147 lines
9.7 KiB
HTML
147 lines
9.7 KiB
HTML
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<title>Highly available Web server cluster on HTTP Server</title>
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<body id="rzaiehighavailability"><a name="rzaiehighavailability"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Highly available Web server cluster on HTTP Server</h1>
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<div><p>This topic provides information on highly available Web server
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clusters.</p>
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<div class="important"><span class="importanttitle">Important:</span> Information
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for this topic supports the latest PTF levels for HTTP Server for i5/OS .
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It is recommended that you install the latest PTFs to upgrade to the latest
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level of the HTTP Server for i5/OS. Some of the topics documented here are
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not available prior to this update. See <a href="http://www-03.ibm.com/servers/eserver/iseries/software/http/services/service.html" target="_blank">http://www.ibm.com/servers/eserver/iseries/software/http/services/service.htm</a> <img src="www.gif" alt="Link outside Information Center" /> for more information. </div>
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<p>If Web serving is a critical aspect of your business, you may want high
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availability and scalability of your Web server environment. High availability
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and scalability of the Web server environment can be achieved through the
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use of iSeries™ clustering.
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</p>
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<p>The Web server cluster solution can provide: </p>
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<ul><li><strong>Planned downtime</strong>: If a Web server requires planned maintenance,
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it is possible to transfer the work to another node without visible service
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interruptions to the client. </li>
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<li><strong>No unplanned downtime</strong>: If a machine fails, the work is transferred
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to another node with no human involvement and without visible service interruptions
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to the client. </li>
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<li><strong>Scalability</strong>: When employing multiple nodes, it is possible to distribute
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the Web site workload over the cluster nodes. </li>
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</ul>
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<p>Clusters are a collection of complete systems that work together to provide
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a single, unified computing capability. For more information on clusters,
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see <a href="../rzaig/rzaigicclust.htm">Clusters</a> in
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the iSeries Information Center. </p>
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<p>A liveness monitor checks the state of the Web server and interacts with
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the Web server and the clustering resource services in the event that a Web
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server fails (failover), or a manual switchover takes place (ensures no interruption
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of Web server services). The clustered hash table (part of the state replication
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mechanism) can be used to replicate highly available CGI program state data
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across the cluster nodes so that the state data is available to all nodes
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in the event that a Web server fails (failover) or is switched-over manually
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(switchover). To take advantage of this capability, an existing CGI program
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must be enabled in a highly available Web Server environment. CGI programs
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write to the CGI APIs to indicate what data is replicated. </p>
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<p>There are three Web server cluster models that are supported: </p>
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<ul><li>Primary/backup with takeover IP model </li>
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<li>Primary/backup with a network dispatcher model </li>
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<li>Peer model </li>
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</ul>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzaieconcepts.htm" title="This topic provides concepts of functions on HTTP Server and IBM Web Administration for i5/OS interface.">Concepts of functions of HTTP Server</a></div>
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</div>
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</div><div class="nested1" id="takeover"><a name="takeover"><!-- --></a><h2 class="topictitle2">Primary/backup with takeover IP model </h2>
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<div><p>In this model, the Web server runs on the primary and all backup nodes.
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The backup node or nodes are in a idle state, ready to become the primary
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Web server should the primary Web server fail (failover), or a switchover
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takes place. All client requests are always served by the primary node. </p>
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<p>The following diagram illustrates a Primary/backup with takeover IP model.
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</p>
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<br /><img src="rzal8505.gif" alt="Picture of primary/backup with IP-takeover model." /><br /><p>When the primary node fails (failover), or is brought down by the administrator,
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the failover/switchover process begins. The following steps are performed
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during failover/switchover: </p>
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<ol><li>One of the backup servers becomes the primary (the first backup in the
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switchover order). </li>
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<li>The client requests are redirected to the new primary node.</li>
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<li>If the new primary receives a user request that belongs to a long-running-session
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(a CGI program that has been updated to be a highly available CGI program),
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the server will restore the request's state. The new primary retrieves that
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highly available CGI program's state information from the clustered hash table.
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The clustered hash table is part of the state replication mechanism. </li>
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<li>After the failed node recovers, the highly available Web server instance
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can be restarted and it will become the backup system. If the system administrator
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wants the failed node to become primary again, a manual switchover must be
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performed (this can be accomplished with the IBM Simple Cluster Management
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interface available through iSeries Navigator or a business partner tool). </li>
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</ol>
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</div>
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</div>
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<div class="nested1" id="dispatcher"><a name="dispatcher"><!-- --></a><h2 class="topictitle2">Primary/backup with a network dispatcher model </h2>
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<div><p>In this model, just like the primary/backup with takeover IP model, the
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Web server runs on the primary and all backup nodes. The backup nodes are
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in an idle state and all client requests are served by the primary node. A
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network dispatcher (for example the IBM WebSphere<sup>®</sup> Edge Server) sends client
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requests to the Web server. </p>
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<p>The following diagram illustrates a Primary/backup with a network dispatcher
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model. </p>
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<br /><img src="rzal8508.gif" alt="Picture of primary/backup with a network dispatcher model." /><br /><p>When the primary node fails (failover), or a switchover takes place, the
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failover/switchover process begins. The following steps are performed during
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failover/switchover: </p>
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<ol><li>One of the backup servers becomes the primary (the first backup in the
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switchover order). </li>
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<li>The client requests are sent to the new primary node by the network dispatcher.
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</li>
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<li>If the new primary receives a user request that belongs to a long-running-session,
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the server needs to restore the request's state. The new primary searches
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for the state either locally or in the clustered hash table. The clustered
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hash table is part of the state replication mechanism. </li>
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<li>After the failed node recovers, the system administrator can restart the
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Web server instance and it will become a backup Web server. If the system
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administrator wants the failed node to become primary again, a manual switchover
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must be performed. </li>
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</ol>
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<div class="note"><span class="notetitle">Note:</span> A node can join a recovery domain as primary only if the cluster resource
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group is in inactive mode. </div>
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</div>
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</div>
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<div class="nested1" id="peer"><a name="peer"><!-- --></a><h2 class="topictitle2">Peer model </h2>
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<div><p>In this model, there is no declared primary node. All nodes are in an active
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state and serve client requests. A network dispatcher (for example the IBM
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WebSphere Edge Server) evenly distributes requests to different cluster nodes.
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This guarantees distribution of resources in case of heavy load. Linear scalability
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is not guaranteed beyond a small number of nodes. After some number of nodes
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are added, scalability can disappear, and the cluster performance can deteriorate.
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</p>
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<p>The following diagram illustrates the peer model.</p>
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<br /><img src="rzal8506.gif" alt="Picture of peer model." /><br /><p>For more information on Clusters, see <a href="../rzaig/rzaigtroubleshoot.htm">Clustering troubleshooting</a>. For instructions
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on how to set up a highly available Web server, see <a href="rzaieconfigha.htm">Set up and administration of a highly available Web server cluster on HTTP Server (powered by Apache)</a> or <a href="rzaiesetha_cl.htm">Set up a highly available Web server using clustering CL commands
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for HTTP Server</a>. </p>
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</div>
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</div>
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</body>
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