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Transaction Locking And Isolation In JDBC

Transaction is a set of instructions which follow atomicity,consistency,Integrity and durability.

1. Atomicity : This states that either all actions should complete or none should occur.

2. Consistency : This imposes certain business rule.

3. Integrity : This states that the two actions shall not interfere with each other or access each others data.

4. Durability : This states that the changes occurred due to a certain action or transaction must be stored in a durable persistent storage.

 

When we talk about the Transaction Locking And Isolation In JDBC, following are the issues we need to address when to concurrent transactions are taking place :

1. Dirty read : This happens when uncommitted data is read.Typical example is :

a. Transaction 1 inserts a row into a table.

b.Transaction 2 reads the new row.

c.Transaction 1 rolls back.

2. Non-repeatable read : This happens when data reads from the same table for the same row are not same.Typical example is

a. Transaction 1 reads a row.

b. Transaction 2 updates the row.

c. Transaction 1 reads the same row a second time and gets a different  results.

 

3. Phantom Read : This occurs due to appearing of new records when same query is run at different times.Typical example is :

a. Transaction 1 reads all rows that satisfy a WHERE clause.

b. Transaction 2 inserts additional rows that satisfies the WHERE clause.

c. Transaction 1 executes  the WHERE condition and picks up the additional row.

There are different transaction isolation levels used from JDBC coding to address these issues.The function to set isolation level is :

 

con.setTranscationIsolationLevel(intvalue);

Different integer values can be provided as follows :

0 – JDBC_TRANSACTION_NONE
1 – JDBC_TRANSACTION_READ_UNCOMMITTED
2 – JDBC_TRANSACTION_READ_COMMITTED
4 – JDBC_TRANSACTION_REPEATABLE_READ
8 – JDBC_TRANSACTION_SERIALIZABLE

Once we set the isolation levels using the above function,we can address the issues of transaction level in JDBC connection.As the isolation     level increases it gets stricter and lower level problems are also addressed.If we have set the value to 8 ,the isolation level with value lesser than 8 ,ie. 0,1,2,4, are also taken  care of.

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How container does the cookie work?

 

A Cookie is a key value pair primarily used by the container to pass the session id in order to maintain session between client and server. There can be user defined cookies as well to use cookie as attributes to pass values from one part of the web application to another.

The user only requests for the session,creating a session object,attaching the session id with the session object and sending it to the client side or the browser with the response is done by the server at the back end. These things the code has nothing to bother about,the code asks for the session and these things happen automatically.

 

The code below shows how to get a session object in servlet :

HttpSession sess = request.getSession();

 

With the help of the above written code,the session can be retrieved from the server.Following are the steps covered by the server as soon as this line of code executes :

1. HttpSession object is created

2. Unique Session id is created

3. A new cookie object is created.

4. Session id is associated with cookie

5. Cookie is set Into the response to be sent to client side

 

These are the steps involved in sending the cookie with the response if session does not exist .

The same code when executed does the work of retrieving a session object from request if it exists.If session already exists,the server finds the session id from the request(since cookie will be sent to the server from client side as part of the url).The server finds out the unique session id from the cookie and retrieves the session object with the matching Session id.

Thus all the cookie work happens behind the scene.

To know whether the session already existed or was just created,we can ask the session object itself as :

sess.isNew();

If it returns true ,it means session is created otherwise it is a preexisting session.

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