Locking Protocol and locks

Locking Protocol

Locking protocols are used in database management systems as a means of concurrency control. Multiple transactions may request a lock on a data item simultaneously. Hence, a mechanism is required to manage the locking requests made by transactions. Such a mechanism is called as Lock Manager. It relies on the process of message passing where transactions and lock manager exchange messages to handle the locking and unlocking of data items.  

Data structure used in Lock Manager –
The data structure required for implementation of locking is called as Lock table.

1. It is a hash table where name of data items are used as hashing index.
2. Each locked data item has a linked list associated with it.
3. Every node in the linked list represents the transaction which requested for lock, mode of lock requested (mutual/exclusive) and current status of the request (granted/waiting).
4. Every new lock request for the data item will be added in the end of linked list as a new node.
5. Collisions in hash table are handled by technique of separate chaining.

Working of Lock Manager –

1. Initially the lock table is table empty as no data item is locked.
2. Whenever lock manger receives a lock request from a transaction T_i on a particular data item Q_i following cases may arise:
• If Q_i is not already locked, a linked list will be created and lock will be granted to the requesting transaction T_i.
• If the data item is already locked, a new node will be added at the end of its linked list containing the information about request made by T_i.
3. If the lock mode requested by T_i is compatible with lock mode of transaction currently having the lock, T_i will acquire the lock too and status will be changed to ‘granted’. Else, status of T_i’s lock will be ‘waiting’.
4. If a transaction T_i wants to unlock the data item it is currently holding, it will send an unlock request to the lock manager. The lock manger will delete T_i’s node from this linked list. Lock will be granted to the next transaction in the list.
5. Sometimes transaction T_i may have to be aborted. In such a case all the waiting request made by T_i will be deleted from the linked lists present in lock table. Once abortion is complete, locks held by T_i will also be released.

Locks

A lock is a data variable which is associated with a data item. This lock signifies that operations that can be performed on the data item. Locks help synchronize access to the database items by concurrent transactions.

All lock requests are made to the concurrency-control manager. Transactions proceed only once the lock request is granted.

Binary Locks: A Binary lock on a data item can either locked or unlocked states.

Shared/exclusive: This type of locking mechanism separates the locks based on their uses. If a lock is acquired on a data item to perform a write operation, it is called an exclusive lock.

1. Shared Lock (S): A shared lock is also called a Read-only lock. With the shared lock, the data item can be shared between transactions. This is because you will never have permission to update data on the data item.

For example, consider a case where two transactions are reading the account balance of a person. The database will let them read by placing a shared lock. However, if another transaction wants to update that account's balance, shared lock prevent it until the reading process is over.

2. Exclusive Lock (X): With an Exclusive Lock, a data item can be read as well as written. This is exclusive and can't be held concurrently on the same data item. X-lock is requested using lock-x instruction. Transactions may unlock the data item after finishing the 'write' operation.