What is WSDL and How Does It Work in XML?
Web Services Description Language (WSDL) is an XML-based standard used to describe the functionality, interface, and network location of a web service. It acts as a formal, machine-readable contract between a client and a service provider, detailing what operations the service offers, the data types it accepts and returns, and the exact protocols and URLs needed to communicate with it. By providing this blueprint, WSDL enables seamless cross-platform integration, allowing systems built on different technologies to discover and interact with SOAP-based web services automatically.
Understanding WSDL
WSDL (pronounced wiz-dul) is predominantly used in Simple Object Access Protocol (SOAP) architectures. Because it is written in standard XML, it remains strictly platform- and language-independent. A client application written in Java or Python can read a WSDL file generated by a .NET server and instantly generate the client-side code (stubs) needed to invoke the remote procedures correctly.
The Core XML Elements of a WSDL Document
A WSDL document breaks down a web service interface into abstract definitions (what the service does) and concrete implementations (how and where to access it). It accomplishes this using five primary XML elements:
1. <types>
The <types> element defines the data types used by
the web service. It typically uses XML Schema Definition (XSD) to
declare complex data structures, custom objects, and validation rules
for the incoming and outgoing data.
2. <message>
The <message> element describes the data being
exchanged. Each message consists of one or more
<part> elements, which represent the parameters
passed to an operation (request) or the return values received from it
(response).
3. <portType>
The <portType> element defines the abstract
interface of the web service. It groups multiple
<operation> elements together, where each operation
represents an action the service can perform (analogous to a method or
function in programming). Each operation specifies its associated input
and output <message> elements.
WSDL supports four basic operation types within
<portType>: * One-Way: The service
receives a message without sending a response. *
Request-Response: The client sends a request, and the
service sends back a response. * Solicit-Response: The
service sends a request to the client and expects a response. *
Notification: The service sends a message to the client
without expecting a response.
4. <binding>
The <binding> element links the abstract
operations defined in <portType> to concrete
protocols and data formats. It specifies the transport protocol
(typically SOAP over HTTP) and defines how message payloads are encoded
(e.g., Document/Literal style).
5. <service> and
<port>
The <service> element represents the actual
physical deployment of the web service. Inside it, one or more
<port> elements specify the concrete network address
(URI/URL) where the service can be reached using the defined
binding.
How WSDL Describes Interfaces: Abstract to Concrete
WSDL organizes service definitions hierarchically:
- Data Definition: The
<types>and<message>tags establish the payload structure. - Behavioral Definition: The
<portType>tag maps messages to specific operations. - Protocol Mapping: The
<binding>tag dictates the network format and transport mechanism. - Endpoint Location: The
<service>tag reveals the exact URL where these operations are hosted.
This strict separation of interface from implementation allows developers to update a service’s physical location or transport protocol without altering the core functional contract.