WSDL: An XML-based Language for Describing Web Services

August 31, 2024 3 min read Technology Information Technology Web Services XML API WSDL Integration

WSDL (Web Services Description Language) is an XML-based language used for describing a web service and specifying the methods by which it can be accessed.

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WSDL (Web Services Description Language) is an XML-based language used to describe web services and define how to access them. The primary function of WSDL is to provide a standard method for syntax and structure in a web service description, facilitating interoperability between disparate systems and applications.

Components of WSDL

Types

Defines the data types used by web services through XML Schema.

Message

Describes the messages used by the web service, including input and output parameters.

PortType

Specifies the collection of operations (functions) that can be performed by the web service.

Binding

Details how the operations are carried out over the network, explaining the protocol and data format.

The Structure of a WSDL Document

A WSDL document typically includes the following key elements represented in XML syntax:

 1<definitions name="service"
 2             targetNamespace="http://www.example.com/wsdl"
 3             xmlns:tns="http://www.example.com/wsdl"
 4             xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
 5             xmlns="http://schemas.xmlsoap.org/wsdl/">
 6
 7  <types> ... </types>
 8  <message name="RequestMessage"> ... </message>
 9  <message name="ResponseMessage"> ... </message>
10  <portType name="ServicePortType"> ... </portType>
11  <binding name="ServiceBinding" type="tns:ServicePortType"> ... </binding>
12  <service name="ServiceName"> ... </service>
13</definitions>

Historical Context

WSDL was first introduced by IBM and Microsoft in 2000 and fast became an industry standard. The language bridges the gap between the technical specifications of a web service and its various consumers, regardless of their platform and language of development.

Applicability and Use Cases

WSDL is essential in the context of SOA (Service-Oriented Architecture), where seamless integration across different services is crucial. It provides a contract that describes the precise nature of the service, fostering great reliability and scalability.

Enterprise Integration
Vendor Interoperability
Complex Service Networks

Comparisons

WSDL vs. REST

WSDL: Primarily used in SOAP (Simple Object Access Protocol) web services, strong typing, and explicit contract definition.
REST (Representational State Transfer): Simpler and more flexible, uses standard HTTP and is typically less verbose than WSDL-described services.

WSDL vs. OpenAPI

WSDL: Focused on XML and SOAP; heavyweight; service definitions in XML.
OpenAPI (formerly Swagger): JSON-based, broadly adopted for modern RESTful API specifications.

SOAP (Simple Object Access Protocol): A protocol for exchanging structured information in web services.
UDDI (Universal Description, Discovery, and Integration): A directory for storing information about web services.
REST (Representational State Transfer): An architectural style for designing loosely coupled web services.

FAQs

How is WSDL different from XML?

WSDL is an XML-based specification that focuses on describing the service interface and providing instructions on how to use the service, whereas XML is a markup language used to form structured documents.

Can WSDL be used with REST services?

Typically, REST services do not use WSDL, opting instead for simpler and more flexible definitions like OpenAPI.

Is WSDL still relevant?

Yes, WSDL remains relevant in systems where complex transactions and rigorous standards are necessary, typically in enterprise environments.

References

Christensen, E., Curbera, F., Meredith, G., & Weerawarana, S. (2001). Web Services Description Language (WSDL) 1.1. World Wide Web Consortium (W3C). Retrieved from https://www.w3.org/TR/wsdl
Gabriel, T. (2018). Understanding Web Services: XML, WSDL, SOAP, and UDDI. Addison-Wesley Professional.

Summary

WSDL plays a pivotal role in modern web services by offering a clear, standardized method to describe and access web-based protocols. Despite newer technologies such as REST and JSON, WSDL remains indispensable for scenarios requiring detailed service contracts, thus ensuring robust and reliable web service operations across diverse computing environments.