The four isolation levels
====================
READ UNCOMMITTED
---------------------------------
With the READ-UNCOMMITTED isolation level there isn’t much isolation present between the transactions at all. The transactions can see changes to data made by other transactions that are not committed yet. That means transactions could be reading data that may not even exist eventually because the other transaction that was updating the data rolled-back the changes and didn’t commit. This is known as dirty read. An application rarely needs to rely on dirty needs, in fact this really can’t be called an isolation level. Simply put, there isn’t really any isolation at all and hence such a system can’t really be called a transactional system.
READ COMMITTED
-----------------------------
With the READ-COMMITTED isolation level, the phenomenon of dirty read is avoided, because any uncommitted changes is not visible to any other transaction, until the change is committed. Within this isolation level each SELECT uses its own snapshot of the committed data that was committed before the execution of the SELECT. Now because each SELECT has its own snapshot, so the same SELECT when run multiple times during the same transaction could return different result sets (in case some one commit from other session). This phenomenon is called non-repeatable read. Each consistent read, even within the same transaction, sets and reads its own fresh snapshot.
REPEATABLE READ
-----------------------------
With the REPEATABLE-READ isolation level, the phenomenon of non-repeatable read is avoided. This isolation level returns the same result set through out the transaction execution for the same SELECT run any number of times. A snapshot of the SELECT is taken the first time that SELECT is run during the transaction and the same snapshot is used through out the transaction when the same SELECT is executed. A transaction running in this isolation level does not take into account any changes to data made by other transactions, regardless of whether the changes have been committed or not. This ensures that reads are always consistent(repeatable). This isolation level is the default for InnoDB. Although this isolation level solves the problem of non-repeatable read, but there is another possible problem phantom reads.
SERIALIZABLE
-----------------------
With the SERIALIZABLE isolation level, the phenomenon of phantom reads is avoided. Transactions when run in this isolation level place locks on all records that are accessed, as well as locks the resource so that records cannot be appended to the table being operated on by the transaction. Transactions when run in this fashion, run in a serialized manner. This isolation level is the strongest possible isolation level.
Note:
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1> The READ-UNCOMMITTED isolation level has the least number of locking done, after that comes the READ-COMMITTED isolation level which removes most of the gap-locking and hence produces fewer deadlocks, also in the case of READ-COMMITTED, locking reads only lock the index records and not the gaps before/after them. REPEATABLE-READ has a higher level of locking as compared to READ-COMMITTED, UPDATE, DELETE use next-key locking, also locking reads also use next-key locking. SERIALIZABLE has the highest level of locking, all the simple SELECTs are automatically converted to SELECT … LOCK IN SHARE MODE, and hence all records have shared locks.
2> SERIALIZABLE and REPEATABLE-READ employ lots of locking and hence creating more deadlock situations, which in turn decreases performance. In fact SERIALIZABLE is the least performant of the isolation levels, as it converts even plain reads into locking reads. REPEATABLE-READ is better in terms of locking and deadlocks but READ-COMMITTED is even better because there are fewer gap-locks. But locking and deadlocks is not the only thing when considering performance, there is another issue of mutex contention that needs consideration.
References:
http://www.ovaistariq.net/597/understanding-innodb-transaction-isolation-levels/#.V8aravl94lI
https://dev.mysql.com/doc/refman/5.7/en/innodb-transaction-isolation-levels.html
====================
READ UNCOMMITTED
---------------------------------
With the READ-UNCOMMITTED isolation level there isn’t much isolation present between the transactions at all. The transactions can see changes to data made by other transactions that are not committed yet. That means transactions could be reading data that may not even exist eventually because the other transaction that was updating the data rolled-back the changes and didn’t commit. This is known as dirty read. An application rarely needs to rely on dirty needs, in fact this really can’t be called an isolation level. Simply put, there isn’t really any isolation at all and hence such a system can’t really be called a transactional system.
READ COMMITTED
-----------------------------
With the READ-COMMITTED isolation level, the phenomenon of dirty read is avoided, because any uncommitted changes is not visible to any other transaction, until the change is committed. Within this isolation level each SELECT uses its own snapshot of the committed data that was committed before the execution of the SELECT. Now because each SELECT has its own snapshot, so the same SELECT when run multiple times during the same transaction could return different result sets (in case some one commit from other session). This phenomenon is called non-repeatable read. Each consistent read, even within the same transaction, sets and reads its own fresh snapshot.
REPEATABLE READ
-----------------------------
With the REPEATABLE-READ isolation level, the phenomenon of non-repeatable read is avoided. This isolation level returns the same result set through out the transaction execution for the same SELECT run any number of times. A snapshot of the SELECT is taken the first time that SELECT is run during the transaction and the same snapshot is used through out the transaction when the same SELECT is executed. A transaction running in this isolation level does not take into account any changes to data made by other transactions, regardless of whether the changes have been committed or not. This ensures that reads are always consistent(repeatable). This isolation level is the default for InnoDB. Although this isolation level solves the problem of non-repeatable read, but there is another possible problem phantom reads.
SERIALIZABLE
-----------------------
With the SERIALIZABLE isolation level, the phenomenon of phantom reads is avoided. Transactions when run in this isolation level place locks on all records that are accessed, as well as locks the resource so that records cannot be appended to the table being operated on by the transaction. Transactions when run in this fashion, run in a serialized manner. This isolation level is the strongest possible isolation level.
Note:
--------
1> The READ-UNCOMMITTED isolation level has the least number of locking done, after that comes the READ-COMMITTED isolation level which removes most of the gap-locking and hence produces fewer deadlocks, also in the case of READ-COMMITTED, locking reads only lock the index records and not the gaps before/after them. REPEATABLE-READ has a higher level of locking as compared to READ-COMMITTED, UPDATE, DELETE use next-key locking, also locking reads also use next-key locking. SERIALIZABLE has the highest level of locking, all the simple SELECTs are automatically converted to SELECT … LOCK IN SHARE MODE, and hence all records have shared locks.
2> SERIALIZABLE and REPEATABLE-READ employ lots of locking and hence creating more deadlock situations, which in turn decreases performance. In fact SERIALIZABLE is the least performant of the isolation levels, as it converts even plain reads into locking reads. REPEATABLE-READ is better in terms of locking and deadlocks but READ-COMMITTED is even better because there are fewer gap-locks. But locking and deadlocks is not the only thing when considering performance, there is another issue of mutex contention that needs consideration.
References:
http://www.ovaistariq.net/597/understanding-innodb-transaction-isolation-levels/#.V8aravl94lI
https://dev.mysql.com/doc/refman/5.7/en/innodb-transaction-isolation-levels.html
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