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<?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">
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<title>Managing Replication Manager Group Membership</title>
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<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">Managing Replication Manager Group Membership</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="rep_newsite.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_filename.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="group_membership"></a>Managing Replication Manager Group Membership</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="group_membership.html#group_mem_add">Adding Sites to a Replication Group</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="group_membership.html#group_mem_remove">Removing Sites from a Replication Group</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="group_membership.html#group_mem_primordialstartup">Primordial Startups</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="group_membership.html#group_mem_upgrade">Upgrading Groups</a>
</span>
</dt>
</dl>
</div>
<p>
A replication group is a collection of two or more database
environments which are configured to replicate with one another.
When operating normally, a replication group consists of a master
site and one or more read-only sites.
</p>
<p>
For Replication Manager applications, the sites comprising the
replication group are recorded in an internal database, so even if
a group member is not available, it counts towards the group's
total site count. This matters for certain replication activities,
such as holding elections and acknowledging replication messages
that require some number of sites to participate in these
activities. Replicated applications will often require all sites,
or a majority of sites, to participate before the activity can be
completed.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
If you are configuring your application to keep replication
metadata in-memory by specifying the <a href="../api_reference/C/repconfig.html#config_DB_REP_CONF_INMEM" class="olink">DB_REP_CONF_INMEM</a> flag to the
<a href="../api_reference/C/repconfig.html" class="olink">DB_ENV->rep_set_config()</a> method, then the internal database containing group
site information is not stored persistently on disk. This
severely limits Replication Manager's ability to automatically
manage group membership. For more information, including some
work-arounds, see
<a class="xref" href="rep_filename.html" title="Managing Replication Files">Managing Replication Files</a>.
</p>
</div>
<p>
Because Replication Manager tracks group members, there are some
administrative activities that you should know about when using BDB
replication.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="group_mem_add"></a>Adding Sites to a Replication Group</h3>
</div>
</div>
</div>
<p>
To add a site to a replication group, you merely start up the
site such that it knows where at least one site in the group is
located. The site new site then joins the group. When this
happens, the new site is recorded in the Replication Manager's
group member database.
</p>
<p>
Note that when you are starting the very first site in the
group for the very first time (called <span class="emphasis"><em>the primordial
start up</em></span>), there are no other existing sites to
help the new site join the group. In fact, a primordial start up
actually creates the group. For this reason, there are some
slight differences on how to perform a primordial start up. For
a description of this, see
<a class="xref" href="group_membership.html#group_mem_primordialstartup" title="Primordial Startups">Primordial Startups</a>.
</p>
<p>
When you add a site to a replication group, you use the
following general procedure:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Make sure your replication group is operating well
enough that write activity can occur.
</p>
</li>
<li>
<p>
Create and open the environment such that it is configured to
use replication.
</p>
</li>
<li>
<p>
Use <a href="../api_reference/C/repmgr_site.html" class="olink">DB_ENV->repmgr_site()</a> to obtain a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle.
Configure this handle for the local site's host and
port information when you create the handle. Then, use
<a href="../api_reference/C/dbsite_set_config.html" class="olink">DB_SITE->set_config()</a> to indicate that this is the local
site by setting the <code class="literal">DB_LOCAL_SITE</code>
parameter.
</p>
</li>
<li>
<p>
Use <a href="../api_reference/C/repmgr_site.html" class="olink">DB_ENV->repmgr_site()</a> to obtain a second <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle.
Configure this handle with the host and port
information for a site that already belongs to the
replication group. Then, use <a href="../api_reference/C/dbsite_set_config.html" class="olink">DB_SITE->set_config()</a> to
indicate this site is a "helper" site by setting the
<code class="literal">DB_BOOTSTRAP_HELPER</code> parameter. By
configuring a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle in this way, your new
site will know how to contact the replication group so
that it can join the group.
</p>
</li>
<li>
<p>
Start replication as normal by configuring an
acknowledgement policy, setting the site's replication
priority, and then calling <a href="../api_reference/C/repmgrstart.html" class="olink">DB_ENV->repmgr_start()</a>.
</p>
</li>
</ul>
</div>
<p>
Note that on subsequent start-ups of your replication code, any
helper site information you might provide is ignored because
the Replication Manager reads the group membership database
in order to obtain this information.
</p>
<p>
Also, be aware that if the new site cannot be added to the
group for some reason (because a master site is not available,
or because insufficient replicas are running to acknowledge the
new site), the attempt to start the new site via <a href="../api_reference/C/repmgrstart.html" class="olink">DB_ENV->repmgr_start()</a>
will fail and return <code class="literal">DB_REP_UNAVAIL</code>. You can
then pause and retry the start up attempt until it completes
successfully.
</p>
<p>
You must use the exact same host string and port number to refer
to a given site throughout your application and on each of its
sites.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="group_mem_remove"></a>Removing Sites from a Replication Group</h3>
</div>
</div>
</div>
<p>
Elections and message acknowledgements require knowledge of the
total number of sites in the group. If a site is shut down, or
is otherwise unable to communicate with the rest of the group,
it still counts towards the total number of sites in the group.
In most cases, this is the desirable behavior.
</p>
<p>
However, if you are shutting down a site permanently, then you
should remove that site from the group. You might also want to
remove a site from the group if you are shutting it down
temporarily, but nevertheless for a very long period of time
(days or weeks). In either case, you remove a site from the
group by:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Make sure your replication group is operating well
enough that write activity can occur.
</p>
</li>
<li>
<p>
On one of the sites in your replication group (this
does not have to be the master site), use <a href="../api_reference/C/repmgr_site.html" class="olink">DB_ENV->repmgr_site()</a>
to obtain a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle. Configure this handle with
the host and port information of the site that you want
to remove.
</p>
<p>
Note that this step can occur at any site —
including the site that you are removing from the
group.
</p>
</li>
<li>
<p>
Call the <a href="../api_reference/C/dbsite_remove.html" class="olink">DB_SITE->remove()</a> method. This removes the
identified site from the replication group database. If
this action is not performed on the master site, the
client sends a request to the master to perform the
operation and awaits confirmation.
</p>
</li>
</ul>
</div>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Upon completing the above procedure, DO NOT call the
<a href="../api_reference/C/dbsite_close.html" class="olink">DB_SITE->close()</a> method. After removing (or even attempting
to remove) a site from the group using a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle,
the handle must never be accessed again.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="group_mem_primordialstartup"></a>Primordial Startups</h3>
</div>
</div>
</div>
<p>
If you have never started a site in a replication group before,
then the replication group membership database does not exist.
In this situation, you must start the site and declare it to be
the group creator. This causes the site to become the master,
create the group membership database, and create a replication
group of size 1. After that, subsequent sites can add
themselves to the group as described in
<a class="xref" href="group_membership.html#group_mem_add" title="Adding Sites to a Replication Group">Adding Sites to a Replication Group</a>.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
It is never incorrect to declare a site the group creator.
This is true even well-after the replication group has been
established. This is because group creator information is
ignored on any site start-up, except for the primoridial
start-up; that is, a start-up where the group membership
database does not exist.
</p>
</div>
<p>
To declare a site as the group creator:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Create and open the environment such that it is configured to
use replication.
</p>
</li>
<li>
<p>
Use <a href="../api_reference/C/repmgr_site.html" class="olink">DB_ENV->repmgr_site()</a> to obtain a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle.
Configure this handle for the local site's host and
port information when you create the handle. Then, use
<a href="../api_reference/C/dbsite_set_config.html" class="olink">DB_SITE->set_config()</a> to indicate that this is the group
creator site by setting the <code class="literal">DB_GROUP_CREATOR</code>
parameter.
</p>
</li>
<li>
<p>
Start replication as normal by configuring
acknowledgement policies, setting replication
priorities for the site, and then calling
<a href="../api_reference/C/repmgrstart.html" class="olink">DB_ENV->repmgr_start()</a>.
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="group_mem_upgrade"></a>Upgrading Groups</h3>
</div>
</div>
</div>
<p>
Prior to the Berkeley DB 11.2.5.2 release, replication group
membership was managed differently than in the way it is
described in the previous sections. For this reason, when you
upgrade from older releases of Berkeley DB to 11.2.5.2 or
later, the upgrade procedure is different than when upgrading
between other releases.
</p>
<p>
To perform an upgrade that takes you from the old way of
managing group membership to the new way of managing group
membership (pre-11.2.5.2 to 11.2.5.2 and later), do the
following:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Update your replication code to use the new <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a>
handle and related methods. Recompile and thoroughly
test your code to make sure it is production-ready.
</p>
</li>
<li>
<p>
Do the following one production machine at a time. Make
sure to do this at the master site LAST.
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Shut down the old replication code.
</p>
</li>
<li>
<p>
Install the new replication code.
</p>
</li>
<li>
<p>
Configure a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle for the local site.
Use <a href="../api_reference/C/dbsite_set_config.html" class="olink">DB_SITE->set_config()</a> to indicate that this
is a legacy site by setting the
<code class="literal">DB_LEGACY</code> parameter.
</p>
</li>
<li>
<p>
Configure a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle for <span class="emphasis"><em>every
other site</em></span> in the replication
group. Set the <code class="literal">DB_LEGACY</code>
parameter for each of these handles.
</p>
<p>
Please pay careful attention to this step. To
repeat: a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle MUST be configured
for EVERY site in the replication group.
</p>
</li>
<li>
<p>
Start replication. The site is upgraded at this
point.
</p>
</li>
</ol>
</div>
<p>
Once you have performed this procedure for each
production site, making sure to upgrade the master only
after every other site has been upgraded, you are done
upgrading your replicated application to use the
current group membership mechanism.
</p>
</li>
</ul>
</div>
<p>
On subsequent restarts of your replication code, you do not
need to specify the <code class="literal">DB_LEGACY</code> parameter, nor
do you need to identify all of the replication group members.
However, it is not an error if you do specify this information
on subsequent start ups.
</p>
</div>
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