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Editing: rep_mgr_ack.html
<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Choosing a Replication Manager Ack Policy</title> <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> <link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" /> <link rel="up" href="rep.html" title="Chapter 12. Berkeley DB Replication" /> <link rel="prev" href="rep_replicate.html" title="Running Replication using the db_replicate Utility" /> <link rel="next" href="rep_elect.html" title="Elections" /> </head> <body> <div xmlns="" class="navheader"> <div class="libver"> <p>Library Version 11.2.5.3</p> </div> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Choosing a Replication Manager Ack Policy</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="rep_replicate.html">Prev</a> </td> <th width="60%" align="center">Chapter 12. Berkeley DB Replication </th> <td width="20%" align="right"> <a accesskey="n" href="rep_elect.html">Next</a></td> </tr> </table> <hr /> </div> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="rep_mgr_ack"></a>Choosing a Replication Manager Ack Policy</h2> </div> </div> </div> <p>Replication Manager allows the user to choose from a variety of acknowledgement policies. There are two characteristics that should be considered when choosing the policy: consistency and durability. Consistency means making sure some number of clients have applied all available master transactions. Durability, in this context, means only indicating success only if enough clients have applied a transaction. The issue of how many is enough depends on the application's requirements and varies per acknowledgement policy. For example, <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_QUORUM" class="olink">DB_REPMGR_ACKS_QUORUM</a> means the data will survive a change in master or a network partition. In most cases, the number of sites for consistency is equal to the number of sites for durability. Replication Manager uses the consistency value to decide whether or not to wait for acknowledgements. Replication manager uses the durability value to decide either the transaction was successfully processed or that a <a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REP_PERM_FAILED" class="olink">DB_EVENT_REP_PERM_FAILED</a> event should be generated.</p> <p>Replication Manager also strives to give the application the answer and return to the application as quickly as possible. Therefore, if it knows that the number of sites connected is insufficient to meet the consistency value, then it does not wait for any acknowledgements and if it knows that the durability value cannot be met, it returns <a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REP_PERM_FAILED" class="olink">DB_EVENT_REP_PERM_FAILED</a> immediately to the user.</p> <p>With one exception, discussed below, all acknowledgement policies combine the consistency and durability values. For most policies the primary purpose is the durability of the data. For example, the <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_QUORUM" class="olink">DB_REPMGR_ACKS_QUORUM</a> policy ensures that, if successful, the transaction's data is safe in the event of a network partition so that a majority of the sites in the group have the data. The <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_NONE" class="olink">DB_REPMGR_ACKS_NONE</a> policy does not consider either consistency or durability, and it is very fast because it does not wait for any acknowledgements and it does not ever trigger the <a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REP_PERM_FAILED" class="olink">DB_EVENT_REP_PERM_FAILED</a> event. Other policies, <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_ALL" class="olink">DB_REPMGR_ACKS_ALL</a> and <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_ALL_PEERS" class="olink">DB_REPMGR_ACKS_ALL_PEERS</a>, have a primary purpose of consistency. These two policies wait for acknowledgements from all (or all electable) sites in the group.</p> <p>In the face of failure, however, the <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_ALL" class="olink">DB_REPMGR_ACKS_ALL</a> and <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_ALL_PEERS" class="olink">DB_REPMGR_ACKS_ALL_PEERS</a> policies can result in a surprising lack of consistency due to the fact that Replication Manager strives to give the answer back to the application as fast as it can. So, for example, with <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_ALL" class="olink">DB_REPMGR_ACKS_ALL</a>, and one site down, Replication Manager knows that disconnected site can never acknowledge, so it immediately triggers <a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REP_PERM_FAILED" class="olink">DB_EVENT_REP_PERM_FAILED</a>. An unfortunate side effect of this policy is that existing, running sites may fall further and further behind the master if the master site is sending a fast, busy stream of transactions and never waiting for any site to send an acknowledgement. The master does not wait because the consistency value cannot be met, and it does trigger the <a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REP_PERM_FAILED" class="olink">DB_EVENT_REP_PERM_FAILED</a> event because the durability value cannot be met, but those actions now affect the consistency of the other running sites.</p> <p>In order to counteract this unfortunate side effect, the <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_ALL_AVAILABLE" class="olink">DB_REPMGR_ACKS_ALL_AVAILABLE</a> acknowledgement policy focuses on the consistency aspect, but also considers durability. This policy uses all sites for consistency, and a quorum of sites for its decision about durability. As long as there is a non-zero number of client replicas to send to, the master will wait for all available sites to acknowledge the transaction. As long as any client site is connected, this policy will prevent the master from racing ahead if one or more sites is down. On the master, this policy will then consider the transaction durable if the number of acknowledgements meets quorum for the group.</p> <p>The following acknowledgement policies determine durability using acknowledgements from electable peers only: <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_QUORUM" class="olink">DB_REPMGR_ACKS_QUORUM</a>, <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_ONE_PEER" class="olink">DB_REPMGR_ACKS_ONE_PEER</a>, <a href="../api_reference/C/repmgrset_ack_policy.html#ackspolicy_DB_REPMGR_ACKS_ALL_PEERS" class="olink">DB_REPMGR_ACKS_ALL_PEERS</a>. An electable peer is a site where the priority value is greater than zero. In replication groups using these policies, an unelectable site does not send acknowledgements and cannot contribute to transaction durability.</p> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="rep_replicate.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="rep.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="rep_elect.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Running Replication using the db_replicate Utility </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Elections</td> </tr> </table> </div> </body> </html>
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