(B)What is an application domain?
Previously “PROCESS” where used as security boundaries. One process has its own
virtual memory and does not over lap the other process virtual memory; due to this one
process can not crash the other process. So any problem or error in one process does not
affect the other process. In .NET they went one step ahead introducing application domains.
In application domains multiple applications can run in same process with out influencing
each other. If one of the application domains throws error it does not affect the other
application domains. To invoke method in a object running in different application domain
.NET remoting is used.
Figure :- 4.1 One process can have multiple Application domains
(B) What is .NET Remoting ?
.NET remoting is replacement of DCOM. Using .NET remoting you can make remote
object calls which lie in different Application Domains. As the remote objects run in
different process client calling the remote object can not call it directly. So the client uses
a proxy which looks like a real object.
When client wants to make method call on the remote object it uses proxy for it. These
method calls are called as “Messages”. Messages are serialized using “formatter” class
and sent to client “channel”. Client Channel communicates with Server Channel. Server
Channel uses as formatter to deserialize the message and sends to the remote object.
4. Remoting and Webservices
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Figure :- 4.2 Channels, Formatters and Proxy in action.
(B) Which class does the remote object has to inherit ?
All remote objects should inherit from System.MarshalbyRefObject.
(I) What are two different types of remote object creation mode in .NET ?
There are two different ways in which object can be created using Remoting :-
√ SAO (Server Activated Objects) also called as Well-Known call mode.
√ CAO (Client Activated Objects)
SAO has two modes “Single Call” and “Singleton”. With Single Call object the object is
created with every method call thus making the object stateless. With Singleton the object
is created only once and the object is shared with all clients.
CAO are stateful as compared to SAO. In CAO the creation request is sent from client
side. Client holds a proxy to the server object created on server.
(A) Describe in detail Basic of SAO architecture of Remoting?
For these types of questions interviewer expects small and sweet answers. He is basically
looking at what you know about the specific subject. For these type of question this book
will provide detail code which is not necessary to be said during interview. Only the basic
steps and overall brief are enough to convince that you have knowledge about the subject.
Even though this question has detail code and answer say only what is needed in interview.
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Remoting has at least three sections :-
√ Common Interface which will be shared between them.
√ Server.
√ Client.
Figure :- 4.3 Solution Explorer of Remoting Project
In CD “RemotingSample(SAO)” project is provided which gives a insight of remoting.
Above is the figure which shows the three important project sections needed to implement
remoting.
First important section is the common interface between Server and
Client.”InterFaceRemoting” project has the interface code. For sample project interface
is very simple with only two methods :- SetValue and GetValue.
Public Interface InterFaceRemoting
Sub SetValue(ByVal value As String)
Function GetValue() As String
End Interface
Second important section is the server.In this sample server is using HTTP channel and
the server object is singleton.
Imports System
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels.Http
Imports System.Runtime.Remoting.Channels
Imports InterFaceRemoting
Public Class RemotingServer
Inherits MarshalByRefObject
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Implements InterFaceRemoting.InterFaceRemoting
Private strData As String
Public Function GetValue() As String Implements
InterFaceRemoting.InterFaceRemoting.GetValue
Return strData
End Function
Sub New()
strData = “testing..”
End Sub
Public Sub SetValue(ByVal value As String) Implements
InterFaceRemoting.InterFaceRemoting.SetValue
strData = value
End Sub
End Class
Module ModuleRemotingStartUp
Sub Main()
Dim objHttpChannel As HttpChannel
Console.WriteLine(“Server Started....”)
objHttpChannel = New HttpChannel(1234)
ChannelServices.RegisterChannel(objHttpChannel)
RemotingConfiguration.RegisterWellKnownServiceType(GetType(RemotingServer),
“RemoteObject”, WellKnownObjectMode.Singleton)
Console.WriteLine(“Server registered and listening waiting
for clients...”)
Console.ReadLine()
End Sub
End Module
Following is detail explanation :-
√ Channel object is created and registered.
Following is the code.
Dim objHttpChannel As HttpChannel
Console.WriteLine(“Server Started....”)
objHttpChannel = New HttpChannel(1234)
ChannelServices.RegisterChannel(objHttpChannel)
√ Server then hosts the object so that client can connect to it. This is the time
when we specify what mode the server object will be created i.e. Singleton or
SingleCall. This is done by the following below given code. Note in sample we
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are hosting the server object in singleton mode that means that the same object
will be shared between all clients. Also note the server object is implementing
“InterFaceRemoting” and inheriting from “MarshalByRefObject”.
RemotingConfiguration.RegisterWellKnownServiceType(GetType(RemotingServer),
“RemoteObject”, WellKnownObjectMode.Singleton)
Now comes the final section that is third section the client which will connect to this
hosted remoting object.
Following is a detail explanation of client code :-
√ First we create the channel i.e. HTTP. Note whatever channel the server is
using same will be used by the client.
ChannelServices.RegisterChannel(objHttpChannel)
√ As said before the common interface i.e.“InterFaceRemoting” will be used
to communicate with client.
√ After that we can get the server object reference using following code
objRemoting = CType(Activator.GetObject(GetType(InterFaceRemoting.InterFaceRemoting),
“http://localhost:1234/RemoteObject”), InterFaceRemoting.InterFaceRemoting)
√ Then the client can make method call as if the object is local. But actually the
object is a proxy.
Console.WriteLine(“Value on server :- “ & objRemoting.GetValue.ToString())
Imports System
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Channels.Http
Imports System.Runtime.Remoting.Channels
Imports InterFaceRemoting
Module ModuleStartClient
Sub Main()
Dim objHttpChannel As New HttpChannel
Dim objRemoting As InterFaceRemoting.InterFaceRemoting
ChannelServices.RegisterChannel(objHttpChannel)
objRemoting =
CType(Activator.GetObject(GetType(InterFaceRemoting.InterFaceRemoting),
“http://localhost:1234/RemoteObject”),
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InterFaceRemoting.InterFaceRemoting)
Console.WriteLine(“Referenced the main object.... Now
displaying Data”)
Console.WriteLine(“Value on server :- “ &
objRemoting.GetValue.ToString())
Console.WriteLine(“Press enter to Terminate”)
Console.ReadLine()
End Sub
End Module
You an run the program and see the output. For running the program run the server
program which is in server directory. Run “Server.exe” from BIN directory. If the EXE
runs properly following will be the screen as shown below.
Figure :- 4.4 Running Server Program of Remoting
Now run “Client.exe” from client folder in BIN directory.Following will be the output
seen.This means that the client connected to the server program and displayed the data in
the server object. In the server object we have initialized value “testing......”. In constructor
of class “RemotingServer” same value is displayed at the client side as shown in figure
below.
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Figure :- 4.5 Client Program output of Remoting
(A) What are the situations you will use singleton architecture in
remoting ?
If all remoting clients have to share the same data singleton architecture will be used.
(A) What is fundamental of published or precreated objects in Remoting
?
In scenarios of singleton or single call the objects are created dynamically. But in situations
where you want to precreate object and publish it you will use published object scenarios.
Dim obj as new objRemote
obj.Initvalue = 100
RemotingServices.Marshal(obj,”RemoteObject”)
As shown in above sample following changes will be needed on server side.
RemotingConfiguration.RegisterWellKnownServiceType is replaced by
RemotingServices.Marshal(obj,”RemoteObject”) where “obj” is the precreated objected
on the server whose value is initialized to 100.
(A) What are the ways in which client can create object on server in CAO
model ?
There are two ways by which you can create Client objects on remoting server :-
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√ Activator.CreateInstance().
√ By Keyword “New”.
(A) Are CAO stateful in nature ?
Yes. In CAO remoting model client creates a instance on server and instance variable set
by client on server can be retrieved again with correct value.
(A) In CAO model when we want client objects to be created by “NEW”
keyword is there any precautions to be taken ?
Remoting Clients and Remoting Server can communicate because they share a common
contract by implementing Shared Interface or Base Class (As seen in previous examples).
But according to OOP’s concept we can not create a object of interface or Base Classes
(Abstract Class). Shipping the server object to client is not a good design practice. In
CAO model we can use SOAPSUDS utility to generate Metadata DLL from server which
can be shipped to client, clients can then use this DLL for creating object on server. Run
the SOAPSUDS utility from visual studio command prompt for syntax see below :-
soapsuds -ia:RemotingServer -nowp -oa:ClientMetaData.dll
Where RemotingServer is your server class name.
ClientMetaData.dll is the DLL name by which you will want to create the metadll.
Server code will change as follows :-
ChannelServices.RegisterChannel(objHttpChannel)
RemotingConfiguration.ApplicationName = “RemoteObject”
RemotingConfiguration.RegisterActivatedServiceType(GetType(InterFaceRemoting.InterFaceRemoting))
Note :- We have to provide applicationname and register the object as ActivatedServiceType.
On client side we have to reference the generated ClientMetaData.dll from SOAPSUDS
utility. Below are changes which are needed to be incorporated at the Remoting Client :-
RemotingConfiguration.RegisterActivatedClientType(typeof(RemoteObject),“http://
localhost:1234/MyServer”)
Dim objRemoteObject as new RemoteObject().
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RemoteObject is class which is obtained from ClientMetaData.dll which we created using
SOAPSUDS utility. Now you can reference the object as normal object.
(I) Is it a good design practice to distribute the implementation to
Remoting Client ?
It’s never advisable to distribute complete implementation at client, due to following
reasons:-
√ Any one can use ILDASM and decrypt your logic.
√ It’s a bad architecture move to have full implementation as client side as any
changes in implementation on server side you have to redistribute it again.
So the best way is to have a interface or SOAPSUDS generated meta-data DLL at client
side rather than having full implementation.
(A) What are LeaseTime, SponsorshipTime, RenewonCallTime and
LeaseManagerPollTime?
This is a very important question from practical implementation point of view. Companies
who have specific requirement for Remoting projects will expect this question to be answered.
In normal .NET environment objects lifetime is managed by garbage collector. But in
remoting environment remote clients can access objects which are out of control of
garbage collector. Garbage collector boundary is limited to a single PC on which framework
is running; any remote client across physical PC is out of control of GC (Garbage
Collector).
This constraint of garbage collector leads to a new way of handling lifetime for remoting
objects, by using concept called as “LeaseTime”. Every server side object is assigned by
default a “LeaseTime” of five minutes. This leasetime is decreased at certain intervals.
Again for every method call a default of two minutes is assigned. When i say method call
means every call made from client. This is called as “RenewalOnCallTime”.
Let’s put the whole thing in equation to make the concept more clear.
Total Remoting object life time = LeaseTime + (Number of method calls) X
(RenewalTime).
If we take NumberOfMethodCalls as one.
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Then default Remote Object Life Time = 5 + (1) X 2 = 10 minutes (Everything is in
minutes)
When total object lifetime is reduced to zero, it queries the sponsor that should the object
be destroyed. Sponsor is an object which decides should object Lifetime be renewed. So
it queries any registered sponsors with the object, if does not find any then the object is
marked for garbage collection. After this garbage collection has whole control on the
object lifetime. If we do not foresee how long a object will be needed specify the
“SponsorShipTimeOut” value. SponsorShipTimeOut is time unit a call to a sponsor is
timed out.
“LeaseManagerPollTime” defines the time the sponsor has to return a lease time extension.
(A) Which config file has all the supported channels/protocol ?
Machine.config file has all the supported channels and formatter supported by .NET
remoting.Machine.config file can be found at
“C:\WINDOWS\Microsoft.NET\Framework\vXXXXX\CONFIG” path. Find
and the formatters. Below is a figure shown which can give a clear idea of how the file
looks like.
Note :- Interviewer will not ask you to name all channels and formatters in machine.config
but will definitely like to know in which file are all the formatter and channels specified, one
sweet answer “Machine.config” can fetch you handsome job.
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Figure :- 4.6 Channels and Formatter in machine.config file
(A) How can you specify remoting parameters using Config files ?
Both remoting server and remoting client parameters can be provided through config
files. Below is a sample of server config file which provides all remoting parameter values
which we where providing through code.
mode=”SingleCall”
type=”Server.ClsServer, Server”
objectUri=”RemoteObject” />
Later this config file can be loaded using the following code.
RemotingConfiguration.Configure(AppDomain.CurrentDomain.SetupInformation.ApplicationBase
& “Server.config”)
Same way we also have client.config file for loading the client remoting parameters.
url = “tcp://localhost:9000/Server/RemoteObject”/>
client remoting can then load the configuration file by using :-
Dim IobjCommon As CommonInterFace.Icommon
Dim StrData As String
Dim objServiceEntries As WellKnownClientTypeEntry()
RemotingConfiguration.Configure(AppDomain.CurrentDomain.SetupInformation.ApplicationBase
& “Client.config”)
objServiceEntries =
RemotingConfiguration.GetRegisteredWellKnownClientTypes()
IobjCommon = Activator.GetObject(GetType(Icommon),
objServiceEntries(0).ObjectUrl.ToString())
StrData = IobjCommon.GetValue()
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Console.WriteLine(“ Serve side Data is “ & StrData)
Console.ReadLine()
Note :- Complete source is provided in CD in folder “RemotingObjectLifeTime”.If you run
Server and Client following output can be seen. All source is compiled using VS2005
BETA1
Figure : - 4.7 Output of Server and Client for RemotingObjectLifeTime project
(A) Can Non-Default constructors be used with Single Call SAO?
Twist :- What are the limitation of constructors for Single call SAO ?
163
Non-Default constructors can not be used with single call objects as object is created
with every method call, there is no way to define Non-default constructors in method
calls.
It’s possible to use Non-Default constructor with Client activated objects as both methods
:-
“NEW” keyword and “Activator.CreateInstance” provide a way to specify Non-Default
constructors.
(I) How can we call methods in remoting Asynchronously ?
All previous examples are a synchronous method calls that means client has to wait until
the method completes the process. By using Delegates we can make Asynchronous method
calls.
(A) What is Asynchronous One-Way Calls ?
One-way calls are a different from asynchronous calls from execution angle that the .NET
Framework does not guarantee their execution. In addition, the methods used in this kind
of call cannot have return values or out parameters. One-way calls are defined by using
[OneWay()] attribute in class.
(B) What is marshalling and what are different kinds of marshalling ?
Marshaling is used when an object is converted so that it can be sent across the network
or across application domains. Unmarshaling creates an object from the marshaled data.
There are two ways to do marshalling :-
√ Marshal-by-value (MBV) :- In this the object is serialized into the channel, and
a copy of the object is created on the other side of the network. The object to
marshal is stored into a stream, and the stream is used to build a copy of the
object on the other side with the unmarshalling sequence.
√ Marshaling-by-reference (MBR):- Here it creates a proxy on the client that is
used to communicate with the remote object. The marshaling sequence of a
remote object creates an ObjRef instance that itself can be serialized across
the network.
Objects that are derived from “MarshalByRefObject” are always marshaled by reference.
All our previous samples have classes inherited from “MarshalByRefObject”
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To marshal a remote object the static method RemotingServices.Marshal() is
used.RemotingServices.Marshal() has following overloaded versions:-
public static ObjRef Marshal(MarshalByRefObject obj)
public static ObjRef Marshal(MarshalByRefObject obj, string objUri)
public static ObjRef Marshal(MarshalByRefObject obj, string objUri,Type
requestedType)
The first argument obj specifies the object to marshal. The objUri is the path that is
stored within the marshaled object reference; it can be used to access the remote object.
The requestedType can be used to pass a different type of the object to the object reference.
This is useful if the client using the remote object shouldn't use the object class but an
interface that the remote object class implements instead. In this scenario the interface is
the requestedType that should be used for marshaling.
(A) What is ObjRef object in remoting ?
All Marshal() methods return ObjRef object.The ObjRef is serializable because it
implements the interface ISerializable, and can be marshaled by value. The ObjRef knows
about :-
√ location of the remote object
√ host name
√ port number
√ object name.
(B) What is a Web Service ?
Web Services are business logic components which provide functionality via the Internet
using standard protocols such as HTTP.
Web Services uses Simple Object Access Protocol (SOAP) in order to expose the business
functionality.SOAP defines a standardized format in XML which can be exchanged
between two entities over standard protocols such as HTTP. SOAP is platform independent
so the consumer of a Web Service is therefore completely shielded from any
implementation details about the platform exposing the Web Service. For the consumer it
is simply a black box of send and receive XML over HTTP. So any web service hosted on
windows can also be consumed by UNIX and LINUX platform.
165
(B) What is UDDI ?
Full form of UDDI is Universal Description, Discovery and Integration. It is a directory
that can be used to publish and discover public Web Services. If you want to see more
details you can visit the http://www.UDDI.org .
(B) What is DISCO ?
DISCO is the abbreviated form of Discovery. It is basically used to club or group common
services together on a server and provides links to the schema documents of the services
it describes may require.
(B) What is WSDL?
Web Service Description Language (WSDL)is a W3C specification which defines XML
grammar for describing Web Services.XML grammar describes details such as:-
√ Where we can find the Web Service (its URI)?
√ What are the methods and properties that service supports?
√ Data type support.
√ Supported protocols
In short its a bible of what the webservice can do.Clients can consume this WSDL and
build proxy objects that clients use to communicate with the Web Services. Full WSDL
specification is available at http://www.w3.org/TR/wsdl.
(A) What the different phase/steps of acquiring a proxy object in
Webservice ?
Following are the different steps needed to get a proxy object of a webservice at
the client side :-
√ Client communicates to UDI node for WebService either through browser or
UDDI's public web service.
√ UDII responds with a list of webservice.
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√ Every service listed by webservice has a URI pointing to DISCO or WSDL
document.
√ After parsing the DISCO document, we follow the URI for the WSDL document
related to the webservice which we need.
√ Client then parses the WSDL document and builds a proxy object which can
communicate with Webservice.
(B) What is file extension of Webservices ?
.ASMX is extension for Webservices.
Note :- After this we are going to deal with a sample of webservice. In VS2005 webproject
is created from the menu itself as compared to 2003 where it was present in the explorer.
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Figure :- 4.8 Create Web project menu in VS2005
(B)Which attribute is used in order that the method can be used as
WebService ?
WebMethod attribute has to be specified in order that the method and property can be
treated as WebService.
(A) What are the steps to create a webservice and consume it ?
Note :- For this question this book will make a attempt by creating a simple webservice and
explaining steps to acheive it. A simple webservice will be created which takes two number
and gives addition result of the two number. In CD sample webservice project with folder
name “MathsWebService” is provided and same will be explained below. Definitely the
168
interviewer will not expect such a detail answer but this book will explain you in detail so
that you are on right track during interview.
This webservice will add two numbers and give to the calling client.All the below steps
are according to VS2005 beta editor :-
√ First create a website by clicking on File -- New WebSite.
√ From “Visual Studio Installed Templates” click on “Asp.NET Web Service”.
See figure below. Name the figure as “Maths Web Service”.
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Figure :- 4.9 Create WebService Project
√ By default the .NET editor has made a default webservice method called as
"HelloWord" which returns a string datatype. Let's rename "Service.vb" to
"Maths.vb" and "Service.asmx" to "Maths.asmx". Let’s replace the
“HelloWorld” with following code below :-
Public Function AddTwoNumbers(ByVal Number1 As Integer, ByVal
Number2 As Integer) As Integer
Return Number1 + Number2
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End Function
Figure :- 4.10 Rename all your default “Service” to “Maths”
√ After the webservice is done click on add Webreference. Normally for
components we do a “Add Reference” and for Webservices we do “Add Web
Reference”.
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Figure :- 4.11 Click on Add Web Reference
√ You will be shown with a list of webservices which are known to the
solutions. As we are looking for our “Maths” webservice which exist in the
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same solution, we click “Webservices in this solution”.
Figure :- 4.12 List of webservices for browsing
√ Your editor has located the “Maths” webservice.Select the webservice.
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Figure :- 4.13 Solution showing the availability of Maths Webservice.
174
√ After you have clicked on “Maths” webservice you will see a search progress
bar as shown in figure below. This process will start the webservice, reference it
and create a proxy for the client, so that using it client can absorb the
webservice.
Figure :- 4.14 Starting the webservice and creating the proxy for your solution.
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√ Finally you are able to see your webservice which is ready for use. Click on
Add Reference and you will see a “Localhost” reference in your .NET solution.
Figure :- 4.15 Starting the webservice and creating the proxy for your solution.
√ We need to make a client who will absorb this “Maths Webservice”. Add
“WebserviceClient.aspx” and create a UI as shown below. In the button click
put in the following code. “LocalHost.ClsMaths” is the proxy object by which
you can make calls to the webservice.
Sub cmdCalculate_Click(ByVal sender As Object, ByVal e As
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System.EventArgs)
Dim pobjMaths As New localhost.ClsMaths
lblResultDisplay.Text =
Convert.ToString(pobjMaths.AddTwoNumbers(Convert.ToInt16(txtNumber1.Text),
Convert.ToInt16(txtNumber2.Text)))
End Sub
Figure :- 4.16 Complete Webservice in action.
Note :- The whole point of creating this “Maths Webservice” step by step was to have a
understanding of practical angle of how webservices are created. It’s very rare that you will
be asked to explain every step of how to write a webservice. But in case your interviewer is
too bend down to also know what are the actual steps in creating a Webservice.
(A) Do webservice have state ?
Twist :- How can we maintain State in Webservices ?
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Webservices as such do not have any mechanism by which they can maintain state.
Webservices can access ASP.NET intrinsic objects like Session, application and so on if
they inherit from “WebService” base class.
<%@ Webservice class="TestWebServiceClass" %>
Imports System.Web.Services
Public class TestWebServiceClass
Inherits WebService
session("Val") = Value
End Sub
end class
Above is a sample code which sets as session object called as “val”. TestWebserviceClass
is inheriting from WebService to access the session and application objects.
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