<div dir="ltr"><span style="font-family:arial,sans-serif;font-size:13px">Hello,</span><div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">I have some clients who are currently using <span class="">pcp_promote_node</span> with DISALLOW_TO_FAILOVER.</div>
<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">They have a two <span class="">node</span> PostgreSQL cluster which is managed by EnterpriseDB's PPFM (Postgres Plus Failover Manager). This is a requirement.</div>
<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">When a failover happens and PPFM promotes the new slave it fences the old master with the <span class="">pcp_promote_node</span>command.</div>
<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">This is desirable because</div><div style="font-family:arial,sans-serif;font-size:13px"><ol><li style="margin-left:15px">
It detaches the old master</li><li style="margin-left:15px">It promotes the promotion target to master </li><li style="margin-left:15px">It does not block while searching for the new master (as would occur with pcp_detach_node)</li>
</ol><div>After <span class="">pcp_promote_node</span> is called, the PostgreSQL failover occurs. This does mean a short period in which the pgpool master is not promoted, but this seems to be tolerated by the software.</div>
</div><div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">Cheers,</div><div><br></div></div><div class="gmail_extra"><br clear="all"><div><div style="color:#b8276a;font:bold 14px Arial,Helvetica,sans-serif">
<br>
<span style="color:#323b62">James Sewell,</span><br>
<span style="color:#a9a9a9"><span style="font:12px Arial,Helvetica,sans-serif">PostgreSQL Team Lead / Solutions Architect </span></span><br>
<span style="color:#a9a9a9">______________________________________</span><br>
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<br><br><div class="gmail_quote">On Thu, Jun 26, 2014 at 4:54 PM, Yugo Nagata <span dir="ltr"><<a href="mailto:nagata@sraoss.co.jp" target="_blank">nagata@sraoss.co.jp</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi all,<br>
<br>
Are you using pcp_promote_node? If so, please tell me your usecase!<br>
<br>
pgpool-II has pcp_promote_node command, which change specified backend<br>
nodes status to 'primary', but this has some problems.<br>
So, if there is no users of pcp_promote_node, we consider to remove<br>
this from pgpool-II. Or not, I want to know usecases and fix<br>
pcp_promote_node to be more usefull and suitable for the usecase.<br>
<br>
The current pcp_promote_node works as below;<br>
<br>
(0) This is enabled only in master-slave / streaming-replication mode.<br>
<br>
(1) This changes pgpool-II's internal status and set the specified<br>
node to primary. The internal status is used in online-recovery<br>
and loadbalancing etc..<br>
** Note that this doesn't control the backend nodes themselves.**<br>
<br>
(2) All nodes other than the new primary are detached from pgpool-II.<br>
<br>
(3) pgpool-II execute follow_master command for all the detached nodes.<br>
follow_master command is mainly used for auto-recovery of standbys<br>
from the new primary.<br>
<br>
Here, I found the some problems:<br>
<br>
(a) Even when DISALLOW_TO_FAILOVER is used, backends node are detached<br>
at step (2).<br>
(b) If pcp_recovery_node is executed in follow_master command at step (3),<br>
recovery can fail because pgpool-II's internal status can be different<br>
from the acutual backend status of primary/standby.<br>
<br>
I want to fix about (a), if there is any user of pcp_promote_node.<br>
I think the design would be:<br>
- When DISALLOW_TO_FAILOVER is used, pcp_promote_node is disabled<br>
because this command can detach some backend nodes.<br>
- or, pcp_promote_node change primary node status, but doesn't detach<br>
any nodes. (In this case, some trick would be required so that<br>
primary node should be unique.)<br>
<br>
About (b), I think this is a restriction of pgpool-II. User have<br>
to note that internal status can be different from the acutual backend<br>
status of primary/standby, after using pcp_promote_node. If you know<br>
other idea, please tell me.<br>
<br>
Any comment and suggestion?<br>
<br>
<br>
Best regards,<br>
<span class="HOEnZb"><font color="#888888">--<br>
Yugo Nagata <<a href="mailto:nagata@sraoss.co.jp">nagata@sraoss.co.jp</a>><br>
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</font></span></blockquote></div><br></div>
<br>
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