2011-09-13 17:44:24 +00:00
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Clock Skew</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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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="rep.html" title="Chapter 12. Berkeley DB Replication" />
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<link rel="prev" href="rep_ryw.html" title="Read your writes consistency" />
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<link rel="next" href="repmgr_channels.html" title="Using Replication Manager message channels" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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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">Clock Skew</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="rep_ryw.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 12.
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Berkeley DB Replication
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="repmgr_channels.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="rep_clock_skew"></a>Clock Skew</h2>
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</div>
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</div>
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</div>
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<p>Since master leases take into account a timeout that is used across
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all sites in a replication group, leases must also take into account
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any known skew (or drift) between the clocks on different machines
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in the group. The guarantees provided by master leases take clock
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skew into account. Consider a replication group where a client's
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clock is running faster than the master's clock and the group has
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a lease timeout of 5 seconds. If clock skew is not taken into
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account, eventually, the client will believe that 5 seconds have
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passed faster than the master and that client may then grant its
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lease to another site. Meanwhile, the master site does not believe
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5 seconds have passed because its clock is slower, and it believes
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it still holds a valid lease grant. For this reason, Berkeley DB compensates
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for clock skew.</p>
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<p>The master of a group using leases must account for skew
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in case that site has the slowest clock in the group. This computation
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avoids the problem of a master site believing a lease grant is valid
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too long. Clients in a group must account for skew in case they
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have the fastest clock in the group. This computation avoids
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the problem of a client site expiring its grant too soon and
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potentially granting a lease to a different site. Berkeley DB uses
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a conservative computation and accounts for clock skew on both
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sides, yielding a double compensation.</p>
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<p>The <a href="../api_reference/C/repclockskew.html" class="olink">DB_ENV->rep_set_clockskew()</a> method takes the values for both the fastest
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and slowest clocks in the entire replication group as parameters.
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The values passed in must be the same for all sites in the group.
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If the user knows the maximum clock drift of their sites, then those
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values can be expressed as a relative percentage. Or, if the user
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runs an experiment then the actual values can be used.</p>
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<p>For example, suppose the user knows that there is a maximum drift
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rate of 2% among sites in the group. The user should pass in
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102 and 100 for the fast and slow clock values respectively.
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That is an unusually large value, so suppose, for example, the
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rate is 0.03% among sites in the group. The user should pass in
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10003 and 10000 for the fast and slow clock values. Those values
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can be used to express the level of precision the user needs.</p>
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<p>An example of an experiment a user can run to help determine skew
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would be to write a program that started simultaneously on all
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sites in the group. Suppose, after 1 day (86400 seconds), one
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site shows 86400 seconds and the other site shows it ran faster
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and it indicates 86460 seconds has passed.
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The user can use 86460 and 86400 for their parameters for the
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fast and slow clock values.</p>
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<p>Since Berkeley DB is using those fast and slow clock values to compute
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a ratio internally, if the user cannot detect or measure any
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clock skew, then the same value should be passed in for both
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parameters, such as 1 and 1.</p>
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</div>
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="rep_ryw.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="rep.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="repmgr_channels.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Read your writes consistency </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"> Using Replication Manager message channels</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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