What are WSDL, SOAP and REST?

Question

A WSDL is an XML document that describes a web service. It actually stands for Web Services Description Language.

SOAP is an XML-based protocol that lets you exchange info over a particular protocol (can be HTTP or SMTP, for example) between applications. It stands for Simple Object Access Protocol and uses XML for its messaging format to relay the information.

REST is an architectural style of networked systems and stands for Representational State Transfer. It's not a standard itself, but does use standards such as HTTP, URL, XML, etc.

Example: In a simple terms if you have a web service of calculator.

WSDL: WSDL tells about the functions that you can implement or exposed to the client. For example: add, delete, subtract and so on.

SOAP: Where as using SOAP you actually perform actions like doDelete(), doSubtract(), doAdd(). So SOAP and WSDL are apples and oranges. We should not compare them. They both have their own different functionality.

Why we use SOAP and WSDL: For platform independent data exchange.

EDIT: In a normal day to day life example:

WSDL: When we go to a restaurant we see the Menu Items, those are the WSDL's.

Proxy Classes: Now after seeing the Menu Items we make up our Mind (Process our mind on what to order): So, basically we make Proxy classes based on WSDL Document.

SOAP: Then when we actually order the food based on the Menu's: Meaning we use proxy classes to call upon the service methods which is done using SOAP. :)

Every time someone mentions SOAP/WSDL, I think of objects and classes defined in xml...

"You use SOAP just the same way that you would any PHP class. However, in this case the class does not exist in the local applications file system, but at a remote site accessed over http." ... "If we think of using a SOAP service as just another PHP class then the WSDL document is a list of all the available class methods and properties. "

http://www.doublehops.com/2009/07/07/quick-tutorial-on-getting-started-with-soap-in-php/comment-page-1/

..and whenever someone talks about REST I think of HTTP's commands (request methods) like POST, GET and DELETE

SOAP -> SOAP(Simple object access protocal) is the application level protocal created for machine to machine interaction. Protocol defines standard rules. All the parties who are using the particular protocol should adhere to the protocol rules. Like TCP, It unwinds at transport layer, The SOAP protocol will be understood by Application layer( any application which supports SOAP - Axis2, .Net).

WSDL -> SOAP message consist of SoapEnevelope->SoapHeader and SoapBody. It doesn't define what would be message format? what are all the transports(HTTP,JMS) it supports? without this info, It is hard for any client who wants to consume the particular web service to construct the SOAP message. Even if they do, they won't be sure, it'll work all the time. WSDL is the rescue. WSDL (Web Service description Language) defines the operations, message formats and transport details for the SOAP message.

REST -> REST(Representational state transfer) is based on the Transport. Unlike SOAP which targets the actions, REST concerns more on the resources. REST locates the resources by using URL (example -http://{serverAddress}/employees/employeeNumber/12345) and it depends on the transport protocol( with HTTP - GET,POST, PUT, DELETE,...) for the actions to be performed on the resources. The REST service locates the resource based on the URL and perform the action based on the transport action verb. It is more of architectural style and conventions based.

You're not going to "simply" understand something complex.

WSDL is an XML-based language for describing a web service. It describes the messages, operations, and network transport information used by the service. These web services usually use SOAP, but may use other protocols.

A WSDL is readable by a program, and so may be used to generate all, or part of the client code necessary to call the web service. This is what it means to call SOAP-based web services "self-describing".

REST is not related to WSDL at all.