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<th colspan="3" align="center">Degrees of isolation</th>
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<h2 class="title" style="clear: both"><a id="transapp_read"></a>Degrees of isolation</h2>
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<dl>
<dt>
<span class="sect2">
<a href="transapp_read.html#snapshot_isolation">Snapshot Isolation</a>
</span>
</dt>
</dl>
</div>
<p>
Transactions can be isolated from each other to different
degrees. <span class="emphasis"><em>Serializable</em></span> provides the most
isolation, and means that, for the life of the transaction,
every time a thread of control reads a data item, it will be
unchanged from its previous value (assuming, of course, the
thread of control does not itself modify the item). By
default, Berkeley DB enforces serializability whenever
database reads are wrapped in transactions. This is also known
as <span class="emphasis"><em>degree 3 isolation</em></span>.
</p>
<p>
Most applications do not need to enclose all reads in
transactions, and when possible, transactionally protected
reads at serializable isolation should be avoided as they can
cause performance problems. For example, a serializable cursor
sequentially reading each key/data pair in a database, will
acquire a read lock on most of the pages in the database and
so will gradually block all write operations on the databases
until the transaction commits or aborts. Note, however, that
if there are update transactions present in the application,
the read operations must still use locking, and must be
prepared to repeat any operation (possibly closing and
reopening a cursor) that fails with a return value of <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>.
Applications that need repeatable reads are ones that require
the ability to repeatedly access a data item knowing that it will
not have changed (for example, an operation modifying a data item based
on its existing value).
</p>
<p>
<span class="emphasis"><em>Snapshot isolation</em></span> also guarantees
repeatable reads, but avoids read locks by using multiversion
concurrency control (MVCC). This makes update operations more
expensive, because they have to allocate space for new
versions of pages in cache and make copies, but avoiding read
locks can significantly increase throughput for many
applications. Snapshot isolation is discussed in detail
below.
</p>
<p>
A transaction may only require <span class="emphasis"><em>cursor
stability</em></span>, that is only be guaranteed that
cursors see committed data that does not change so long as it
is addressed by the cursor, but may change before the reading
transaction completes. This is also called <span class="emphasis"><em>degree 2
isolation</em></span>. Berkeley DB provides this level of
isolation when a transaction is started with the
<a href="../api_reference/C/dbcget.html#dbcget_DB_READ_COMMITTED" class="olink">DB_READ_COMMITTED</a> flag. This flag may also be specified when
opening a cursor within a fully isolated transaction.
</p>
<p>
Berkeley DB optionally supports reading uncommitted data;
that is, read operations may request data which has been
modified but not yet committed by another transaction. This is
also called <span class="emphasis"><em>degree 1 isolation</em></span>. This is
done by first specifying the <a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a> flag when
opening the underlying database, and then specifying the
<a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a> flag when beginning a transaction,
opening a cursor, or performing a read operation. The
advantage of using <a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a> is that read
operations will not block when another transaction holds a
write lock on the requested data; the disadvantage is that
read operations may return data that will disappear should the
transaction holding the write lock abort.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="snapshot_isolation"></a>Snapshot Isolation</h3>
</div>
</div>
</div>
<p>
To make use of snapshot isolation, databases must first
be configured for multiversion access by calling <a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a>
with the <a href="../api_reference/C/dbopen.html#dbopen_DB_MULTIVERSION" class="olink">DB_MULTIVERSION</a> flag. Then transactions or
cursors must be configured with the <a href="../api_reference/C/txnbegin.html#txnbegin_DB_TXN_SNAPSHOT" class="olink">DB_TXN_SNAPSHOT</a>
flag.
</p>
<p>
When configuring an environment for snapshot isolation,
it is important to realize that having multiple versions
of pages in cache means that the working set will take up
more of the cache. As a result, snapshot isolation is best
suited for use with larger cache sizes.
</p>
<p>
If the cache becomes full of page copies before the old
copies can be discarded, additional I/O will occur as
pages are written to temporary "freezer" files. This can
substantially reduce throughput, and should be avoided if
possible by configuring a large cache and keeping snapshot
isolation transactions short. The amount of cache required
to avoid freezing buffers can be estimated by taking a
checkpoint followed by a call to <a href="../api_reference/C/logarchive.html" class="olink">DB_ENV->log_archive()</a>. The amount
of cache required is approximately double the size of logs
that remains.
</p>
<p>
The environment should also be configured for sufficient
transactions using <a href="../api_reference/C/envset_tx_max.html" class="olink">DB_ENV->set_tx_max()</a>. The maximum number of
transactions needs to include all transactions executed
concurrently by the application plus all cursors
configured for snapshot isolation. Further, the
transactions are retained until the last page they created
is evicted from cache, so in the extreme case, an
additional transaction may be needed for each page in the
cache. Note that cache sizes under 500MB are increased by
25%, so the calculation of number of pages needs to take
this into account.
</p>
<p>
So when <span class="emphasis"><em>should</em></span> applications use
snapshot isolation?
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
There is a large cache relative to size of
updates performed by concurrent transactions;
and
</li>
<li>
Read/write contention is limiting the throughput
of the application; or
</li>
<li>
The application is all or mostly read-only, and
contention for the lock manager mutex is limiting
throughput.
</li>
</ul>
</div>
<p>
The simplest way to take advantage of snapshot isolation
is for queries: keep update transactions using full
read/write locking and set <a href="../api_reference/C/txnbegin.html#txnbegin_DB_TXN_SNAPSHOT" class="olink">DB_TXN_SNAPSHOT</a> on read-only
transactions or cursors. This should minimize blocking of
snapshot isolation transactions and will avoid introducing
new <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> errors.
</p>
<p>
If the application has update transactions which read
many items and only update a small set (for example,
scanning until a desired record is found, then modifying
it), throughput may be improved by running some updates at
snapshot isolation as well.
</p>
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