Isolation (database systems)

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In database systems, isolation is a property that defines how/when the changes made by one operation become visible to other concurrent operations. Isolation is one of the ACID (Atomicity, Consistency, Isolation, Durability) properties.

Isolation levels

Of the four ACID properties in a DBMS (Database Management System), the isolation property is the one most often relaxed. When attempting to maintain the highest level of isolation, a DBMS usually acquires locks on data or implements multiversion concurrency control, which may result in a loss of concurrency. This requires adding additional logic for the application to function correctly.
Most DBMS's offer a number of transaction isolation levels, which control the degree of locking that occurs when selecting data. For many database applications, the majority of database transactions can be constructed to avoid requiring high isolation levels (e.g. SERIALIZABLE level), thus reducing the locking overhead for the system. The programmer must carefully analyze database access code to ensure that any relaxation of isolation does not cause software bugs that are difficult to find. Conversely, if higher isolation levels are used, the possibility of deadlock is increased, which also requires careful analysis and programming techniques to avoid.
The isolation levels defined by the ANSI/ISO SQL standard are listed as follows.

SERIALIZABLE

This is the highest isolation level. It specifies that all transactions occur in a completely isolated fashion, or, in other words, as if all transactions in the system had executed serially, one after the other. The DBMS may execute two or more transactions at the same time only if the illusion of serial execution can be maintained.
With a lock-based concurrency control DBMS implementation, serializability requires read and write locks (acquired on selected data) to be released at the end of the transaction. Also range-locks must be acquired when a SELECT query uses a ranged WHERE clause, especially to avoid the phantom reads phenomenon (see below).
When using non-lock based concurrency control, no locks are acquired; however, if the system detects a write collision among several concurrent transactions, only one of them is allowed to commit. See snapshot isolation for more details on this topic.

REPEATABLE READS

In this isolation level, a lock-based concurrency control DBMS implementation keeps read and write locks (acquired on selected data) until the end of the transaction. However, range-locks are not managed, so the phantom reads phenomenon can occur (see below).

READ COMMITTED

In this isolation level, a lock-based concurrency control DBMS implementation keeps write locks (acquired on selected data) until the end of the transaction, but read locks are released as soon as the SELECT operation is performed (so the non-repeatable reads phenomenon can occur in this isolation level, as discussed below). As in the previous level, range-locks are not managed.

READ UNCOMMITTED

This is the lowest isolation level. In this level, dirty reads are allowed (see below), so one transaction may see not-yet-committed changes made by other transactions.

Default isolation level

The default isolation level of different DBMS's varies quite widely. Most databases that feature transactions allow the user to set any isolation level. Some DBMS's also require additional syntax when performing a SELECT statement to acquire locks (e.g. SELECT ... FOR UPDATE to acquire exclusive write locks on accessed rows).
However, the definitions above have been criticised in the paper A Critique of ANSI SQL Isolation Levels as being ambiguous, and as not accurately reflecting the isolation provided by many databases:

This paper shows a number of weaknesses in the anomaly approach to defining isolation levels. The three ANSI phenomena are ambiguous. Even their broadest interpretations do not exclude anomalous behavior. This leads to some counter-intuitive results. In particular, lock-based isolation levels have different characteristics than their ANSI equivalents. This is disconcerting because commercial database systems typically use locking. Additionally, the ANSI phenomena do not distinguish among several isolation levels popular in commercial systems.

There are also other criticisms concerning ANSI SQL's isolation definition, in that it encourages implementors to do "bad things":

... it relies in subtle ways on an assumption that a locking schema is used for concurrency control, as opposed to an optimistic or multi-version concurrency scheme. This implies that the proposed semantics are ill-defined.^[1]

Read phenomena

The ANSI/ISO standard SQL 92 refers to three different read phenomena when Transaction 1 reads data that Transaction 2 might have changed.
In the following examples, two transactions take place. In the first, Query 1 is performed. Then, in the second transaction, Query 2 is performed and committed. Finally, in the first transaction, Query 1 is performed again.
The queries use the following data table:

users

id	name	age
1	Joe	20
2	Jill	25

Dirty reads

A dirty read occurs when a transaction is allowed to read data from a row that has been modified by another running transaction and not yet committed.
Dirty reads work similarly to non-repeatable reads; however, the second transaction would not need to be committed for the first query to return a different result. The only thing that may be prevented in the READ UNCOMMITTED isolation level is updates appearing out of order in the results; that is, earlier updates will always appear in a result set before later updates.
In our example, Transaction 2 changes a row, but does not commit the changes. Transaction 1 then reads the uncommitted data. Now if Transaction 2 rolls back his changes (already read by Transaction 1) or updates different changes to the database, then the view of the data may be wrong in the records of Transaction 1.

Transaction 1	Transaction 2
/* Query 1 */ SELECT * FROM users WHERE id = 1;
	/* Query 2 */ UPDATE users SET age = 21 WHERE id = 1; /* No commit here */
/* Query 1 */ SELECT * FROM users WHERE id = 1;
	ROLLBACK; /* lock-based DIRTY READ */

Non-repeatable reads

A non-repeatable read occurs, when during the course of a transaction, a row is retrieved twice and the values within the row differ between reads.
Non-repeatable reads phenomenon may occur in a lock-based concurrency control method when read locks are not acquired when performing a SELECT, or when the acquired locks on affected rows are released as soon as the SELECT operation is performed. Under the multiversion concurrency control method, non-repeatable reads may occur when the requirement that a transaction affected by a commit conflict must roll back is relaxed.

Transaction 1	Transaction 2
/* Query 1 */ SELECT * FROM users WHERE id = 1;
	/* Query 2 */ UPDATE users SET age = 21 WHERE id = 1; COMMIT; /* in multiversion concurrency control, or lock-based READ COMMITTED */
/* Query 1 */ SELECT * FROM users WHERE id = 1; COMMIT; /* lock-based REPEATABLE READ */

In this example, Transaction 2 commits successfully, which means that its changes to the row with id 1 should become visible. However, Transaction 1 has already seen a different value for age in that row. At the SERIALIZABLE and REPEATABLE READ isolation levels, the DBMS must return the old value. At READ COMMITTED and READ UNCOMMITTED, the DBMS may return the updated value; this is a non-repeatable read.
There are two basic strategies used to prevent non-repeatable reads. The first is to delay the execution of Transaction 2 until Transaction 1 has committed or rolled back. This method is used when locking is used, and produces the serial schedule T1, T2. A serial schedule does not exhibit non-repeatable reads behaviour.
In the other strategy, as used in multiversion concurrency control, Transaction 2 is permitted to commit first, which provides for better concurrency. However, Transaction 1, which commenced prior to Transaction 2, must continue to operate on a past version of the database — a snapshot of the moment it was started. When Transaction 1 eventually tries to commit, the DBMS checks if the result of committing Transaction 1 would be equivalent to the schedule T1, T2. If it is, then Transaction 1 can proceed. If it cannot be seen to be equivalent, however, Transaction 1 must roll back with a serialization failure.
Using a lock-based concurrency control method, at the REPEATABLE READ isolation mode, the row with ID = 1 would be locked, thus blocking Query 2 until the first transaction was committed or rolled back. In READ COMMITTED mode, the second time Query 1 was executed, the age would have changed.
Under multiversion concurrency control, at the SERIALIZABLE isolation level, both SELECT queries see a snapshot of the database taken at the start of Transaction 1. Therefore, they return the same data. However, if Transaction 1 then attempted to UPDATE that row as well, a serialization failure would occur and Transaction 1 would be forced to roll back.
At the READ COMMITTED isolation level, each query sees a snapshot of the database taken at the start of each query. Therefore, they each see different data for the updated row. No serialization failure is possible in this mode (because no promise of serializability is made), and Transaction 1 will not have to be retried.

Phantom reads

A phantom read occurs when, in the course of a transaction, two identical queries are executed, and the collection of rows returned by the second query is different from the first.
This can occur when range locks are not acquired on performing a SELECT ... WHERE operation. The phantom reads anomaly is a special case of Non-repeatable reads when Transaction 1 repeats a ranged SELECT ... WHERE query and, in the middle of both operations, Transaction 2 creates (i.e. INSERT) new rows (in the target table) fulfilling that WHERE clause.

Transaction 1	Transaction 2
/* Query 1 */ SELECT * FROM users WHERE age BETWEEN 10 AND 30;
	/* Query 2 */ INSERT INTO users VALUES ( 3, 'Bob', 27 ); COMMIT;
/* Query 1 */ SELECT * FROM users WHERE age BETWEEN 10 AND 30;

Note that Transaction 1 executed the same query twice. If the highest level of isolation were maintained, the same set of rows should be returned both times, and indeed that is what is mandated to occur in a database operating at the SQL SERIALIZABLE isolation level. However, at the lesser isolation levels, a different set of rows may be returned the second time.
In the SERIALIZABLE isolation mode, Query 1 would result in all records with age in the range 10 to 30 being locked, thus Query 2 would block until the first transaction was committed. In REPEATABLE READ mode, the range would not be locked, allowing the record to be inserted and the second execution of Query 1 to include the new row in its results.

Isolation Levels, Read Phenomena and Locks

Isolation Levels vs Read Phenomena

Isolation level	Dirty reads	Non-repeatable reads	Phantoms
Read Uncommitted	may occur	may occur	may occur
Read Committed	-	may occur	may occur
Repeatable Read	-	-	may occur
Serializable	-	-	-

"may occur" means that the isolation level suffers that phenomenon, while "-" means that it does not suffer it.

Isolation Levels vs Locks

Isolation level	Write Lock	Read Lock	Range Lock
Read Uncommitted	-	-	-
Read Committed	V	-	-
Repeatable Read	V	V	-
Serializable	V	V	V

"V" indicates that the method locks for that operation, keeping that lock till the end of the transaction containing that operation.
Note: Read (i.e. SELECT) operations can acquire read (shared) locks in the Read Committed isolation level, but they are released immediately after the read operation is performed.