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<h1><img src="../../backarrow2.gif" alt="Back" width="15"
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(multiple-node systems) — Concepts and terms</h1>
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<p>Use of multiple nodes can provide high system availability
and a degree of fault-tolerance, as the failure or offline
status of one node does not affect the ability of users
connected to another node to continue working.</p>
<p>When a server node is running, the server appears as an
instance of the <cite>Data-Admin-Nodes</cite> class.</p>
<p>You do not need to define instances of this class. The
system creates an instance each time that Process Commander
starts on that server. (You can start and stop the engine
software on each node independent of the state of other
nodes.)</p>
<p>All the nodes share a single common PegaRULES database. The
PegaRULES engine software on each node must be a compatible
version — preferably an identical version — of
Process Commander. </p>
<p>Each node contains an in-memory cache of recently accessed
rules that may, in multiple-node operations, contain different
instances. (The correct and most recent versions of the rules
are always in the database.) Periodic pulse processing
synchronizes the contents of the rule cache on a node to the
latest copy in the PegaRULES database. Optionally, you can
associate a description of each node using
<cite>Data-Admin-Node</cite> instances. The information you
enter appears on the System Nodes Detail display.</p>
<p><img src="../../shared/notestar2.gif" alt="Note" width="16"
height="16" class="inline" />Node names are limited to 32
characters. </p>
<p><img src="../../shared/notestar2.gif" alt="Note" width="16"
height="16" class="inline" />In a multinode Process Commander
system, ensure that the <b>clocks</b> of every node are
synchronized, and the clock on the server hosting the PegaRULES
database is synchronized. Most vendor operating systems offer a
means to accomplish this. Many calculations and decisions such
as time-qualified rules, service level rules, and management
reporting depend on clock settings and intervals. See the Pega
Developer Network article <i><img src="../../shared/pdn.gif"
alt="PDN" width="16" height="16" class="inline" />PRKB-10206
How to synchronize server clocks and database clocks with NTP
for correct service level computations ion</i> for more
information.</p>
<p><img src="../../shared/notestar2.gif" alt="Note" width="16"
height="16" class="inline" />UNIX and Linux-based systems
require special configuration settings to ensure that the JVM
recognizes Daylight Savings time. If not set, the times
recorded in the Process Commander log may not match the server
clock time. See IBM Tech Note "Incorrect time stamps
displayed by an application or in log files" about the TZ
environment variable, and additional details for IBM
WebSphere.</p>
<p><img src="../../shared/notestar2.gif" alt="Note" width="16"
height="16" class="inline" />When two instances of Process
Commander are installed within a single application server (for
example, one for testing and one for production), only one copy
of the <code>pr3native.dll</code> or <code>pr3native.so</code>
library can be loaded. The second copy is not loaded; some
Performance tool statistics may not be available on the copy
started second. </p>
<h3>Other resources</h3>
<p><img src="../../shared/pdn.gif" alt="PDN" width="16" height=
"16" class="inline" />These Pega Developer Network articles are
relevant:</p>
<ul>
<li>PRKB-21845 <i>How to test whether a Process Commander
node is running</i></li>
<li>PRKB-12247 <i>How to configure a reverse proxy
server</i></li>
<li>PRKB-20001 <i>Load balancing a multi-node
cluster</i></li>
<li>PRKB-19914 <i>Recommendations for load balancing and
WebSphere session replication</i></li>
</ul>
<table width="100%" border="0" class="tablegray">
<tr valign="top">
<td width="3%"><img src="../../shared/definition3.gif" alt=
"Definitions" width="13" height="16" /></td>
<td><a href="../../definitions/a/aes.htm">AES</a>, <a href=
"../../definitions/c/cluster.htm">cluster</a>, <a href=
"../../definitions/l/loadbalancing.htm">load-balancing</a>,
<a href=
"../../definitions/h/horizontalcluster.htm">horizontal
cluster</a>, <a href=
"../../definitions/n/node.htm">node</a>, <a href=
"../../definitions/n/nodeid.htm">node ID</a>, <a href=
"../../definitions/p/pulseinterval.htm">pulse</a>, <a href=
"../../definitions/s/systemid.htm">system ID</a>, <a href=
"../../definitions/t/timequalifiedrule.htm">time-qualified
rules</a>, <a href=
"../../definitions/v/verticalcluster.htm">vertical
cluster</a></td>
</tr>
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<td><img src="../../shared/topic.gif" alt="Related topics"
width="13" height="16" /></td>
<td><a href=
"../../data-/data-admin-/data-admin-system/main.htm">About
the System data instance</a><br />
<a href=
"../../data-/data-admin-/data-admin-nodes/main.htm">About
System Node data instances</a></td>
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