WCF and REST Interview Questions

<system.serviceModel>

   <client>

      <endpoint name = "MyEndpoint"

         address  = "http://localhost:8000/MyService/"

         binding  = "wsHttpBinding"

         contract = "IMyContract"

      />

   </client>

</system.serviceModel>

[ServiceContract]

interface IMyContract

{

   [OperationContract]

   string MyMethod( );

}

class MyService : IMyContract

{

   public string MyMethod( )

   {

      return "Hello World";

   }

}

[DataContract]

class Contact

{

   [DataMember]

   public string FirstName;

   [DataMember]

    public string LastName;

}

[ServiceContract]

interface IStock

{

[OperationContract]

[WebGet]

int GetStock(string StockId);

}

1.) How does Windows Communication Foundation address Service Oriented Architecture (SOA)?

WCF is the first programming model built from the ground up to provide implicit service-oriented application development, enabling developers to work autonomously and build applications that are more version independent, thereby increasing application resilience to change.

2.) How to deal with operation overloading while exposing the WCF services?

By default overload operations  are not supported in WSDL based

operation. However by using Name property of OperationContract attribute, we can deal with operation overloading scenario.

[ServiceContract]

interface Isum

{

[OperationContract(Name = "MultiplyInt")]

int Multiply(int arg1,int arg2);

[OperationContract(Name = "MultiplyDouble")]

double Multiply(double arg1,double arg2);

}

Notice that both method name in the above interface is same (Add), however the Name property of the OperationContract is different. In this case client proxy will have two methods with different name MultiplyInt and MultiplyDouble.

3.) Is Windows Communication Foundation going to interoperate with my existing applications?

The current plan is for Windows Communication Foundation to provide wire-level interoperability with WSE3, System.Messaging, .NET Enterprise Services, and ASMX applications. With minimal or no changes to code, applications built with these technologies will be able to call Windows Communication Foundation services and be callable by Windows Communication Foundation services.

4.) How to configure Reliability while communicating with WCF Services?

Reliability can be configured in the client config file by adding reliableSession under binding tag.

Reliability is supported by following bindings only:

NetTcpBinding


WSHttpBindingWSFederationHttpBinding


WSDualHttpBinding

5.) Will Windows Communication Foundation applications interoperate with Web services built with other technologi

es?

Yes. By default,services built with WCF will communicate with other services based on the interoperable Web services specifications. This means that WCF services will communicate with any application built on an infrastructure that also conforms to these standards. Microsoft is deeply committed to p

latform interoperability and is an active member of key standards organizations defining the latest Web services standards.

6.) How to set the timeout property for the WCF Service client call?

The timeout property can be set for the WCF Service client call using binding tag. If no timeout has been specified, the default is considered as 1 minute.

7.) What are the core components of an Windows Communication Foundation service?

A host environment—an application domain and process—in which the service runs;

A service class, implemented in C# or VB.NET or another CLR-based language that implements one or more methods;

One or more endpoints that allow clients to access the service.

8.) What are different elements of WCF Srevices Client configuration file?

WCF Services client configuration file contains endpoint, address, binding and contract.

The IHttpHandler and IHttpHandlerFactory interfaces ?

The IHttpHandler interface is implemented by all the handlers. The interface consists of one property called IsReusable. The IsReusable property gets a value indicating whether another request can use the IHttpHandler instance. The method ProcessRequest() allows you to process the current request. This is the core place where all your code goes. This method receives a parameter of type HttpContext using which you can access the intrinsic objects such as Request and Response. The IHttpHandlerFactory interface consists of two methods - GetHandler and ReleaseHandler. The GetHandler() method instantiates the required HTTP handler based on some condition and returns it back to ASP.NET. The ReleaseHandler() method allows the factory to reuse an existing handler.

What is REST?

REST is a term coined by Roy Fielding in his Ph.D. dissertation [1] to describe an architecture style of networked systems. REST is an acronym standing for Representational State Transfer.

Why is it called Representational State Transfer?

The Web is comprised of resources. A resource is any item of interest.

Motivation for REST

The motivation for REST was to capture the characteristics of the Web which made the Web successful. Subsequently these characteristics are being used to guide the evolution of the Web.

REST - An Architectural Style, Not a Standard

REST is not a standard. You will not see the W3C putting out a REST specification. You will not see IBM or Microsoft or Sun selling a REST developer's toolkit. Why? Because REST is just an architectural style. You can't bottle up that style. You can only understand it, and design your Web services in that style. (Analogous to the client-server architectural style. There is no client-server standard.)

While REST is not a standard, it does use standards:

• HTTP



• URL



• XML/HTML/GIF/JPEG/etc (Resource Representations)



• text/xml, text/html, image/gif, image/jpeg, etc (MIME Types)

The Classic REST System

The Web is a REST system! Many of those Web services that you have been using these many years - book-ordering services, search services, online dictionary services, etc - are REST-based Web services. Alas, you have been using REST, building REST services and you didn't even know it.

REST is concerned with the "big picture" of the Web. It does not deal with implementation details (e.g., using Java servlets or CGI to implement a Web service). So let's look at an example of creating a Web service from the REST "big picture" perspective.

Parts Depot Web Services

Parts Depot, Inc (fictitious company) has deployed some web services to enable its customers to:

• get a list of parts



• get detailed information about a particular part



• submit a Purchase Order (PO)

Let's consider how each of these services are implemented in a RESTful fashion.

Get Parts List

The web service makes available a URL to a parts list resource. For example, a client would use this URL to get the parts list:

http://www.parts-depot.com/parts

Note that "how" the web service generates the parts list is completely transparent to the client. All the client knows is that if he/she submits the above URL then a document containing the list of parts is returned. Since the implementation is transparent to clients, Parts Depot is free to modify the underlying implementation of this resource without impacting clients. This is loose coupling.

Here's the document that the client receives:

<?xml version="1.0"?>

<p:Parts xmlns:p="http://www.parts-depot.com" 

         xmlns:xlink="http://www.w3.org/1999/xlink">

      <Part id="00345" xlink:href="http://www.parts-depot.com/parts/00345"/>

      <Part id="00346" xlink:href="http://www.parts-depot.com/parts/00346"/>

      <Part id="00347" xlink:href="http://www.parts-depot.com/parts/00347"/>

      <Part id="00348" xlink:href="http://www.parts-depot.com/parts/00348"/>

</p:Parts>

[Assume that through content negotiation the service determined that the client wants the representation as XML (for machine-to-machine processing).] Note that the parts list has links to get detailed info about each part. This is a key feature of REST. The client transfers from one state to the next by examining and choosing from among the alternative URLs in the response document.

Get Detailed Part Data

The web service makes available a URL to each part resource. Example, here's how a client requests part 00345:

http://www.parts-depot.com/parts/00345

Here's the document that the client receives:

<?xml version="1.0"?>

<p:Part xmlns:p="http://www.parts-depot.com"   

        xmlns:xlink="http://www.w3.org/1999/xlink">

      <Part-ID>00345</Part-ID>

      <Name>Widget-A</Name>

      <Description>This part is used within the frap assembly</Description>

      <Specification xlink:href="http://www.parts-depot.com/parts/00345/specification"/>

      <UnitCost currency="USD">0.10</UnitCost>

      <Quantity>10</Quantity>

</p:Part>

Again observe how this data is linked to still more data - the specification for this part may be found by traversing the hyperlink. Each response document allows the client to drill down to get more detailed information.

Submit PO

The web service makes available a URL to submit a PO. The client creates a PO instance document which conforms to the PO schema that Parts Depot has designed (and publicized in a WSDL document). The client submits PO.xml as the payload of an HTTP POST.

The PO service responds to the HTTP POST with a URL to the submitted PO. Thus, the client can retrieve the PO any time thereafter (to update/edit it). The PO has become a piece of information which is shared between the client and the server. The shared information (PO) is given an address (URL) by the server and is exposed as a Web service.

Logical URLs versus Physical URLs

A resource is a conceptual entity. A representation is a concrete manifestation of the resource. This URL:

http://www.parts-depot.com/parts/00345

is a logical URL, not a physical URL. Thus, there doesn't need to be, for example, a static HTML page for each part. In fact, if there were a million parts then a million static HTML pages would not be a very attractive design.

[Implementation detail: Parts Depot could implement the service that gets detailed data about a particular part by employing a Java Servlet which parses the string after the host name, uses the part number to query the parts database, formulate the query results as XML, and then return the XML as the payload of the HTTP response.]

As a matter of style URLs should not reveal the implementation technique used. You need to be free to change your implementation without impacting clients or having misleading URLs.

REST Web Services Characteristics

Here are the characteristics of REST:

• Client-Server: a pull-based interaction style: consuming components pull representations.



• Stateless: each request from client to server must contain all the information necessary to understand the request, and cannot take advantage of any stored context on the server.



• Cache: to improve network efficiency responses must be capable of being labeled as cacheable or non-cacheable.



• Uniform interface: all resources are accessed with a generic interface (e.g., HTTP GET, POST, PUT, DELETE).



• Named resources - the system is comprised of resources which are named using a URL.



• Interconnected resource representations - the representations of the resources are interconnected using URLs, thereby enabling a client to progress from one state to another.



• Layered components - intermediaries, such as proxy servers, cache servers, gateways, etc, can be inserted between clients and resources to support performance, security, etc.

Principles of REST Web Service Design

1. The key to creating Web Services in a REST network (i.e., the Web) is to identify all of the conceptual entities that you wish to expose as services. Above we saw some examples of resources: parts list, detailed part data, purchase order.

2. Create a URL to each resource. The resources should be nouns, not verbs. For example, do not use this:

http://www.parts-depot.com/parts/getPart?id=00345

Note the verb, getPart. Instead, use a noun:

http://www.parts-depot.com/parts/00345

3. Categorize your resources according to whether clients can just receive a representation of the resource, or whether clients can modify (add to) the resource. For the former, make those resources accessible using an HTTP GET. For the later, make those resources accessible using HTTP POST, PUT, and/or DELETE.

4. All resources accessible via HTTP GET should be side-effect free. That is, the resource should just return a representation of the resource. Invoking the resource should not result in modifying the resource.

5. No man/woman is an island. Likewise, no representation should be an island. In other words, put hyperlinks within resource representations to enable clients to drill down for more information, and/or to obtain related information.

6. Design to reveal data gradually. Don't reveal everything in a single response document. Provide hyperlinks to obtain more details.

7. Specify the format of response data using a schema (DTD, W3C Schema, RelaxNG, or Schematron). For those services that require a POST or PUT to it, also provide a schema to specify the format of the response.

8. Describe how your services are to be invoked using either a WSDL document, or simply an HTML document.

What is a REST Web Service

The acronym REST stands for Representational State Transfer, this basically means that each unique URL is a representation of some object. You can get the contents of that object using an HTTP GET, to delete it, you then might use a POST, PUT, or DELETE to modify the object (in practice most of the services use a POST for this).

Who's using REST?

All of Yahoo's web services use REST, including Flickr, del.icio.us API uses it, pubsub, bloglines, technorati, and both eBay, and Amazon have web services for both REST and SOAP.

Who's using SOAP?

Google seams to be consistent in implementing their web services to use SOAP, with the exception of Blogger, which uses XML-RPC. You will find SOAP web services in lots of enterprise software as well.

REST vs SOAP

As you may have noticed the companies I mentioned that are using REST api's haven't been around for very long, and their apis came out this year mostly. So REST is definitely the trendy way to create a web service, if creating web services could ever be trendy (lets face it you use soap to wash, and you rest when your tired). The main advantages of REST web services are:

• Lightweight - not a lot of extra xml markup



• Human Readable Results



• Easy to build - no toolkits required

SOAP also has some advantages:

• Easy to consume - sometimes



• Rigid - type checking, adheres to a contract



• Development tools

For consuming web services, its sometimes a toss up between which is easier. For instance Google's AdWords web service is really hard to consume (in CF anyways), it uses SOAP headers, and a number of other things that make it kind of difficult. On the converse, Amazon's REST web service can sometimes be tricky to parse because it can be highly nested, and the result schema can vary quite a bit based on what you search for.