Windows Communication Foundation(WCF) FAQ: Part I

Introduction

In this section we will run through a quick FAQ for WCF. I am sure after reading this you will get a good understanding of the fundamentals of WCF.

Enjoy… the article

What is .NET 3.0?

In one simple equation .NET 3.0 = .NET 2.0 + Windows Communication Foundation + Windows Presentation Foundation + Windows Workflow Foundation + Windows Card Space.

What is Windows Card Space?

It was previously known by its codename Info Card. It is a framework by Microsoft, which securely stores digital identities of a user and provides a unified interface to choose the identity for a particular transaction, such as logging in to a website. Windows Card Space is a central part of Microsoft's effort to create an identity met system, or a unified, secure and interoperable identity layer for the internet.

What is WCF?

First let us give a short answer to this: - "WCF (Indigo was the code name for WCF) is a unification of .NET framework communication technologies ".WCF is a unification technology, which unites the following technologies:-

• NET remoting
• MSMQ
• Web services
• COM+.

Below figure depicts WCF fundamentals pictorially.

Figure: 1 - WCF Components

What are the important principles of SOA (Service oriented Architecture)?

WCF is based on SOA. All big companies are playing big bets on SOA. So how can Microsoft remain behind? So in order to implement SOA architecture easily you need to use WCF.

SOA is based on four important concepts:-

• Boundaries are well defined
  In SOA, everything is formalized. The client who is consuming the service does not need to know how the implementation of the service is done. If you look at some old methodologies of communication like DCOM. Any changes at server level the client also has to change. Therefore, the server and client implementation was so much bound that changes need to be done at all places. In SOA, the rule is if you do enhancement you do not need to change anything at the client. SOA based application only understands that there is an end point, contract, and bindings.
  
  Note: - Just to clarify shortly about end point and contract. Any SOA service is exposed through an end point. End point defines three important aspects What, Where and How. We will understand more details of the same in the later questions.
  
  
• Services evolve
  Change is the law of nature and services will evolve. In SOA, services can be versioned and you can host those services in new ends. For instance, you have a service called as "Search Tickets (Ticket Number) "which gives details based on Ticket Number and its exposed on end point "ep1". Tomorrow you want make your Search Tickets service more useful by also providing an extra option of allowing him to search by passenger name. Therefore, you just declare a new end "ep2" with service "Search Tickets (Ticket Number, Passenger Name)". So the client who is consuming the service at end ep1 continues and at the other end, we have evolved our service by adding new ends ep2.
  
  
• Services share only schemas and contracts
  Services use Schemas to represent data and contracts to understand behavior. They do not use language dependent types or classes in order to understand data and behavior. XML is used to define schemas and contracts. Due to this, there is not heavy coupling between environments.
  
  
• Service compatibility is policy based
  Policy describes the capabilities of the system. Depending on policies, the service can degrade to match the service for the client. For instance your service needs to be hosted for two types of client one which uses Remoting as the communication methodology while other client uses DCOM. An ideal SOA service can cater to both of them according to there communication policies.  

Note: - Many people assume Web services are the base for SOA. The answer is 50 % right. What web services lack is the policy based Service compatibility. If you host a web service it can only serve with HTTP communication channel and SOAP message. Any other type of client trying to communicate he will not degrade it self. This is what is provided by WCF. You can host the service in one or more mode. For instance you can host a WCF service using remoting and ASMX.

What are ends, contract, address, and bindings?

The above terminologies are the core on which SOA stands. Every service must expose one or more ends by which the service can be available to the client. End consists of three important things where, what and how:-

• Contract (What)
  Contract is an agreement between two or more parties. It defines the protocol how client should communicate with your service. Technically, it describes parameters and return values for a method.
  
  
• Address (Where)
  An Address indicates where we can find this service. Address is a URL, which points to the location of the service.
  
  
• Binding (How)
  Bindings determine how this end can be accessed. It determines how communications is done. For instance, you expose your service, which can be accessed using SOAP over HTTP or BINARY over TCP. So for each of these communications medium two bindings will be created.

Below figure, show the three main components of end. You can see the stock ticker is the service class, which has an end hosted on www.soa.com with HTTP and TCP binding support and using Stock Ticker interface type.

Figure 2: - Endpoint Architecture

Note: - You can also remember the end point by ABC where A stands for Address, B for bindings and C for Contract.

Which specifications does WCF follow?

WCF supports specifications defined by WS-* specifications. WS-* specifications are defined together by Microsoft, IBM, SUN and many other big companies so that they can expose there service through a common protocol. WCF supports all specifications defined we will understand them one by one.

• Messaging (WS-Addressing):- SOAP is the fundamental protocol for web services. WS Addressing defines some extra additions to SOAP headers, which makes SOAP free from underlying transport protocol. One of the good things about Message transmission is MTOM, also termed as Message Transmission Optimization Mechanism. They optimize transmission format for SOAP messages in XML-Binary formant using XML optimized packaging (XOP). Because the data will sent in binary and optimized format, it will give us huge performance gain.
• Security (WS-Security, WS-Trust, and WS-Secure Conversation):- All the three WS- define authentication, security, data integrity and
  privacy features for a service.
• Reliability (WS-Reliable Messaging):- This specification ensures end-to-end communication when we want SOAP messages to be traversed back and forth many times.
• Transactions (WS-Coordination and WS-Atomic Transaction):- These two specifications enable transaction with SOAP messages.
• Metadata (WS-Policy and WS-Metadata exchange):- WSDL is a implementation of WS-Metadata Exchange protocol. WS-Policy defines more dynamic features of a service, which cannot be expressed by WSDL.

We have stressed on the WS-* specification as it is a specification which a service has to follow to be compatible with other languages. Because WCF follows WS-* specifications other languages like JAVA , C++ can also exploit features like Messaging , Security , Reliability and transactions written in C# or VB.NET. This is the biggest achievement of WCF to integrate the above features with other languages. 

Note: - During interview the interviewer expects that you know what WS-* specification are supported by WCF and its advantages with respect to interacting with other languages.

What are the main components of WCF?

We need to define three main components in WCF:-

• Service class.
• Hosting environment
• End point 

Explain how Ends, Contract, Address, and Bindings are done in WCF?

what is a service class?

what is a service contract, operation contract and Data Contract?

In this example, we will make simple service, which displays the total cost of the complete product group. In simple words, this service will take three parameters per product cost, number of products and the product name. In return the service will return the total cost of all the products by multiplying number of products * cost per product. As we go ahead in this explanation, we will try to understand all the terminologies, which are asked in the above question.

First, you need to create a Winfx service project. You can see in the below figure we have selected the Winfx project.

Figure 3: - Create new WinFX Service class

In this project, we add a new class and name it as "serviceGetCost.cs". This class will have our core implementation and this is the class, which has all the action. The service class, which has to be exposed to the external client. We need to use the Service Contract attribute to mark it as a service class.

Service Contract attribute define saying which application interface will be exposed as a service.

You can see in the below code snippet we have made an interface and marked it as Service Contract. It is not essential that you need to use an interface you can also use a simple class and mark it as Service but interface represent a contract and do not have implementation. In short, they stand at a very higher level of abstraction. So as a good design practice-using interface to represent a service contract makes more sense.

The next thing to note is the Operation Contract attribute.

Operation Contract dictates which methods should be exposed to the external client using this service.

It defines individual exchange or request and replies. In the current sample, we have defined GetTotalCost method, which will be used by the end client to get the total cost results.

The next thing to note in the code snippet is the Data Contract attribute. In the previous two steps, we have exposed class as a service by using Service Contract and methods by using Operation Contract. Every operation will definitely do some kind of data transfer.

Data Contract attributes defines which type of complex data will be exchanged between the client and the service. They determine which parameters to be serialized.

When you are using simple data types like int, bolo etc it is not necessary that you need to mark the data contract attribute. Because you will always find matching types on the client. However, complex structure like one shown in the below code snippet you will need to define a data contract. Remember data contract define how this data will be passed during transmission. In short data contract attribute define how data will be serialized will transmission.

In the below sample we have marked the structure product data to be serialized.

Figure 4:- The Service class

As data contract are all about serialization you need to import System.Runtime.Serialization name space.

In the next step, we implement the GetTotalCost function. It just returns a simple string with product name and the total cost of all products.

Once our service class is done its time to host this service. There are various ways of hosting a WCF service we will look in to the same in the next question. For the current example, we will host in their own process. 

Figure 5: - Hosting the service

Hosting the WCF service needs two things one is the config file and second is the hosting code on startup. Because we are hosting this service in its own application process this needs to be a windows application. So first let us have a look what entries do, we need to make in the App.config file. In the above figure, everything is clear but let us understands all the section defined in the App.config file.

In the configuration section, we need to add a new section <system.serviceModel>. The most important part of <system.serviceModel> is the endpoint tag. As said in the previous answer End gives three important answers Where, What and How. In short where is the service, what the contract of the service is and how do we communicate with the service.

In the above code snippet, we have only defined the contract i.e. what and how that is bindings. The where is defined in the application entry point static void main ().

Therefore, the contract attribute defines the interface and binding says that the end clients can communicate using "HTTP" protocol.

In Static void Main method, we create an object of Service Host class and use the open method to host the service. We have used the URI object to define the address where the service will be hosted.

Figure 6: - Service Started

If you compile the project, you will see something as shown in the above figure. This says that the service is up and running and ready to serve any WCF client. Now its time to develop consumer, which will consume this WCF service. Microsoft has provided a decent automation to generate the client. Therefore, below figure depicts the various steps.

Figure 7: - svcutil in action

Go to command prompt of windows SDK and run the following command:-

Svcutil <Service hosted URI>

In the above command <Service Hosted URI> is the URI on which the service is hosted. One you run the command against the URI it will generate two files one is the config file and the other is the proxy. You can see in the above figure two files are generated serviceGetCost.cs and output.config file. With the help of these two files, we will make our client.

Figure 8: - Client code walkthrough

You can see in the above figure we have made WFCClientGetCost project. In that, we have added output.config and serviceGetCost.cs to the client project. We have renamed output.config to app.config.

Once we have done with everything, its time to write the client code, which calls the proxy who in turn will call the service hosted. In the above figure, you can see we have the client code also. It is a simple code we first created the object of the data structure set the values. Then we create the object of the service and call the GetTotalCost function.

If everything is compiled and you run the server and client, you should get your output as shown below.

Figure 9: - Output of WCF service

what are the various ways of hosting a WCF service?

There are three major ways to host a WCF service:-

• Self-hosting the service in his own application domain. This we have already covered in the first section. The service comes in to existence when you create the object of Service Host class and the service closes when you call the Close of the Service Host class.
• Host in application domain or process provided by IIS Server.
• Host in Application domain and process provided by WAS (Windows Activation Service) Server. 

How do we host a WCF service in IIS?

Note: - The best to know how to host a WCF in IIS is by doing a small sample. So what we will do is host the same GetCost sample which was self hosted in the previous question.

First thing you will need is to create the SVC file, which exposes the service class. SVC file contains the pointer to the class. You can see from the figure below the class attribute points to the class whose interface is exposed by the service.svc.cs file. Also, note the actual interface is in service.svc.cs file. Below figure, have both the files service.svc, which has the class attribute which points to the service class, and the interface, which resides in service.svc.cs file. We have taken the same sample, which was self-hosted in the previous question.

Figure 10: - The SVC file and the behind code

We also need to provide implementation for the interface. So we have made a class ServiceGetCost which has the actual implementation. Below figure shows the same in detail. In the below figure you can also see the solution files.

Figure 11: - Implementation of Service.svc.cs

We also need to specify the service type and endpoint in web.config file. Also, note we have specified HTTP binding because we will be hosting the service on IIS.

Figure 12: - Web.config file for hosting service on IIS

Now that we are done with the coding part. We need to create the virtual directory in IIS. In the below figure in Step1 and Step2 we have shown how to create the virtual directory in IIS. One important thing to note while creating virtual directory set the access permission to execute.

Figure 13:- IIS Configuration

In the third step, we will publish the website to our virtual directory. Note the fourth step in which we have copied the svc file so that the service can be requested.

Note: - ASP.NET compilation has changed in ASP.NET 2.0. In 2.0 there is no concept of solution files. So if you want to have full compiled DLL you need to publish the project to a virtual directory.

Once you have hosted the SVC file you can test the same by request the service.svc file. If everything works fine you will get something as shown in the below figure

Figure 14:- IIS WCF client

Using the Svcutil.exe, you will need to generate the proxy class and the config file. The proxy and config will be same, as we had done for self-hosting. The one important change is the address. The config file URL now points to the service.svc, which is hosted on IIS. You can run the same client, which we had created for self-hosting. The only change you will need to do is change the endpoint address.

Figure 15:- Output of WCF client at IIS

LOL…You should get the same output, which we had received, for self-hosting.

what are the advantages of hosting WCF Services in IIS as compared to self-hosting?

There are two main advantages of using IIS over self-hosting:-

Automatic activation

IIS provides automatic activation that means the service is not necessary to be running in advance. When any message is received by the service it then launches and fulfills the request. But in case of self hosting the service should always be running.

Process recycling

If IIS finds that a service is not healthy that means if it has memory leaks etc, IIS recycles the process. Ok let us try to understand what is recycling in IIS process. For every browser instance, a worker process is spawned and the request is serviced. When the browser disconnects the worker, process stops and you loose all information. IIS also restarts the worker process. By default, the worker process is recycled at around 120 minutes. So why does IIS recycle. By restarting the worker process it ensures any bad code or memory leak do not cause issue to the whole system.

In case of self-hosting both the above features, you will need to code yourself. Lot of work right!!. That is why IIS is the best option for hosting services until you are really doing something custom.

Below figure shows where the recycle option is located in IIS. You need to click on the DefaultAppool and then Properties.

Figure 16:- IIS recycle option

what are the major differences between services and Web services?

What is the difference WCF and Web services?

Web services can only be invoked by HTTP. While Service or a WCF component can be invoked by any protocol and any transport type. Second web services are not flexible. However, Services are flexible. If you make a new version of the service then you need to just expose a new end. Therefore, services are agile and which is a very practical approach looking at the current business trends.

What are different bindings supported by WCF?

WCF includes predefined bindings. They cover most of bindings widely needed in day-to-day application. However, just incase you find that you need to define something custom WCF does not stop you. So let us try to understand what each binding provides.

BasicHttpBinding: - This binding is used when we need to use SOAP over HTTP. This binding can also be configured to be used as HTTPS. It can be also configured to send data in plain text or in optimized form like MTOM.

Note: - MTOM is discussed in one of the pervious questions in this chapter.

WsHttpBinding: - It is same like BasicHttpBinding. In short, it uses SOAP over HTTP. But with it also supports reliable message transfer, security and transaction. WS-Reliable Messaging, security with WS-Security, and transactions with WS-Atomic Transaction supports reliable message.

NetTcpBinding: - This binding sends binary-encoded SOAP, including support for reliable message transfer, security, and transactions, directly over TCP. The biggest disadvantage of NetTcpBinding is that both server and client should be also made in .NET language.

NetNamedPipesBinding:-Ths binding Sends binary-encoded SOAP over named pipes. This binding is only usable for WCF-to-WCF communication between processes on the same Windows-based machine.

Note: - An interprocess control (IPC) protocol is used for exchanging information between two applications, possibly running on different computers in a network. The difference between Named pipes and TCP is that named pipes have good performance in terms of communication with in processes. But when it comes to communicate across network TCP holds the best choice. So if you are using WCF to communicate with process it's the best choice to use in terms for performance. Named pipes do not perform when the traffic is heavy as compared to TCPIP.

NetMsmqBinding: - This binding sends binary-encoded SOAP over MSMQ. This binding can only be used for WCF-to-WCF communication.

Which are the various programming approaches for WCF?

What is one-way operation?
 
IsOneWay equal to true ensures that the client does not have to wait for the response. So methods marked by IsOneWay to true should always return void. In this, the caller does not get anything in return so it is called as one-way communication.

In order to understand one-way implementation in WCF lets make a code walkthrough of a sample.

Note: - You can find code for the same in "WCFIsOneWay" folder in CD.

Figure 17: - One-Way in action

Above is the code snippet, which describes practically how one way works in WCF. The above given code snippet is numbered. Below is the explanation according to the numbers marked in figure:-

1 - This is the code snippet of the server service. We have created a method called as doHugeTask. DoHugeTask makes the method sleep for 5000 MS and then displays the time when the task is completed.

2 - This code snippet is for client. It creates a proxy object of serviceIsOneWay and calls the doHugeTask method. After calling the doHugeTask, the client execution continues ahead. So as a proof, we display the time when the method calling was completed.

3 - This screen shot shows the output given by both server and client. The top window displays the server output and the below windows displays the client output. 

Note: - You can find the code for the same in WCFIsOneWay folder. For generating the proxies you have to follow the same steps which are shown in the previous steps.

So run the server program first i.e. ServiceIsOneWay and run the client later. You will see the client runs the doHugeTask and moves ahead. Therefore, the client completion time is less than the server is. One more thing to understand is that one way does not give any notification back of completion. Therefore, it is like fire and forgets.

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