Chapter 5. Understanding Bindings in WSDL

Abstract

Bindings map the logical messages used to define a service into a concrete payload format that can be transmitted and received by an endpoint.

Overview

Bindings provide a bridge between the logical messages used by a service to a concrete data format that an endpoint uses in the physical world. They describe how the logical messages are mapped into a payload format that is used on the wire by an endpoint. It is within the bindings that details such as parameter order, concrete data types, and return values are specified. For example, the parts of a message can be reordered in a binding to reflect the order required by an RPC call. Depending on the binding type, you can also identify which of the message parts, if any, represent the return type of a method.

Port types and bindings

Port types and bindings are directly related. A port type is an abstract definition of a set of interactions between two logical services. A binding is a concrete definition of how the messages used to implement the logical services will be instantiated in the physical world. Each binding is then associated with a set of network details that finish the definition of one endpoint that exposes the logical service defined by the port type.
To ensure that an endpoint defines only a single service, WSDL requires that a binding can only represent a single port type. For example, if you had a contract with two port types, you could not write a single binding that mapped both of them into a concrete data format. You would need two bindings.
However, WSDL allows for a port type to be mapped to several bindings. For example, if your contract had a single port type, you could map it into two or more bindings. Each binding could alter how the parts of the message are mapped or they could specify entirely different payload formats for the message.

The WSDL elements

Bindings are defined in a contract using the WSDL binding element. The binding element consists of attributes like, name, that specifies a unique name for the binding and type that provides reference to PortType. The value of this attribute is used to associate the binding with an endpoint as discussed in Chapter 4, Defining Your Logical Interfaces.
The actual mappings are defined in the children of the binding element. These elements vary depending on the type of payload format you decide to use. The different payload formats and the elements used to specify their mappings are discussed in the following chapters.

Adding to a contract

Apache CXF provides command line tools that can generate bindings for predefined service interfaces.
The tools will add the proper elements to your contract for you. However, it is recommended that you have some knowledge of how the different types of bindings work.
You can also add a binding to a contract using any text editor. When hand editing a contract, you are responsible for ensuring that the contract is valid.

Supported bindings

Apache CXF supports the following bindings:
  • SOAP 1.1
  • SOAP 1.2
  • CORBA
  • Pure XML