Locking Protocols

Locking Protocols   

AU: Dec-15,17, May-16, Marks 16

Why Do We Need Locks?

• One of the method to ensure the isolation property in transactions is to require that data items be accessed in a mutually exclusive manner. That means, while one transaction is accessing a data item, no other transaction can modify that data item.

• The most common method used to implement this requirement is to allow a transaction to access a data item only if it is currently holding a lock on that item.

• Thus the lock on the operation is required to ensure the isolation of transaction.

Simple Lock Based Protocol

• Concept of Protocol: The lock based protocol is a mechanism in which there is exclusive use of locks on the data item for current transaction.

• Types of Locks: There are two types of locks used -

i) Shared Lock: The shared lock is used for reading data items only. It is denoted by Lock-S. This is also called as read lock.

ii) Exclusive Lock: The exclusive lock is used for both read and write operations. It is denoted as Lock-X. This is also called as write lock.

• The compatibility matrix is used while working on set of locks. The concurrency control manager checks the compatibility matrix before granting the lock. If the two modes of transactions are compatible to each other then only the lock will be granted.

• In a set of locks may consists of shared or exclusive locks. Following matrix represents the compatibility between modes of locks.

Here T stands for True and F stands for False. If the control manager get the compatibility mode as True then it grant the lock otherwise the lock will be denied.

• For example: If the transaction T₁ is holding a shared lock in data item A, then the no control manager can grant the shared lock to transaction T₂ as compatibility is True.

But it cannot grant the exclusive lock as the compatibility is false. In simple words if transaction T₁ is reading a data item A then same data item A can be read by another transaction T₂ but cannot be written by another transaction.

• Similarly if an exclusive lock (i.e. lock for read and write operations) is hold on the data item in some transaction then no other transaction can acquire Share or exclusive lock as the compatibility function denotes F. That means of some transaction is writing a data item A then another transaction can not read or write that data item A.

Hence the rule of thumb is

i) Any number of transactions can hold shared lock on an item.

ii) But exclusive lock can be hold by only one transaction.

• Example of a schedule denoting shared and exclusive locks: Consider following schedule in which initially A=100. We deduct 50 from A in T, transaction and Read the data item A in transaction T2. The scenario can be represented with the help of locks and concurrency control manager as follows:

Review Questions

1.State and explain the lock based concurrency control with suitable example.  AU: Dec-17, Marks 13, May-16, Marks 16

2. What is Concurrency control? How is implemented in DBMS? Illustrate with suitable example.    AU: Dec-15, Marks 8