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Sunday, October 9, 2016

Concurrency Control in DBMS-Tutorial

Concurrency Control

As we have seen above, when there are multiple transactions executing at the same time on same data, it may affect the result of the transaction. Hence it is necessary to maintain the order of execution of those transactions. In addition, it should not alter the ACID property of a transaction.
In order to maintain the concurrent access of transactions, two protocols are introduced.
  • Lock Based Protocol: - Lock is in other words called as access. In this type of protocol any transaction will not be processed until the transaction gets the lock on the record. That means any transaction will not retrieve or insert or update or delete the data unless it gets the access to that particular data.
These locks are broadly classified as Binary locks and shared / exclusive locks.
In binary lock data can either be locked or unlocked. It will have only these two states. It can be locked for retrieve or insert or update or delete the data or unlocked for not using the data.
In shared / exclusive lock technique the data is said to be exclusively locked if for insert / update /delete. When it is exclusively locked no other transaction can read or write the data. When a data is read from the database, then its lock is shared i.e.; the data can be read by other transaction too but it cannot be changed while retrieving the data.

Lock based protocols are of 4 types

Simplistic Lock Protocol: -As the name suggests it is the simplest way of locking the data during the transaction. This protocol allows all the transaction to get the lock on the data before insert / update /delete on it. After completing the transaction, it will unlock the data.
Pre-claiming Protocol: - In this protocol, it evaluates the transaction to list all the data items on which transaction needs lock. It then requests DBMS for the lock on all those data items before the transaction begins. If DBMS gives the lock on all the data, then this protocol allows the transaction to begin. Once the transaction is complete, it releases all the locks. If all locks are given by DBMS, then it reverts the transactions and waits for the lock.

For example, if we have to calculate total marks of 3 subjects, then this protocol will evaluate the transaction and list the locks on subject1 marks, subject2 marks and then subject3 marks. Once it gets all the locks, it will start the transaction. 
Two Phase Locking Protocol (2PL): - In this type of protocol, as the transaction begins to execute, it starts requesting for the locks that it needs. It goes on requesting for the locks as and when it is needed. Hence it has a growing phase of locks. At one stage it will have all the locks. Once the transaction is complete it goes on releasing the locks. Hence it will have descending phase of locks. Thus this protocol has two phases – growing phase of locks and shrinking phase of locks.

For example, if we have to calculate total marks of 3 subjects, then this protocol will go on asking for the locks on subject1 marks, subject2 marks and then subject3 marks. As and when it gets the locks on the subject marks it reads the marks. It does not wait till all the locks are received. Then it will have total calculation. Once it is complete it release the lock on subject3 marks, subject2 marks and subject1 marks.
In this protocol, if we need to have exclusive lock on any data for writing, then we have to first get the shared lock for reading. Then we have to request / modify the lock to exclusive lock.
Strict Two Phase Locking (Strict 2PL): - This protocol is similar to 2PL in the first phase. Once it receives the lock on the data, it completes the transaction. Here it does not release the locks as it is used and no more required. It waits till whole transaction to complete and commit, then it releases all the locks at a time. This protocol hence does not have shrinking phase of lock release.

In the example of calculating total marks of 3 subjects, locks are achieved at growing phase of the transaction and once it receives all the locks, it executes the transaction. Once the transaction is fully complete, it releases all the locks together.
  • Time Stamp Based Protocol: - As we have seen above in lock based protocol, it acquires locks at the time of execution. But in this method, as soon as a transaction is created it assigns the order of the transaction. The order of the transaction is nothing but the ascending order of the transaction creation. The priority for older transaction is given to execute first. This protocol uses system time or logical counter to determine the time stamp of the transaction.
Suppose there are two transactions T1 and T2. Suppose T1 has entered the system at time 0005 and T2 has entered the system at 0008 clock time. Priority will be given to T1 to execute first as it is entered the system first.
In addition to the timestamp of a transaction, this protocol also maintains the timestamp of last ‘read’ and ‘write’ operation on a data. Based on the timestamp of transaction and the data which it is accessing a timestamp ordering protocol is defined.
According to this protocol:

  • If a transaction T is a read transaction on data X then

This algorithm states that if there is an active write operation on data X when a transaction T is requesting for X, then reject the transaction T. If the transaction T is started as soon as write is complete or no going write operation on X, then execute T. 
For example, if there is an update on marks1 on MARKS table and meanwhile there is a request to read marks1, then do not perform read marks1. This is because there is an update being happening on marks1. If there was an update on marks1 which is executed long back or it is complete just now and there is a request to read marks1, then system will allow reading marks1.
  • If a transaction T is a write transaction on data X then

This algorithm describes about write operation. If there is an active read or write on data X, and at the same time if the transaction T is requesting for X, then the transaction is rejected. If there is no active read / write operation on X, then execute the transaction.
Suppose T1 is reading marks1 from MARKS table. Meanwhile transaction T2 begins and tries to update marks1 in MARKS. Then the transaction T2 is rejected and rolled back.


DBMS Deadlock


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