform operations upon (e.g. GET, PUT, POST, DELETE). You are not limited to picking XML to represent data, you could pick anything really (JSON included)
JSON and XML, are functionally equivalent, and common choices. There are also RPC-based frameworks like GRPC based on Protobufs, and Apache Thrift that can be used for communication between the API producers and consumers. The most common format used by web APIs is JSON because of it is easy to use and parse in every language.