Locking Mechanisms

Explanation Locking mechanisms are a way for databases to produce sequential data output without the sequential steps. The locks provide a method for securing the data that is being used so no anomalies can occur like lost data or additional data that can be added because of the loss of a transaction. Problems that Locks Solve Lost Update Problem- An update can get lost if two or more transactions try to update the same data. The two transactions are unaware of each other, and data can be overwritten. Temporary Update Problem- If one transaction updates the database and then fails, another transaction can read the incorrect value which lowers the integrity of the database. Incorrect Summary Problem- If one transaction is calculating an aggregate while another transaction is updating the same data the integrity would be compromised. Phantom Reads- When an insert or delete is performed on a row that is being used by another transaction the integrity of the data is compromised.

Different Types of Locks There are three primary types of locks that are used in a database. Read Locks- These types of locks make it so that data can only be read. Depending on the database system and restrictions on the read lock, this can make it so only one user can read the data to allowing every user access to reading the data but not being able to modify anything. The read lock can be applied to a single row, a section of rows, or an entire table. This can also be dependent on the type of database system that is being used that could limit the amount of data that can be locked for reading. Write Locks- This type of lock is used for updates in the database system. When this lock is applied it prevents any other transactions from changing the records that are being accessed. This does allow transactions to read the data before it is modified and the changes are made permanent. Exclusive Write Locks- This type of lock is similar to a write lock. The only difference is that with an exclusive write lock, the only things that can look at the data or modify the data is the original transaction. No other transaction can read the data while this lock is applied. There are also many other locks that signal intention to request another type of lock. These locks are called multi-level-locks. This is useful to show other transactions what types of locks the transaction has throughout the hierarchy of data levels.

Multi-Level-Locks Update Lock- Signals intention to request an exclusive lock in the near future. IS Lock- Signals intent to request shared (read) lock in the near future. IX Lock- Signals intent to request an exclusive (write) lock in the near future.

Two-Phase Locking Protocol The Two Phase Commit is designed to coordinate the transactions of the requests to the system. The idea behind the protocol is to produce serialized results from a non-serialized system. This protocol requires that each transaction issues lock and unlock requests in two phases: the shrinking phase and the growing phase. During the growing phase transactions may obtain locks, but cannot release any. During the shrinking phase transactions may release locks but may not obtain any new locks. By following this protocol any update problems with the transaction can be detected and one transaction gets rolled back. It also can raise the priority of the affected transaction to prevent a repeat of the problem.

--Nels5093 06:15, 5 December 2008 (UTC)

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