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Sunday, June 26, 2011

[ C# .net remoting ] - Client activated remoting objects

1. Introduction

In the previous article I had written about server activated remote objects. Also provided a separate article for Single Call and Singleton on the Server activated remote objects. In this post we will explore how do we use the Client activated remote objects.

Before we go on Client Activated remote objects a short note about what is Activation and where the object lives. The obvious thing is whether it is server activated or client activated, the remote object lives in the remote pool of the server.  Client activation means, the client creates the object on the server’s remote pool using the operator new. So, if you are developing a class for the client activated remote object, you have the liberty of using the overloaded constructors.

In Server activated – Single call technique, the remote object is created during the call and destroyed when the call is over.
In Server activated – Singleton technique, a single object is maintained for client requests between the call. All clients shares the object state
In Client Activated technique, the client creates the object on the remote pool the object is dedicated to the client who created it. So here the object state is different for objects created by client/clients.

Now let us go ahead start creating an example for the Client Activated remote object.

2. About the Sample

Below is the screen shot:

The console window is the server exe that waits for the client’s activation request to host the dedicated object in the remote pool. The client is a windows application, which demonstrates the use of the client activation.

Set First Ac button creates a remote object on the remote pool and Display Ac1 retrieves the Object’s state information from the remote machine. Similarly the other button pair second account does the same. This will give an idea that each object created by the client maintains its own state as Display Ac1 and Display Ac2 button clicks are going to retrieve different information.

3. Remote Server – Account Class

The remote server is created as the C# console application project. The project is named as SercerCA (spell mistake! What to do?). After creating the project dot reference for system.runtime.remoting is added to the project.

1) First the required namespace provided in the top. Below is the code:

//001 ServerCA : Name space for Client Activated Server
using System.Runtime.Remoting;

2) Then the Account class is created. And, this class is derived from the MarshalByRefObject to support the remoting of the object created from it. Then some variables related to accounts are declared. These variable (in objects world, the object states) are useful to check the each cleint activated objects mantains their states.

    //002 ServerCA : Remote Object, that will be Activated by Client
    public class Account : MarshalByRefObject
        private int AcNumber;
        private string AcHolderName;
        private int AcBalance;

3) The constructor shown below initializes the data members of the Account class. I hope no more explanation is required here.

    //003 ServerCA : Initilize the Ac with zero, noname
    public Account()
        AcNumber = 0;
        AcHolderName = "";
        AcBalance = 0;
        Console.WriteLine("Account Object Created - Default Ctor");

4) One more constructor is provided for the Account class that helps to initialize the data members when the object is created. Notice that the Server Activated remote class does not support these kind of overloaded constructors. Refer my previous articles to know about the server activated remote objects. As the object of this class is going to be activated by the client (using the new operator) it is now possible to have the overloaded constructors. That means your class can have constructors that accepts parameters to it. Below is the code for it:

//004 ServerCA : Accout with Data. Note Constructor is Overloaded as the activation
// type we are going to set is Client
public Account(int AcNo, string Name, int Bal)
    AcNumber = AcNo;
    AcHolderName = Name;
    AcBalance = Bal;
    Console.WriteLine("Account Object Created - Overloaded Ctor");

5) OK. Now the account class has data members and constructors to initialize the data memebers. When the default contructor that is a constructor without any parameters is invoked there should be a way to initialize the members. So we will provide a get and set methods for the members. Below is the code:
//005 ServerCA : Set methods for the Private data members
public void SetAcNumber(int AcNo)
    AcNumber = AcNo;
public void SetAcName(string Name)
    AcHolderName = Name;
public void SetBalance(int Bal)
    AcBalance = Bal;

//006 ServerCA : Get methods for the Private data members
public int GetAcNumber()
    Console.WriteLine("Account Number Requested");
    return AcNumber;

public string GetAcName()
    Console.WriteLine("Account Name Requested");
    return AcHolderName;

4. Remote Server – Program Main

1) In the program.cs file, required namespaces are first included

//007 ServerCA: Required Namespaces
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.Runtime.Remoting.Channels.Http;

2) Register the required communication channel. Here the sample code taken the Http cumunication channel and registeres the port 10038 for communication.

//008 ServerCS : Setup the Http Channel and Register it
HttpChannel port = new HttpChannel(10038);
ChannelServices.RegisterChannel(port, false);

3) RegisterActivatedServiceType function call tells the server that the Account class can be activated from the client.  Now our account class will kept in the remote pool when the client creates the object of it using the new keyword. Application name is used by the client to make a service request of the server object. Below code registeres the Account object as a client activated remote object:

//009 ServerCS : Set the App Name and Register the object type that
// need to be activated by the client.
RemotingConfiguration.ApplicationName = "ServerCA";

4) Finally the server waits for the key stroke after printing the information message on the console window to terminate itself.

//010 ServerCS : Inform Server is ready
Console.WriteLine("Server is ready to Demo Client Activated Objects...");

The server is ready now. First we created the Account class, which a remote class as it is derived from the MarshalByRefObject. We provided some data members to it and get and set function to those members. Also we provided two constructors to it as it is going to be a client activated remote object. Then in the main program entry we registered an http communication channel claiming a port 10038 for dedicated use. Right! Let us move to the client and see how it activates the Account object from this running server.

5. Client Project

The client is a windows application. I named the project ClientCA. To know the form setup, download the code and check the properties for each controls that appears in bold. What the client will do is already explained in section 2 of this article.

1) As a first step, the project is created on the same server project using File->Add->New project. Then the dot net runtime for remoting is provided through the reference. The reference to System.Runtime.remoting is done through the .Net tab of the reference dialog. Also for this windows project, add sever project is added as the reference using the same Add reference dialog’s projects tab.

2) In the form main class, the following name spaces are included.

//ClientCA 001: Set the Required Namespaces.
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.Runtime.Remoting.Channels.Http;
using System.Runtime.Remoting.Activation;
using SercerCA;

3) Two instances of Account class are declared in the Form class. Note that we already provided the Server project as reference. When the solution (Created for you when you added the second project) is built, it will give two exes. One for Client (Windows App) and other one is for Server (Console App). You may deploy the application in server and client machines to test it.

//ClientCA 002 : Declare the Objects that need to be created on server
//and activation is done by the client
Account account1, account2;

4) In the constructor for the form provided by the IDE, the client application registers the Account class of the Remote server. Note that the server already registered the Account class as the Client Activated by using the utility function RegisterActivatedService type in Section 4 point 3. The below specified client code claims the server that it can activate the Account class any time using the utility function RegisterActivatedClientType. Once both and client are agreed upon the activation of the Account, the client can instantiate the Account object just like how it creates the other normal class objects.

//ClientCA 003 : Register the Client Activated type from the remote Pool.
//Note, Server already registered Account as the Activated Type.

6) Set First Ac Button Click – Client

The first account is created using the default constructor and then setting the variables through the set functions. Note the Account object in the server is activated and kept in the remote pool when the client creates the object using the new keyword. So now there is an Account object in the server remote pool, which is activated by the client. Read my Previous articles to know how the client requests Server Activated remote object.

private void btnAc1Set_Click(object sender, EventArgs e)
    //ClientCA 004 : Create the Account Object1
    account1 = new Account();
    account1.SetAcNumber( Int32.Parse(txtAcNo.Text));

    //Clear the Fields and Say Account created
    txtAcName.Text = "";
    txtAcNo.Text = "";
    txtBalance.Text = "";
    MessageBox.Show("Account Created");

7) Set Second Ac Button Click – Client

The second account is created using the overloaded constructor. And this object also sits in the remote pool of the server. So now, there are two objects in the server and their life are activated by the client. Also note that these two objects maintain their object state independently. That is, for Example Account1 may have a balance of 1000 and Account2 may have different balance of 1200. To see that, next two button click events are handled.

private void btnAc2Set_Click(object sender, EventArgs e)
    //ClientCA 005 : Create the Account object2 using overloaded Constructor
    int AcNo = Int32.Parse(txtAcNo.Text);
    int Bal = Int32.Parse(txtBalance.Text );
    account2 = new Account(AcNo, txtAcName.Text, Bal);

    //Clear the Fields and Say Account created
    txtAcName.Text = "";
    txtAcNo.Text = "";
    txtBalance.Text = "";
    MessageBox.Show("Account Created");

8) Retrieving the State information of the two Accounts

The next two button events make use the accounts created previously to get the account details. The code is given below:

private void btnAc1Get_Click(object sender, EventArgs e)
    //ClientCA 006 : Get the Details of First Object.
    txtAcName.Text = account1.GetAcName();
    txtAcNo.Text = account1.GetAcNumber().ToString();
    txtBalance.Text = account1.GetBalance().ToString();

private void btnAc2Get_Click(object sender, EventArgs e)
    //ClientCA 007 : Get the Details of First Object.
    txtAcName.Text = account2.GetAcName();
    txtAcNo.Text = account2.GetAcNumber().ToString();
    txtBalance.Text = account2.GetBalance().ToString();

9) Testing the App

On Same Machine 
1) Right click the solution and Click rebuild all.
2) Now nevigate to both the exe using windows explore.
3) Run the Console Server
4) Run the Windows application
5) Click the button on the Windows application and Observe the result on the Client. 

On Different Machines
1) Copy Server to a Machine say Mahesh. Here, mahesh is the name of the machine.
2) Have the client on your machine itself. Before that Search for the keyword Localhost and change it to Mahesh.
3) Run the server at machine mahesh.
4) Run the client at your machine
5) Click the Get Details button.

You may have a Question that when will these client activated object removed from the remote pool. How does the GC works?. Well, Collecting the information for Remote Object Life time. See you in the next article.

Note: The samples are created using the VS2005 IDE

Source Code and Running App Demo: Download

Extracted Zip gives you Source Code and Video of Testing the App. To see the video, set the resolution to 800x600 and use full screen option from the video.

Thursday, June 09, 2011

[ SQL 2005 ] - How do I use Joins in SQL Server?

1. Introduction

Joins are useful for bringing data together from different tables based on their database relations. First we will see how the join operates between tables. Then we will explore the Order of Execution when Join and where condition both exists. Finally we will move our exploration to the Importance of the Join order.

2. Run the attached script

The downloaded script has the three table and table data for this article. You should also run the Northwnd script as some example here uses the NorthWnd DB. Once you downloaded the script CreateObject.zip run the script in the NorthWnd Db.

Below is the three tables created by the Script:

We are going to use these tables to perform the joins. These tables are just for demo purpose and So I do not have proper table relationship in terms of Primary and Foreign keys. OK, Let us move on.

3. Cartesian Product of Table

Usually join will be performed between two tables based on the key columns between two tables those together constitutes the database table relationship. For Example DeptId in the employee table and DeptId in the Department table make the relationship between these two tables.

The below one is the example Joining the tables without using the key columns. Here, TableA and TableB are clubbed together to form the whole result set based on Cartesian Product. The Cartesian product will take a single record in the first table and attaches with all the records in the second table. Then takes the Second records in the first table and attaches with all the records the second table and this continue till the end of the records on the first table.

The result of the Cartesian join is shown below:

4. Joining Two tables

When joining the two tables to avoid the bulk number of records that results as shown in the previous example, we should chose a join column from both the tables. The example given below joins Table_A and Table_B based on the Column called ID. Since the column in both the tables mapped based on the ID column, we will reduce huge records that are logically not useful and not related the ID map.

Below is the Result of the Join:

Note that the Row Number 1 and Row number 5 are returned as the join result as they satisfies the mapping condition A.Id = B.Id. In the query it is shown in the Red Box

5. Joining multiple tables

The above example joint two tables. To join multiple tables, we should use the result of the previous join and pick a column from it, then pick a column in the new table then specify the join condition as the previous example. This way we can join multiple numbers of tables. Consider whatever joint so far as the single table and join it with the new one.

First Table_A is joins with Table_B, which is nothing but the previous example. Then the joint result of A and B is considered as single table say AB. Then the AB is joint with the Table_C. This is shown in the below picture:

5. Join Types

There are three type of join available based the way we join columns on two different tables.
1) Full Join
2) Inner Join
3) Left outer Join
4) Right outer Join

What we saw in the previous two examples are the inner joins. If we join the same table we called it as Self join and it is special category do not get confuse it with the join types. Let us see an example for the join types in next coming examples.

Before we go into those examples, remember that the result computed so for is considered as LEFT and the new table coming to join the existing result is RIGHT. This is useful when we are joining multiple tables with different type of joins.

6. Full Join

A full join is somewhat different from the Cartesian product. Cartesian product will get all the possible row combination between the two joining tables. Full join takes the matching columns plus all table rows from the left table that doest match the right and all tables rows in the right that does not match the left. It applies null for unmatched row on the other end when doing so. The below example shows the full join between Table_A and Table_C

1. In the above picture the Blue Row the matching row on both the table.
2. Second row (Green First, red next) is the unmatched one. Row exists on the Left table and null substituted for all the columns in the Right.
3. Third row (Red First, Green next) is also the unmatched one. Row exists on the Right side table, null returned for the left one.

Look at the from clause,
The Table_A is taken first and joint with Table_C. Here, The result set computed so for always treated as Left side of join and the new table going to be joint is treated as Right side of join.

7. Left Join

Left join makes sure to take all the rows on the left side of the table by placing the null entries for the table joining on the right side when there is unmatched row on the right side.

In the above example, Id value of 2 in the Left table does not exist on the right side table Table_C.Id. But, we still got the 2,BBB row from the Table_A by placing the null entries for the right side table. This is shown in Green and red boxes above.

Also note that when SQL is processing, it takes the rows for the Table_A first (So Left) then joins it with the Table_C (Right  side). It does not matter whether we provide A.Id = C.Id or C.Id = A.Id

8. Right Join

It is the reverse of the left join. It implies take all the rows on the right side of table by placing the null on the left table for unmatched rows. Below is the example for it:

Blue Box : Matched rows.
Green : Row exits on the right side table Table_B and match (Based on Id column) not available on the left
Red : Null placement for the columns

9. Inner Join

In inner join only the matched rows are retrieved. Please refer the section four. Inner join returns same result and hence one no need to worry about the placing a table on left or right.

10. Self Join

Joining the table with the same table is called the Self Join. To explain this let us go the table on the Northwnd database (Attached with this article). Have a look at the columns in the employee table. The EmployeeId column is the Primary key column and each row belongs to a single employee. The reports to column refer some other row in the same table stating that referred row is the manager for the referring row. But the referred row (manager) is also an employee possible having a valid entry on its ReportsTo column. So in the NorthWnd database this relationship achieves a hierarchical reporting structure.

Now have look at the below example:

Here, the row pointed by ReportTo column is Manager. So the table on the left hand side is employee and table on the Right hand side is Manager. When the FirstName is referred on the left table of the joint result, it is Employee name and the same FirstName on the right table of the join result is Manager name.

11. Execution Sequence

When the query involves the combination of the outer and inner join the execution sequence is important. If you have only inner join the execution sequence is not much important as they are going to provide the same result. Well, What I am taking about?

Let is Say you have a query, which has both inner join and outer join (Left or Right). Also let us assume you have where clause that filters the records based on column that is not a joining id column. The question comes which operation is performed first. We have two options:
1) Apply the where clause first because joining later will improve the performance then perform the join
2) Apply the Join then perform the where clause

The above two option returns same result when all the joins involved are inner joins. But the result may differ when we have at least one outer join. OK. SQL chose the second option because as per its operation Sequence From clause is completed first, then applies the where clause.

Given below is an Example and the result:

How the Sequence differs is shown below:

Option 1

Option 2:

So keep in mind the operation sequence as SQL first completes the join first then applies the where clause when the query has one or more outer joins.

12. Order of the Joins

Like the Operation sequence the Order of the join also important when you want mix the inner joins with outer (Left or Right) joins. Again, if the entire join involved between the tables are inner joins the order of the join does not matter. But it matters when we mix the inner and outer joins.

What is Order of the Join?  If my query joins three tables like [X inner Y] Left Z, the order here is inner join performed first, and then the left join

OK. Let us go back to the NorthWnd Database again. The result you want to achieve is Get all the customers names whether they have order or not. Also list the Qty of order placed by the customer if they actually has some order.

Look at the Query and result below: [Outer Join then Inner Join]

From the above query, you can see the order of join as mentioned below:
1) A right join between Orders and Customers. SQL first queries the Orders table (As it appears first) and treats the result as Left. Then it queries the Customers table next and treats the result set as Right. Finally from both the result set Right join is performed that means SQL ensures you that it will not lose any rows on the Right side result set that is it will not lose any rows from the Customers table. So you will get all customers including the two who don’t placed any orders and since a matching records for those two rows are not available you will get null columns for the Orders. Now the resulting join result is available for the next join and this join result is now treated as Left.

2) Now the above returned result (Left side) is joint with the Order Details table. SQL knows it already has the Left result set so it query the table Order Details to have the Right. Finally an Inner join is performed between Left and Right based on the Order Id. But note that we have two null entries for the ordered column for which there are no corresponding customers in the Left side result. So the Inner join just skips those records. So we got a total of 2155 missing the two customers who does not place any orders, which is not the result we need. Read the Underlines text at the top of this section.

Now look at the Query and Result below: [Inner Join then Outer Join]

Here, Inner join based on the OrderId between Orders and Order Details is performed first. This result (Left side) is then Right joint against the Cutomers table [Right].

Now let us analyze how this is giving the result we want.

The inner join between Order and Order Details brings all the matching records based on the order id. Note that we are not losing any order id here by null values. Then by keeping this already brought result on the left, Customers table is queried and kept in the Right. Then the right join is performed between Customers and Left side result based on the Customer Id. Now we get all the customers including the two for which we don’t have any matching records on the Left side table.

Keep in mind that join order is important when you mix the inner join with outer join.

13. Other way of achieving the same result

When I had a talk with one of my office friend (VaraPrasad), he told that the result you are expecting could be achieved without using the Right Join. How? That is the question I asked him. He told that Crystal reports does it, I will show you. Good, now this section is added to this article based on what I got from him.

OK. Now let us see how this works and gives the expected result of not losing any customers. Note that the Rule remains same, whatever computed so for is Left and the Joining table is on the Right.

1) SQL first queries the table Customers and keeps it as the result on the Left.
2) It reads the Open parenthesis, and queries the table Orders and keeps it Left again. Why? SQL Says “Boss, I know that I am not going to join this table and Table I am going join is not ready yet. So I kept is Left side result.
3) Now the Order Details table is queried and kept as Right side of join as Left side is already available.
4) A join between Order and Order Details is performed based on the Order Id. The resultant records are treated as right because the Customer table is already queried and kept in the Left. Now thee left join between the Left and Right side of result set brings all customers as the join type left outer join.

Note: The scripts for creating the Demo tables and NorthWnd database is available as download.

Sample script for Download 

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The opinions expressed herein are my own personal opinions and do not represent my current or previous employer's view in anyway