2011-09-13 17:44:24 +00:00
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<title>Degrees of isolation</title>
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="transapp.html" title="Chapter 11. Berkeley DB Transactional Data Store Applications" />
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<link rel="prev" href="transapp_inc.html" title="Isolation" />
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<link rel="next" href="transapp_cursor.html" title="Transactional cursors" />
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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Degrees of isolation</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="transapp_inc.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 11.
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Berkeley DB Transactional Data Store Applications
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="transapp_cursor.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="transapp_read"></a>Degrees of isolation</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="transapp_read.html#snapshot_isolation">Snapshot Isolation</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>Transactions can be isolated from each other to different degrees.
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<span class="emphasis"><em>Serializable</em></span> provides the most isolation, and means that, for
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the life of the transaction, every time a thread of control reads a data
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item, it will be unchanged from its previous value (assuming, of course,
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the thread of control does not itself modify the item). By default,
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Berkeley DB enforces serializability whenever database reads are wrapped in
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transactions. This is also known as <span class="emphasis"><em>degree 3 isolation</em></span>.</p>
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<p>Most applications do not need to enclose all reads in transactions, and
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when possible, transactionally protected reads at serializable isolation
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should be avoided as they can cause performance problems. For example,
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a serializable cursor sequentially reading each key/data pair in a
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database, will acquire a read lock on most of the pages in the database
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and so will gradually block all write operations on the databases until
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the transaction commits or aborts. Note, however, that if there are
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update transactions present in the application, the read operations must
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still use locking, and must be prepared to repeat any operation
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(possibly closing and reopening a cursor) that fails with a return value
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of <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>.
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Applications that need repeatable reads
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are ones that require the ability to repeatedly access a data item
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knowing that it will not have changed (for example, an operation
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modifying a data item based on its existing value).</p>
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<p><span class="emphasis"><em>Snapshot isolation</em></span> also guarantees repeatable reads, but
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avoids read locks by using multiversion concurrency control (MVCC).
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This makes update operations more expensive, because they have to
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allocate space for new versions of pages in cache and make copies, but
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avoiding read locks can significantly increase throughput for many
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applications. Snapshot isolation is discussed in detail below.</p>
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<p>A transaction may only require
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<span class="emphasis"><em>cursor stability</em></span>, that is only
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be guaranteed that cursors see committed data that does not change so
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long as it is addressed by the cursor, but may change before the reading
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transaction completes. This is also called <span class="emphasis"><em>degree 2
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isolation</em></span>. Berkeley DB provides this level of isolation when a transaction
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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
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be specified when opening a cursor within a fully isolated
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transaction.</p>
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<p>Berkeley DB optionally supports reading uncommitted data; that is, read
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operations may request data which has been modified but not yet
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committed by another transaction. This is also called <span class="emphasis"><em>degree
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1 isolation</em></span>. This is done by first specifying the
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<a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a> flag when opening the underlying database,
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and then specifying the <a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a> flag when beginning
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a transaction, opening a cursor, or performing a read operation. The
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advantage of using <a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a> is that read operations
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will not block when another transaction holds a write lock on the
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requested data; the disadvantage is that read operations may return data
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that will disappear should the transaction holding the write lock
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abort.</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="snapshot_isolation"></a>Snapshot Isolation</h3>
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</div>
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</div>
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</div>
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<p>To make use of snapshot isolation, databases must first be configured
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for multiversion access by calling <a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a> with the
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<a href="../api_reference/C/dbopen.html#dbopen_DB_MULTIVERSION" class="olink">DB_MULTIVERSION</a> flag. Then transactions or cursors must be
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configured with the <a href="../api_reference/C/txnbegin.html#txnbegin_DB_TXN_SNAPSHOT" class="olink">DB_TXN_SNAPSHOT</a> flag.</p>
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<p>When configuring an environment for snapshot isolation, it is important
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to realize that having multiple versions of pages in cache means that
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the working set will take up more of the cache. As a result, snapshot
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isolation is best suited for use with larger cache sizes.</p>
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<p>If the cache becomes full of page copies before the old copies can be
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discarded, additional I/O will occur as pages are written to temporary
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"freezer" files. This can substantially reduce throughput, and should
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be avoided if possible by configuring a large cache and keeping snapshot
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isolation transactions short. The amount of cache required to avoid
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freezing buffers can be estimated by taking a checkpoint followed by a
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call to <a href="../api_reference/C/logarchive.html" class="olink">DB_ENV->log_archive()</a>. The amount of cache required is
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approximately double the size of logs that remains.</p>
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<p>The environment should also be configured for sufficient transactions
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using <a href="../api_reference/C/envset_tx_max.html" class="olink">DB_ENV->set_tx_max()</a>. The maximum number of transactions
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needs to include all transactions executed concurrently by the
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application plus all cursors configured for snapshot isolation.
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Further, the transactions are retained until the last page they created
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is evicted from cache, so in the extreme case, an additional transaction
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may be needed for each page in the cache. Note that cache sizes under
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500MB are increased by 25%, so the calculation of number of pages needs
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to take this into account.</p>
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<p>So when <span class="emphasis"><em>should</em></span> applications use snapshot isolation?</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>There is a large cache relative to size of updates performed by
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concurrent transactions; and</li>
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<li>Read/write contention is limiting the throughput of the application;
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or</li>
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<li>The application is all or mostly read-only, and contention for the lock
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manager mutex is limiting throughput.</li>
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</ul>
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</div>
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<p>The simplest way to take advantage of snapshot isolation is for queries:
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keep update transactions using full read/write locking and set
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<a href="../api_reference/C/txnbegin.html#txnbegin_DB_TXN_SNAPSHOT" class="olink">DB_TXN_SNAPSHOT</a> on read-only transactions or cursors. This
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should minimize blocking of snapshot isolation transactions and will
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avoid introducing new
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<a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>
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errors.</p>
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<p>If the application has update transactions which read many items and
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only update a small set (for example, scanning until a desired record is
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found, then modifying it), throughput may be improved by running some
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updates at snapshot isolation as well.</p>
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</div>
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<td width="40%" align="left"><a accesskey="p" href="transapp_inc.html">Prev</a> </td>
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<a accesskey="u" href="transapp.html">Up</a>
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<td width="40%" align="right"> <a accesskey="n" href="transapp_cursor.html">Next</a></td>
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<td width="40%" align="left" valign="top">Isolation </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Transactional cursors</td>
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</tr>
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