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From "Torsten Mielke (Confluence)" <conflue...@apache.org>
Subject [CONF] Apache ActiveMQ > Pluggable storage lockers
Date Thu, 20 Mar 2014 08:23:01 GMT
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                                                            <a href="    https://cwiki.apache.org/confluence/display/~tmielke
" style="color:#326ca6;text-decoration:none;; color: #326ca6; text-decoration: none">Torsten
Mielke</a> edited the page:
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                                                    <td valign="top" style="font-size:
13px"> <h2 id="email-title-heading" style="font-size: 16px; line-height: 20px; min-height:
20px; margin: 0; padding: 0"> <a href="https://cwiki.apache.org/confluence/display/ACTIVEMQ/Pluggable+storage+lockers"
style="color: #326ca6; text-decoration: none"> <img class="icon" src="cid:page-icon"
alt="" style="border: 0; padding: 0 5px 0 0; text-align: left; vertical-align: middle" />
<strong style="font-size:16px;line-height:20px;vertical-align:top;">Pluggable storage
lockers</strong> </a> </h2> </td>
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                                            <p class="aui-message info" style="font-size:
13px; margin: 1em 0; padding: 8px; background-color: #e0f0ff; border: 1px solid #9eb6d4">
<b>Comment:</b> Tried to make it clear that the default db locker uses a long
running tx against the db, which also consumes resources on the db. </p>
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                                                    <p class="diff-context-placeholder"
style="font-size: 13px">...</p>
                                                    <p class="diff-block-target diff-block-context"
style="font-size: 13px">The Database Locker uses its&nbsp;<code style="font-size:
13px">keepAlive</code> method to ensure the broker still holds the lock. You can
set the keep alive period using the <code style="font-size: 13px">lockKeepAlivePeriod</code>
property. The default period is 30000 ms. <span class="diff-html-added" id="added-diff-0"
style="font-size: 100%; background-color: #ddfade;">If a broker fails to acquire the lock
on the database, it will retry every lockAcquireSleepInterval milliseconds.</span> </p>
                                                    <p class="diff-block-target diff-block-context"
style="font-size: 13px"> <span class="diff-html-added" id="added-diff-1" style="font-size:
100%; background-color: #ddfade;">This locker opens a JDBC transaction against a database
table (activemq_lock) that lasts as long as the broker remains alive. This locks the entire
table and prevents another broker from accessing the store. In most cases this will be a fairly
long running JDBC transaction which occupies resources on the database over time.</span>
</p>
                                                    <p class="diff-block-target diff-block-context"
style="font-size: 13px">A problem with this locker can occur when the master broker crashes
or loses its connection to the database. The information about the lock remains in the database,
until the database responds to the half closed socket connection via a tcp timeout. The database
lock expiry requirement can prevent the slave from starting for a period. In addition, <span
class="diff-html-removed" id="removed-diff-0" style="font-size: 100%; background-color: #ffe7e7;
text-decoration: line-through;">where </span><span class="diff-html-added" id="added-diff-2"
style="font-size: 100%; background-color: #ddfade;">if </span>the database supports
failover, and the connection is dropped in the event of a replica failover, <span class="diff-html-removed"
id="removed-diff-1" style="font-size: 100%; background-color: #ffe7e7; text-decoration: line-through;">the
</span><span class="diff-html-added" id="added-diff-3" style="font-size: 100%; background-color:
#ddfade;">that JDBC transaction will be rolled back. The </span>broker sees this
as a failure and both master and slave will again compete for a lock.</p>
                                                    <p class="diff-context-placeholder"
style="font-size: 13px">...</p>
                                                    <p class="diff-block-target" style="font-size:
13px">The Lease Database Locker solves the master/slave problems of the default Database
Locker. It does <span class="diff-html-removed" id="removed-diff-2" style="font-size: 100%;
background-color: #ffe7e7; text-decoration: line-through;">so by having </span><span
class="diff-html-added" id="added-diff-4" style="font-size: 100%; background-color: #ddfade;">not
open a long running JDBC transaction but lets </span>the master broker acquire a lock
that's only valid for a short period. To retain the lock the master broker must periodically
extend the lock's lease. The slave broker also checks periodically to see if the lease has
expired. The lease can survive a db replica failover. You can configure it like this:</p>
                                                    <p class="diff-context-placeholder"
style="font-size: 13px">...</p>
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