In database systems, validation protocols are used to ensure that the changes made to the database by a transaction do not violate the integrity constraints or consistency rules defined for the database. The validation process is performed after the transaction has completed its execution and is in the partially committed state, but before it is allowed to commit its changes to the database.
One example of a validation protocol is the strict two-phase locking (S2PL) protocol, which is commonly used in many database systems. The S2PL protocol ensures that a transaction acquires all the necessary locks on the data items it accesses before it is allowed to make changes to them. The locks are held by the transaction until it has completed its execution and has committed its changes or has been rolled back.
During the validation phase, the S2PL protocol checks whether the transaction's changes violate any of the integrity constraints defined for the database. For example, consider a database that contains a table of bank accounts, where each account has a balance that cannot be negative. If a transaction attempts to withdraw an amount that exceeds the account balance, the validation phase will detect this violation and prevent the transaction from committing its changes to the database.
Another example of a validation protocol is the timestamp ordering protocol, which uses timestamp values to determine the order in which transactions are executed and committed to the database. In this protocol, each transaction is assigned a unique timestamp value that reflects the time at which it started its execution. The timestamp values are used to order the transactions and to determine which transactions are allowed to commit their changes to the database. During the validation phase, the timestamp ordering protocol checks whether the transaction's changes violate any of the consistency rules defined for the database. For example, if a transaction updates a record that has already been modified by another transaction with a higher timestamp value, the validation phase will detect this violation and prevent the transaction from committing its changes to the database.
Overall, validation protocols are an essential mechanism for maintaining the consistency and correctness of the data in a database, and for ensuring that the transactions executed on the database do not violate its integrity constraints or consistency rules.
