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Reference : Locking and Isolation Levels : Isolation Levels

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Isolation Levels

An isolation level represents a particular locking strategy employed in the database system to improve data consistency. The higher the isolation level, the more complex the locking strategy behind it. The isolation level provided by the database determines whether a transaction will encounter the following behaviors in data consistency:
Dirty reads
User 1 modifies a row. User 2 reads the same row before User 1 commits. User 1 performs a rollback. User 2 has read a row that has never really existed in the database. User 2 may base decisions on false data.
Non-repeatable reads
User 1 reads a row, but does not commit. User 2 modifies or deletes the same row and then commits. User 1 rereads the row and finds it has changed (or has been deleted).
Phantom reads
User 1 uses a search condition to read a set of rows, but does not commit. User 2 inserts one or more rows that satisfy this search condition, then commits. User 1 rereads the rows using the search condition and discovers rows that were not present before.
Isolation levels represent the database system’s ability to prevent these behaviors. The American National Standards Institute (ANSI) defines four isolation levels:
*Read uncommitted (0)
*Read committed (1)
*Repeatable read (2)
*Serializable (3)
In ascending order (0–3), these isolation levels provide an increasing amount of data consistency to the transaction. At the lowest level, all three behaviors can occur. At the highest level, none can occur. The success of each level in preventing these behaviors is due to the locking strategies they use, which are as follows:
Read uncommitted (0)
Locks are obtained on modifications to the database and held until end of transaction (EOT). Reading from the database does not involve any locking.
Read committed (1)
Locks are acquired for reading and modifying the database. Locks are released after reading but locks on modified objects are held until EOT.
Repeatable read (2)
Locks are obtained for reading and modifying the database. Locks on all modified objects are held until EOT. Locks obtained for reading data are held until EOT. Locks on non-modified access structures (such as indexes and hashing structures) are released after reading.
Serializable (3)
All data read or modified is locked until EOT. All access structures that are modified are locked until EOT. Access structures used by the query are locked until EOT.
The following table shows what data consistency behaviors can occur at each isolation level.
Table 21. Isolation Levels and Data Consistency
Dirty Read
Nonrepeatable Read
Phantom Read
0, Read uncommitted
1, Read committed
2, Repeatable read
3, Serializable
Although higher isolation levels provide better data consistency, this consistency can be costly in terms of the concurrency provided to individual users. Concurrency is the ability of multiple users to access and modify data simultaneously. As isolation levels increase, so does the chance that the locking strategy used will create problems in concurrency.
The higher the isolation level, the more locking involved, and the more time users may spend waiting for data to be freed by another user. Because of this inverse relationship between isolation levels and concurrency, you must consider how people use the database before choosing an isolation level. You must weigh the trade-offs between data consistency and concurrency, and decide which is more important.