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<h1><a name="kanchor23"></a>Isolation</h1>
<p>Isolation
imposes rules which ensure that transactions do not interfere with each other
even if they are executed at the same time. If two or more transactions are
executed at the same time, they must be executed in a way so that transaction A
won't be impacted by the intermediate data of transaction B. Note that
isolation does not dictate the order of the transactions. Another important
thing to understand about isolation is serializability of transactions. If the
effect on the database is the same when transactions are executed concurrently
or when their execution is interleaved, these transactions are called
serializable.</p>
<p>There are
several degrees of isolation to be distinguished:</p>
<ul>
<li value="1">Degree 0: A
transaction does not overwrite data updated by another user or process
("dirty data") of other transactions.</li>
<li value="2">Degree 1:
Degree 0 plus a transaction does not commit any writes until it completes all
its writes (until the end of transaction).</li>
<li value="3">Degree 2:
Degree 1 plus a transaction does not read dirty data from other transactions.</li>
<li value="4">Degree 3:
Degree 2 plus other transactions do not dirty data read by a transaction before
the transaction commits.</li>
</ul>
<p>The more
common classification is by isolation levels:</p>
<ol>
<li value="1"><strong>Serializable</strong>: In this
case, transactions are executed serially so that there is no concurrent data
access. Transactions can also be executed concurrently but only when the
illusion of serial transactions is maintained (i.e. no concurrent access to
data occurs). If the system uses locks,
a lock should be obtained over the whole range of selected data. If the system does not use locks, no lock is acquired; however,
if the system detects a concurrent transaction in progress, which would violate
the serializability illusion, it must force that transaction to rollback, and
the application will have to restart the transaction.</li>
<li value="2"><strong>Repeatable Read</strong>: In this case, a lock is acquired
over all the data retrieved from a database. Phantom reads can occur (i.e. new
data from the other committed transactions included in the result)</li>
<li value="3"><strong>Read Committed</strong>: In this case, read locks are
acquired on the result set, but released immediately. Write locks are acquired
and released only at the end of the transaction. Non-repeatable reads can occur, i.e. deletions or modifications from the other committed transactions will be visible by the current transaction. Phantom
reads are also possible. This is the model db4o uses.</li>
<li value="4"><strong>Read Uncommitted</strong>: With this isolation level dirty reads are
allowed. Uncommitted modifications from
the other transactions are visible. Both phantom and nonrepeatable reads can
occur.</li>
</ol>
<p>Read more about <a href="http://en.wikipedia.org/wiki/Isolation_level" target="_parent">isolation levels on Wikipedia</a>. </p>
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