Question about Web services..

 what are web services? 

The simplest answer would be that a web service is any piece of software available over the Internet that uses a standardized XML messaging system. To drill down, XML is a language used to encode all communications to a Web service. Because all communication is in XML, Web services are not limited to any one operating system or programming language. For instance, Java can talk with Per, Windows applications can talk with Unix applications, and so forth.
Beyond this basic definition, a Web service may further be defined as such:

1. A web service can have a public interface, defined in a common XML grammar. The interface describes all the methods available to clients and specifies the signature for each method. The interface definition is currently created using the Web Service Description Language (WSDL).

2. If you create a Web service, there should be some relatively simple mechanism for you to publish it. This would be the UDDI, Universal Description, Discovery, and Integration mechanism that allows the creators to house and service its public interface. Web services currently run a diversified range of production from news syndication and stock-market data to weather reports and package-tracking systems.

Remember these definitions as interviewers are likely to ask how you classify web services. Now let’s move on to frequently asked questions during web services interviews.

What are the components of web services?

There are four main components: XML, SOAP, UDDI, AND WSDL.

XML: eXtensible Markup Language is a uniform data representation and exchange mechanism.

SOAP: Simple Object Access Protocol, SOAP is a standard way of using XML vocabulary to enable programs on separate computers to interact across any network and describing messages between applications.

UDDI: Universal Description, Discovery, and Integration specification, UDDI is a mechanism to register and located WS based application.

WSDL: Web Services Description Language, this is a standard meta language to describe the services offered. Specifically, WSDL states what a request message much contain and what the response will look like in a clear notation. WSDL also defines where the service is available and what communications protocol is used to talk to that service.

What is new about Web services?

The answer is simply, XML. XML lies at the core of Web services, and provides a common language for describing Remote Procedure Calls, Web services, and Web service directories. Prior to XML, one could share data among different applications, but XML makes this so much easier to do. In the same vein, one can share services and code without Web services, but XML makes it easier to do these as well. By standardizing with XML, different applications can more easily talk to one another, and this makes software a whole lot more interesting. Try this course on getting started with XML to master this separate language.

What’s an example of a real web service in action?

IBM Web Services Browser, available on the IBM Alphaworks site, is a good and intuitive example of a real web service. The browser provides a series of web services demonstrations. Behind the scenes, it ties together SOAP, WSDL, and UDDI to provide a simple plug-and-play interface for finding and integrating web services. For example, you can find a stock-quote service, a traffic-report service, and a weather service. Each service is independent, and you can stack services like building blocks.

Explain the web services architecture.

The operations between different software applications, which are running on a variety of platforms and frame-working are supported by web services. This web services architecture provides the concepts, modeling, and understanding web services and relationships among the components. The WSA specifies the minimal characteristics that are very common for all web services and a number of characteristics to the needed web services. WSA is called interoperability architecture that means the global elements of a global web service network are identified by this architecture in order to perform the interoperability between the web services.

What are the steps to get a proxy object of a web service at the client side?

There are three steps to get a proxy object of a web service at the client side.

1. Access UDDI node for a list of web services.

2. Services thus responded by UDDI have URL pointing to DISCO or WSDL document.

3. Parse DISCO and WSDL document and build a proxy object which can communicate with the web service.

What types of operations are available in WSDL?

There are four operations available:

1. One-way, where the operation can receive a message but will not return a response.

2. Request-response, where the operation can receive a request and will return a response.

3. Solicit-response, where the operation can send a request and will wait for a response.

4. Notification, where the operation can send a message but will not wait for a response.

Define a REST web service.

REST is Representational State Transfer and it is a network of web pages where the client progresses through an application by selecting links. REST is an architectural style that uses existing standards such as HTTP.

Why do we need .Net Web Services?

We have a number of heterogeneous technologies available on internet. The demand for reusable components across platforms and programming languages are high. Most of the components have the limitation that they can't share or exchange data across different platforms, they are mostly language specific or platform specific. The technologies like COM, RMI, CORBA etc. contributed best to fulfill requirements to some extent, but components result from these said technologies are mostly either language specific or platform specific. 

To avoid above problem, we need to have web services. Through web services we have overcome the problem of interoperability between languages and platforms. Web services uses SOAP as transport protocol which uses a text based messaging model, i.e. XML to communicate between disparate systems. 

Deploying a Web Service

Deploying the .Net Web Services is as simple as any ASP.NET application. Similar to ASP.NET applications, you need to copy or upload the .ASMX file and the .DISCO files to the appropriate directories, and that's it.

What is new about Web services? 

People have been using Remote Procedure Calls (RPC) for some time now, and they long ago discovered how to send such calls over HTTP.
So, what is really new about Web services? The answer is XML.
XML lies at the core of Web services, and provides a common language for describing Remote Procedure Calls, Web services, and Web service directories.
Prior to XML, one could share data among different applications, but XML makes this so much easier to do. In the same vein, one can share services and code without Web services, but XML makes it easier to do these as well.
By standardizing on XML, different applications can more easily talk to one another, and this makes software a whole lot more interesting.

Explain WebMethod Attribute in Asp.Net Web Service..?

Attaching the WebMethod attribute to a Public method indicates that you want the method exposed as part of the XML Web service. The WebMethod attribute tells .NET that a particular public method should be exposed as a web-callable method. The WebMethod attribute has six associated properties to document and change the behavior of your web method. They are:

Description
MessageName
EnableSession
CacheDuration
TransactionOption
BufferResponse

The first two properties are used to document a web method, while the others affect its behavior.

Explain the protocols a .Net Web Service uses...??

In .Net, a web service is bind with three different protocols such as HTTP/POST, HTTP/GET, and SOAP. This allows client with three options to choose for communication. The protocols are included in the WSDL file that is automatically generated in .NET.

Http-Get and Http-Post can only be used when name/value pairs of data is dealt with. But when data is complex in nature such as ASP.NET dataset, XML notes etc, then we can use SOAP that serializes data in simpler form before sending.  

I keep reading about Web services, but I have never actually seen one. Can you show me a real Web service in action? 

If you want a more intuitive feel for Web services, try out the IBM Web Services Browser, available on the IBM Alphaworks site. The browser provides a series of Web services demonstrations. Behind the scenes, it ties together SOAP, WSDL, and UDDI to provide a simple plug-and-play interface for finding and invoking Web services. For example, you can find a stock-quote service, a traffic-report service, and a weather service. Each service is independent, and you can stack services like building blocks. You can, therefore, create a single page that displays multiple services--where the end result looks like a stripped-down version of my.yahoo or my.excite.

What is the Web service protocol stack?

The Web service protocol stack is an evolving set of protocols used to define, discover, and implement Web services. The core protocol stack consists of four layers:
Service Transport: This layer is responsible for transporting messages between applications. Currently, this includes HTTP, SMTP, FTP, and newer protocols, such as Blocks Extensible Exchange Protocol (BEEP).
XML Messaging: This layer is responsible for encoding messages in a common XML format so that messages can be understood at either end. Currently, this includes XML-RPC and SOAP.
Service Description: This layer is responsible for describing the public interface to a specific Web service. Currently, service description is handled via the WSDL.
Service Discovery: This layer is responsible for centralizing services into a common registry, and providing easy publish/find functionality. Currently, service discovery is handled via the UDDI.
Beyond the essentials of XML-RPC, SOAP, WSDL, and UDDI, the Web service protocol stack includes a whole zoo of newer, evolving protocols. These include WSFL (Web Services Flow Language), SOAP-DSIG (SOAP Security Extensions: Digital Signature), and USML (UDDI Search Markup Language). For an overview of these protocols, check out Pavel Kulchenko's article, Web Services Acronyms, Demystified, on XML.com.
Fortunately, you do not need to understand the full protocol stack to get started with Web services. Assuming you already know the basics of HTTP, it is best to start at the XML Messaging layer and work your way up.

What is XML-RPC? 

XML-RPC is a protocol that uses XML messages to perform Remote Procedure Calls. Requests are encoded in XML and sent via HTTP POST; XML responses are embedded in the body of the HTTP response.
More succinctly, XML-RPC = HTTP + XML + Remote Procedure Calls.
Because XML-RPC is platform independent, diverse applications can communicate with one another. For example, a Java client can speak XML-RPC to a Perl server.
To get a quick sense of XML-RPC, here is a sample XML-RPC request to a weather service (with the HTTP Headers omitted):
<?xml version="1.0" encoding="ISO-8859-1"?>
<methodCall>
<methodName>weather.getWeather</methodName>
<params>
<param><value>10016</value></param>
</params>
</methodCall>
The request consists of a simple element, which specifies the method name (getWeather) and any method parameters (zip code).

Here is a sample XML-RPC response from the weather service:

<?xml version="1.0" encoding="ISO-8859-1"?>
<methodResponse>
<params>
<param>
<value><int>65</int></value>
</param>
</params>
</methodResponse>
The response consists of a single element, which specifies the return value (the current temperature). In this case, the return value is specified as an integer.
In many ways, XML-RPC is much simpler than SOAP, and therefore represents the easiest way to get started with Web services.
The official XML-RPC specification is available at XML-RPC.com. Dozens of XML-RPC implementations are available in Perl, Python, Java, and Ruby. See the XML-RPC home page for a complete list of implementations.

1. What are Windows services?

Windows services, previously known as NT services, are applications that are installed on the system as system services. In other words, Windows services are applications that run in the background with the Windows operating system. The primary use of Windows services is to reduce the consumption of memory required for performing backend operations. Let's take an example to understand this easily. Suppose you want to perform a variety of functions, such as monitor the performance of your computer or application, check the status of an application, and manage various devices, such as printers.

In such a case, you can use Windows services to reduce memory consumption. In addition, Windows services can run on your system even if you have not logged on to your computer. In addition, these services do not have any user interface.

2. Can you share a process between Windows services?

Yes, you can share a process between Windows services.

3. In .NET, which is the parent class to create all Windows services?

The ServiceBase class is the parent class to create all Windows services.

4. Which class in .NET is used to install a Windows service?

The ServiceInstaller class, also known as the project installer class, is used to install a Windows service.

5. While installing a Windows service, an EventLogInstaller class is automatically created to install the event log related to the particular service. Is it true?

Yes, it is true.

6. Which property of the ServiceBase class can be used to specify whether a service can be paused and resumed?

The CanPauseAndContinue property provides such type of service.

7. Describe the services that UDDI provides to Web applications.

UDDI provides the following types of services to a Web application:

• XML Schema for business descriptions - Includes information about the service publisher (contact name, address, and so on) and specifications on the Web service
• Web registry of Web services - Includes business, service, and binding information for the Web service

8. Write the file extension for a Web service.

A Web service file extension is .asm file. For example, service1.asmx is a Web service file.

9. Which method is used to uninstall the Windows services?

The Uninstall() method is used to uninstall the Windows services.

10. What is the use of the mustUnderstand attribute in the Header element of a SOAP message?

The mustUnderstand attribute indicates that a header entry is either required or optional for the recipient to process further.

11. Explain the WSDL.

WSDL is a short form for Web Services Description Language, which is used to describe a Web service in terms of the messages that it creates and accepts. The WSDL document is an XML file that contains the interface schema for the Web service. It identifies the methods that are used during the exchange between a Web service consumer and a Web service provider. The following are the elements contained in the WSDL document:

• Types - Describe the variations of data types that are used to exchange messages between the user and the provider.
• Message - Describes the actual message or method call.
• portType - Describes the set of operations and each related message.
• binding - Describes the protocol details.
• service - Used to make groups a set of related ports together.

12. What advantage UDDI has over DISCO?

The UDDI directory has an advantage over a DISCO file, as it provides a single location where a client can find the Web services offered by different organizations.

13. How can you ensure that only authorized users access your Web service?

You should use the <authorization> element to ensure that only authorized users access your Web service. This element allows or denies access to your Web service according to their role.

14. Describe the EventLog class.

The EventLog class is used to access the Windows event logs from Windows services. Using EventLog, you can also customize Windows event logs that record information about important software and hardware events, such as the events of the .NET controls, keyboard, or other hardware devices.

The EventLog class allows you to read or write to event logs, delete logs, and create as well as delete event sources. You can use the EventLog class to create event logs while creating an event source. An event source can be used to write to only one event log at a particular time. However, it is possible to associate one event log to multiple sources.

15. How can you prevent your Web services from unauthorized access?

The following are the ways to prevent your Web service from unauthorized access:

• Using encryption and message-based security.
• Using authentication and access controls for the Web service.

16. Explain the concept of Web services in brief.

A Web service may be defined as an independent and self-sustained unit of a software application that is hosted on the Web and implement specific functionalities to execute the business logic. A Web service provides so many functionalities, such as generating pay slips for employees, computing tax, broadcasting weather report, and providing updated news. The Web service allows application to share information or exchange data with other applications across different operating systems and hardware.

Therefore, the work of a Web service is to unite software by exchanging data irrespective of their operating systems, supported hardware, and programming language used in their development. The Web services transfer data in the XML format and use Simple Object Access Protocol (SOAP) to communicate. It is an XML based protocol. The Web services use Web Services Description Language (WSDL) and Universal Description, Discovery, and Integration (UDDI) to describe itself.

17. What advantages have Web services over Component Object Model (COM) and Distributed Component Object Model (DCOM)?

The advantages of Web services over COM and DCOM are as follows:

• Web services are simple to use and can be implemented on varied platforms.
• Web services are loosely coupled; as a result, their interfaces and methods can be extended.
• Web services do not carry any state information with them so that multiple requests can be processed simultaneously.

18. Mention the namespace that you must import in code to build a Web service.

System.Web.Services is the elementary namespace, which must be imported to develop code of a Web service.

19. What does the portType element of a WSDL document contain?

The portType element contains the operations exposed by the Web service, and the messages involved in the communication between the Web service and its consumers.

20. What is DISCO?

DISCO is a technology developed by Microsoft to publish and discover Web services. It discovers URLs of all XML Web services located on a Web server and creates a list of these Web services in a file called as a DISCO file.

21. Which two methods are used to discover the URL of Web services?

The two methods to discover the URL of Web services are Web service discovery tool (Disco.exe) and UDDI.

22. Which step is necessary to perform before a Web service can be consumed?

It is necessary to build a proxy class by using the wsdl.exe utility before a Web service can be consumed.

23. Which property of the WebMethod attribute allows you to maintain the state of objects across sessions in a Web method?

The WebMethod attribute's EnableSession property enables you to enable session state for a Web method.

24. Write the names of public properties defined in the WebService class.

There are many properties defined in the WebServices class:

• Application - Obtains the application object for the current HTTP request
• Context - Obtains the HttpContext object for the current request, which encapsulates all HTTP-specific context used by the HTTP server to process Web requests
• Server - Obtains the HttpServerUtility object for the current request
• Session - Obtains the HttpSessionState object for the current request
• SoapVersion - Obtains the version of the SOAP protocol used to make the SOAP request to a Web service
• User - Obtains the Server User Object. This property can be used to authenticate whether a user is authorized to execute the request.

25. What do you understand by SOAP encoding?

The Serialization of the types, such as integers and strings, inside a SOAP message is called encoding. The SOAP objects use XML elements and attributes to serialized data, for example, encodingStyle is an attribute of theEnvelop element, which is used to specify the encoding rules for a SOAP object.

26. What is the use of a .disco file?

A client application uses a .disco file to locate or discover the documents that contain the description of a Web service. The .disco file contains links to other resources, which describe essential features, such as capabilities of a Web service. The links contained in a .disco file can refer to other discovery documents or XSD schemas. The description about the services and capabilities of a Web service is written in Web services Description Language (WSDL). A .disco file can also contain the information about other XML Web services that reside on the same or a different Web server.

27. Mention the name of the directory where it is necessary to locate the proxy file to use a Web service.

The proxy file must be stored in the /bin directory. This directory is situated under the root directory of the application.

28. Does a Web service have state?

The Web services do not have any technique to maintain state. However, it can access ASP.NET objects, such as application and session if they extend from the WebService base class.

29. Which namespace must be included in a code that enables a XML Web service to write events in an event log file?

The System.Diagnostics is the namespace, which must be included in a code to enable a Web service for writing events in an event log file.

30. Which tool installs the DLL on your local computer and installs the Windows service in a transactional manner?

The Installutil.exe tool.