Types of Lock

What is a Lock?

A Lock is a variable assigned to any data item to keep track of the status of that data item so that isolation and non-interference are ensured during concurrent transactions....

Lock Based Protocols

A lock is a variable associated with a data item that describes the status of the data item to possible operations that can be applied to it. They synchronize the access by concurrent transactions to the database items. It is required in this protocol that all the data items must be accessed in a mutually exclusive manner. Let me introduce you to two common locks that are used and some terminology followed in this protocol....

Types of Lock

Shared Lock (S): Shared Lock is also known as Read-only lock. As the name suggests it can be shared between transactions because while holding this lock the transaction does not have the permission to update data on the data item. S-lock is requested using lock-S instruction. Exclusive Lock (X): Data item can be both read as well as written.This is Exclusive and cannot be held simultaneously on the same data item. X-lock is requested using lock-X instruction....

Lock Compatibility Matrix

A transaction may be granted a lock on an item if the requested lock is compatible with locks already held on the item by other transactions. Any number of transactions can hold shared locks on an item, but if any transaction holds an exclusive(X) on the item no other transaction may hold any lock on the item. If a lock cannot be granted, the requesting transaction is made to wait till all incompatible locks held by other transactions have been released. Then the lock is granted....

Concurrency Control Protocols

Concurrency Control Protocol allow concurrent schedules, but ensure that the schedules are conflict/view serializable, and are recoverable and maybe even cascadeless. These protocols do not examine the precedence graph as it is being created, instead a protocol imposes a discipline that avoids non-serializable schedules. Different concurrency control protocols provide different advantages between the amount of concurrency they allow and the amount of overhead that they impose....

Types of Lock-Based Protocols

1. Simplistic Lock Protocol...

Upgrade / Downgrade locks

A transaction that holds a lock on an item Ais allowed under certain condition to change the lock state from one state to another. Upgrade: A S(A) can be upgraded to X(A) if Ti is the only transaction holding the S-lock on element A. Downgrade: We may downgrade X(A) to S(A) when we feel that we no longer want to write on data-item A. As we were holding X-lock on A, we need not check any conditions....

Problem With Simple Locking

Consider the Partial Schedule:...

Conclusion

In conclusion, lock-based concurrency control in a database management system (DBMS) uses locks to control access, avoid conflicts, and preserve the integrity of the database across multiple users. The protocol seeks to achieve a balance between concurrency and integrity by carefully controlling the acquisition and deletion of locks by operations....

Frequently Asked Question on Lock Based Protocol – FAQs

Why do we use a lock-based protocol?...