Sindbad~EG File Manager
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<h2 class="title" style="clear: both"><a id="rep_elect"></a>Elections</h2>
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<p>
Replication Manager automatically conducts elections when
necessary, based on configuration information supplied to the
<a href="../api_reference/C/reppriority.html" class="olink">DB_ENV->rep_set_priority()</a> method, unless the application turns off
automatic elections using the <a href="../api_reference/C/repconfig.html" class="olink">DB_ENV->rep_set_config()</a> method.
</p>
<p>
It is the responsibility of a Base API application to
initiate elections if desired. It is never dangerous to hold
an election, as the Berkeley DB election process ensures there
is never more than a single master database environment.
Clients should initiate an election whenever they lose contact
with the master environment, whenever they see a return of
<a href="../api_reference/C/repmessage.html#repmsg_DB_REP_HOLDELECTION" class="olink">DB_REP_HOLDELECTION</a> from the <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a> method, or when,
for whatever reason, they do not know who the master is. It is
not necessary for applications to immediately hold elections
when they start, as any existing master will be discovered
after calling <a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a>. If no master has been found after a
short wait period, then the application should call for an
election.
</p>
<p>
For a client to win an election, the replication group must
currently have no master, and the client must have the most
recent log records. In the case of clients having equivalent
log records, the priority of the database environments
participating in the election will determine the winner. The
application specifies the minimum number of replication group
members that must participate in an election for a winner to
be declared. We recommend at least ((N/2) + 1) members. If
fewer than the simple majority are specified, a warning will
be given.
</p>
<p>
If an application's policy for what site should win an
election can be parameterized in terms of the database
environment's information (that is, the number of sites,
available log records and a relative priority are all that
matter), then Berkeley DB can handle all elections
transparently. However, there are cases where the application
has more complete knowledge and needs to affect the outcome of
elections. For example, applications may choose to handle
master selection, explicitly designating master and client
sites. Applications in these cases may never need to call for
an election. Alternatively, applications may choose to use
<a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a>'s arguments to force the correct outcome to an
election. That is, if an application has three sites, A, B,
and C, and after a failure of C determines that A must become
the winner, the application can guarantee an election's
outcome by specifying priorities appropriately after an
election:
</p>
<pre class="programlisting">on A: priority 100, nsites 2
on B: priority 0, nsites 2</pre>
<p>
It is dangerous to configure more than one master
environment using the <a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a> method, and applications
should be careful not to do so. Applications should only
configure themselves as the master environment if they are the
only possible master, or if they have won an election. An
application knows it has won an election when it receives the
<a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REP_ELECTED" class="olink">DB_EVENT_REP_ELECTED</a> event.
</p>
<p>
Normally, when a master failure is detected it is desired
that an election finish quickly so the application can
continue to service updates. Also, participating sites are
already up and can participate. However, in the case of
restarting a whole group after an administrative shutdown, it
is possible that a slower booting site had later logs than any
other site. To cover that case, an application would like to
give the election more time to ensure all sites have a chance
to participate. Since it is intractable for a starting site to
determine which case the whole group is in, the use of a long
timeout gives all sites a reasonable chance to participate. If
an application wanting full participation sets the <a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a>
method's <span class="bold"><strong>nvotes</strong></span> argument to
the number of sites in the group and one site does not reboot,
a master can never be elected without manual intervention.
</p>
<p>
In those cases, the desired action at a group level is to
hold a full election if all sites crashed and a majority
election if a subset of sites crashed or rebooted. Since an
individual site cannot know which number of votes to require,
a mechanism is available to accomplish this using timeouts. By
setting a long timeout (perhaps on the order of minutes) using
the <span class="bold"><strong>DB_REP_FULL_ELECTION_TIMEOUT</strong></span> flag to the
<a href="../api_reference/C/repset_timeout.html" class="olink">DB_ENV->rep_set_timeout()</a> method, an application can allow Berkeley DB to
elect a master even without full participation. Sites may also
want to set a normal election timeout for majority based
elections using the <span class="bold"><strong>DB_REP_ELECTION_TIMEOUT</strong></span>
flag to the <a href="../api_reference/C/repset_timeout.html" class="olink">DB_ENV->rep_set_timeout()</a> method.
</p>
<p>
Consider 3 sites, A, B, and C where A is the master. In the
case where all three sites crash and all reboot, all sites
will set a timeout for a full election, say 10 minutes, but
only require a majority for <span class="bold"><strong>nvotes</strong></span> to
the <a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a> method. Once all
three sites are booted the election will complete immediately
if they reboot within 10 minutes of each other. Consider if
all three sites crash and only two reboot. The two sites will
enter the election, but after the 10 minute timeout they will
elect with the majority of two sites. Using the full election
timeout sets a threshold for allowing a site to reboot and
rejoin the group.
</p>
<p>
To add a database environment to the replication group with
the intent of it becoming the master, first add it as a
client. Since it may be out-of-date with respect to the
current master, allow it to update itself from the current
master. Then, shut the current master down. Presumably, the
added client will win the subsequent election. If the client
does not win the election, it is likely that it was not given
sufficient time to update itself with respect to the current
master.
</p>
<p>
If a client is unable to find a master or win an election,
it means that the network has been partitioned and there are
not enough environments participating in the election for one
of the participants to win. In this case, the application
should repeatedly call <a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a> and <a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a>, alternating
between attempting to discover an existing master, and holding
an election to declare a new one. In desperate circumstances,
an application could simply declare itself the master by
calling <a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a>, or by reducing the number of participants
required to win an election until the election is won. Neither
of these solutions is recommended: in the case of a network
partition, either of these choices can result in there being
two masters in one replication group, and the databases in the
environment might irretrievably diverge as they are modified
in different ways by the masters.
</p>
<p>
Note that this presents a special problem for a replication
group consisting of only two environments. If a master site
fails, the remaining client can never comprise a majority of
sites in the group. If the client application can reach a
remote network site, or some other external tie-breaker, it
may be able to determine whether it is safe to declare itself
master. Otherwise it must choose between providing
availability of a writable master (at the risk of duplicate
masters), or strict protection against duplicate masters (but
no master when a failure occurs). Replication Manager offers
this choice via the <a href="../api_reference/C/repconfig.html" class="olink">DB_ENV->rep_set_config()</a> method
<a href="../api_reference/C/repconfig.html#config_DB_REPMGR_CONF_2SITE_STRICT" class="olink">DB_REPMGR_CONF_2SITE_STRICT</a> flag. Base API applications can
accomplish this by judicious setting of the <span class="bold"><strong>nvotes</strong></span> and <span class="bold"><strong>nsites</strong></span>
parameters to the <a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a> method.
</p>
<p>
It is possible for a less-preferred database environment to
win an election if a number of systems crash at the same time.
Because an election winner is declared as soon as enough
environments participate in the election, the environment on a
slow booting but well-connected machine might lose to an
environment on a badly connected but faster booting machine.
In the case of a number of environments crashing at the same
time (for example, a set of replicated servers in a single
machine room), applications should bring the database
environments on line as clients initially (which will allow
them to process read queries immediately), and then hold an
election after sufficient time has passed for the slower
booting machines to catch up.
</p>
<p>
If, for any reason, a less-preferred database environment
becomes the master, it is possible to switch masters in a
replicated environment. For example, the preferred master
crashes, and one of the replication group clients becomes the
group master. In order to restore the preferred master to
master status, take the following steps:
</p>
<div class="orderedlist">
<ol type="1">
<li>
The preferred master should reboot and re-join the
replication group as a client.
</li>
<li>
Once the preferred master has caught up with the
replication group, the application on the current master
should complete all active transactions and reconfigure
itself as a client using the <a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a> method.
</li>
<li>
Then, the current or preferred master should call
for an election using the <a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a> method.
</li>
</ol>
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Sindbad File Manager Version 1.0, Coded By Sindbad EG ~ The Terrorists