Concurrency Control in Database Systems

Lock-based protocols are a mechanism to control concurrent access to a data item. Data items can be locked in exclusive (X) mode, where the data item can be both read and written, or shared (S) mode, where the data item can only be read. Lock requests are made to the concurrency-control manager, and a transaction can proceed only after the request is granted.

If a lock cannot be granted, the requesting transaction is made to wait until all incompatible locks held by other transactions have been released.

The concurrency-control manager is responsible for handling lock requests. Transactions can proceed only after their lock requests are granted by the concurrency-control manager.

<p>Data items can be locked in exclusive (X) mode, where the data item can be both read and written, or shared (S) mode, where the data item can only be read.</p> Signup and view all the answers

<p>A lock-compatibility matrix determines if a transaction may be granted a lock on an item based on whether 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 lock on the item, no other transaction may hold any lock on the item.</p> Signup and view all the answers