Answer: -
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.
WCF Components |
In one simple equation .NET 3.0 = .NET 2.0 + Windows Communication Foundation + Windows Presentation Foundation + Windows Workflow Foundation + Windows Card Space.
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Answer: -
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.
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Answer: - 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.
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Answer: - 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.
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Answer: - 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. |
Answer: -
We need to define three main components in WCF:-
• Service class.
• Hosting environment
• End point
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Answer: -
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.
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Answer: -
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.
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Answer: - 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.
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.
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Answers:- There are scenarios in the project when you want the message to deliver in proper time. The timely delivery of message is more important than losing message. In these scenarios, Volatile queues are used.
Below is the code snippet, which shows how to configure Volatile queues. You can see the binding Configuration property set to Volatile Binding. This code will assure that message will deliver on time but there is a possibility that you can lose data.
<appSettings>
<!-- use appSetting to configure MSMQ queue name -->
<add key="queueName" value=".\private$\ServiceModelSamplesVolatile" />
</appSettings>
<system.serviceModel>
<services>
<service name="Samples.StockTickerServ"
behaviorConfiguration="CalculatorServiceBehavior">
...
<!-- Define NetMsmqEndpoint -->
<endpoint address="net.msmq://localhost/private/ServiceModelSamplesVolatile"
binding="netMsmqBinding"
bindingConfiguration="volatileBinding"
contract="Samples.InterfaceStockTicker" />
...
</service>
</services>
<bindings>
<netMsmqBinding>
<binding name="volatileBinding"
durable="false"
exactlyOnce="false"/>
</netMsmqBinding>
</bindings>
...
</system.serviceModel>
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The main use of queue is that you do not need the client and the server running at one time. Therefore, it is possible that a message will lie in queue for long time until the server or client picks it up. But there are scenarios where a message is of no use after a certain time. Therefore, these kinds of messages if not delivered within that time span it should not be sent to the user.
Below is the config snippet, which defines for how much time the message should be in queue.
<bindings>
<netMsmqBinding>
<binding name="MyBindings"
deadLetterQueue="Custom"
customDeadLetterQueue="net.msmq://localhost/private/ServiceModelSamples"
timeToLive="00:00:02"/>
</netMsmqBinding>
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In the previous code we have use ‘TransactionFlowOption’. We can specify transaction in 3 ways in WCF:-
TransactionFlowOption.NotAllowed
This is a default option. Using this option no transaction will be propagated across the binding. If any client attempts to call the WCF service in a transaction it will be ignored for this option.
TransactionFlowOption.Allowed
This option specifies that client can call this WCF service in a transaction. It’s not compulsory that the service needs to be called in a transaction. You can call without the transaction also.
TransactionFlowOption.Mandatory
This option specifies that client must call the WCF service in a transaction mode. If the WCF service is called without transaction, ‘FaultException’ will be raised.
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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
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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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