The Top 15 Web Service Description Language (WSDL) Interview Questions for Web Services Jobs

Web services have become an integral part of software development and enterprises today. As a web service developer, you need to have a solid understanding of web services standards like WSDL, SOAP, REST etc. In this article, we will focus on WSDL and go through the most frequently asked WSDL interview questions for web service developer roles.

What is WSDL?

WSDL stands for Web Services Description Language. It is an XML format for describing web services and how to access them. The WSDL document provides key details about a web service like operations, input and output messages, binding details and service endpoint.

WSDL enables separation of interface from implementation, so the client applications interested in using the web service, only need the WSDL file. They don’t need to know the underlying implementation language and platform details.

What are the key components of a WSDL document?

A WSDL document has the following key sections:

• Types – Defines the data types used in messages. Usually it’s an XML schema.
• Message – Defines the data elements for each operation.
• Operation – Defines one method of the web service.
• Port Type – Groups a set of operations together.
• Binding – Specifies protocol and data format details for each port type.
• Service – Specifies the port and binding details for a web service.

What are the different styles of WSDL?

There are two styles of WSDL document – RPC style and Document style

In RPC style, the request and response messages are defined using method name and simple data types. There are no schemas associated hence can’t be validated.

In Document style the request and response messages are defined as XML elements that can be validated against associated schema. This provides better interoperability.

How is SOAP binding specified in WSDL?

SOAP binding details are specified inside the <binding> element of a WSDL document. It contains protocol information and data format details

For example:

<binding name="ServiceNamePortBinding" type="tns:ServicePortType">  <soap:binding style="document"     transport="http://schemas.xmlsoap.org/soap/http"/>  ...</binding>

Here soap:binding specifies that the binding is for SOAP protocol.

What is the purpose of namespaces in WSDL?

Namespaces help avoid naming collisions between different WSDL documents. The namespaces are defined using xmlns attributes.

Some pre-defined namespaces like SOAP, XML Schema etc. are commonly imported in the <definitions> element. Other namespaces for custom types, services etc. are defined in their respective elements.

How is the SOAP Action specified in WSDL?

The SOAP action is specified on the input and output elements inside the WSDL operation:

<operation name="GetUser">  <input wsaw:Action="http://example.com/GetUser" .../>  <output wsaw:Action="http://example.com/GetUserResponse" .../> </operation>

The SOAP action helps identify the intent of the SOAP message.

What is the difference between a PortType and Binding in WSDL?

A portType in WSDL specifies the abstract interface of a web service by grouping a set of abstract operations. It does not have implementation or protocol specific details.

A binding provides the implementation and protocol specific details for a portType. It defines the message format and transmission protocols.

What is the purpose of importing XML Schemas in WSDL?

XML Schemas are used to define custom data types in WSDL documents. These data types are used in messages for operation requests and responses.

By importing XML schemas, the WSDL document can validate messages against data type definitions provided in the schemas. This ensures interoperability.

How are WSDL documents accessed by client applications?

WSDL documents are hosted at a network accessible URL, usually on the web server where the web service is deployed.

The WSDL URL is usually structured like:
http://hostname/webserviceName?wsdl

Client applications use this URL to download the WSDL document and read the details of the web service.

What is the relationship between WSDL and SOAP?

WSDL is used to describe web services interfaces. SOAP is one of the protocols that can be used to access web services.

WSDL provides abstract description of operations supported by a web service. The communication protocol binding like SOAP binding provides protocol specific details on how to access the service.

What is UDDI and its significance for WSDL?

UDDI stands for Universal Description Discovery and Integration. It provides a registry and repository for storing WSDL documents and inquiry API for searching services.

Businesses can register their web service details like WSDL URL, access point URLs, service category etc. on UDDI registry. Client applications can then search this registry to discover various web services.

Can a WSDL document contain multiple services?

Yes, a single WSDL document can contain the description for multiple web services in SOA architectures.

Each web service will have its own <portType>, <binding> and <service> elements defined in the WSDL document, while the <types> and <messages> can be reused.

What are the limitations of WSDL?

Some key limitations of WSDL are:

• Only supports XML Schema data types
• Limited support for REST services
• Does not describe non-functional aspects like security
• Does not provide semantics of operations

How are faults specified in WSDL?

Faults can be specified in WSDL inside the <operation> element. A <fault> element is used to define an exception that could be thrown by the operation:

<operation>   ...  <fault name="InvalidInputException"          message="invalidInputMsg"/> </operation>

The fault message refers to a message element that provides details for that exception.

What is RPC style web service? How is it defined in WSDL?

In RPC style web service, the request and response messages are defined using method name and simple data types like strings, integers.

The RPC style WSDL does not contain XML schema definitions in <types> element. The style is specified on binding:

<binding>  <soap:binding style="rpc" ... /></binding>

Can we create WSDL first or code first in web service development?

WSDL document can be created before service implementation or after it.

In the code first approach, the service code is written first and then WSDL is auto-generated using tools.

In the WSDL first approach, WSDL contract is created first. Then code is written to implement the service according to the WSDL contract.

Both approaches have their own pros and cons. Code first is easier to implement but WSDL first enables defining service contract upfront.