109 lines
6.4 KiB
HTML
109 lines
6.4 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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<meta name="DC.Title" content="Guidelines for writing high availability CGIs" />
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<meta name="abstract" content="By understanding the guidelines for writing cluster-enabled CGIs, you can avoid potential problems with your CGI code." />
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<meta name="description" content="By understanding the guidelines for writing cluster-enabled CGIs, you can avoid potential problems with your CGI code." />
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<meta name="copyright" content="(C) Copyright IBM Corporation 2002,2006" />
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<title>Guidelines for writing high availability CGIs</title>
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</head>
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<body id="rzag3ch4guidelines"><a name="rzag3ch4guidelines"><!-- --></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">Guidelines for writing high availability CGIs</h1>
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<div><p>By understanding the guidelines for writing cluster-enabled CGIs,
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you can avoid potential problems with your CGI code.</p>
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<p>All CGI programs existing today are cluster-disabled. A CGI program developer
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should follow the following rules when writing cluster-enabled CGI programs:</p>
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<ul><li>Write the CGI in such a way that running them with the same state more
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than once does not cause any problem.</li>
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<li>Store the CGI program’s state between client’s requests only in the Web
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server.</li>
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<li>Use only cluster-safe mechanisms. For example, if a CGI program uses shared
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memory (which is currently not a cluster-safe mechanism), the macro is almost
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certainly cluster-disabled.</li>
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</ul>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 1. CGI problems and solutions. This
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table identifies potential problem areas and suggests a solution:</caption><thead align="left"><tr><th valign="top" id="d0e31">Potential problems</th>
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<th valign="top" id="d0e33">Solutions</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e31 ">When the stateful CGI is run with the same state more
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than once, its correctness is not ensured.</td>
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<td valign="top" headers="d0e33 ">Rewrite the CGI so that it can run with the same state
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more than once.</td>
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</tr>
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<tr><td valign="top" headers="d0e31 ">The stateful CGI accesses shared memory.</td>
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<td valign="top" headers="d0e33 ">Eliminate the use of shared memory.</td>
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</tr>
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<tr><td valign="top" headers="d0e31 ">The stateful CGI generates session handles ignoring
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session handles passed by the Web server.</td>
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<td valign="top" headers="d0e33 ">Rewrite the CGI to use session handles passed by the
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Web server.</td>
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</tr>
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</tbody>
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</table>
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</div>
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<p>There are two categories of high availability Web server programming models
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to consider when writing high availability CGI programs or enabling an existing
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CGI program for use as a high availability CGI program. The two categories
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are:</p>
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<ul><li>Primary/backup</li>
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<li>Peer model</li>
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</ul>
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<p>For the primary/backup, follow these additional guidelines:</p>
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<p>The stateful data is saved by the high availability CGI program by calling
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the QzhbCgiSendState API. To retrieve any stateful data that has been stored
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use the QzhbCgiRecvState API. The QzhbCgiRecvState API returns stateful information
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when the environment variable QZHBRECOVERY is set and QZHBHA_MODEL is equal
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to PRIMARYBACKUP. If the QZHBRECOVERY is not set, then the CGI program should
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not use the QzhbCgiRecvState API. You must write a persistent
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CGI that maintains the data in static variables. If the environment variable
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QZHBRECOVERY is set, retrieve the data using the QzhbCgiRecvState API and
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restore the static variables.</p>
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<p>The Web server still limits the total number of persistent CGIs (both cluster-enabled
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and cluster-disabled ) using the MaxPersistentCGI directive.</p>
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<p>For the Peer model, follow these additional guidelines:</p>
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<p>The stateful data is saved by the high availability CGI program by calling
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the QzhbCgiSendState API. To retrieve any stateful data that has been stored
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use the QzhbCgiRecvState API. The QzhbCgiRecvState API must be used with each
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new request to retrieve any stateful data that has been stored for a previous
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high availability CGI program invocation. In this model your CGI program must
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not save stateful data in static variables.</p>
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<p>If QZHBHA_MODEL is PUREPEER the CGI is expected to restore its state, to
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serve the request, and to return its new state to the Web server. When the
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Web server receives the new CGI’s state, it stores the state (which will be
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passed to the CGI with the subsequent request), returns the response to the
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client, and terminates the CGI job. Consider a cluster-enabled persistent
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CGI program. The concept of persistency is kept, but its implementation is
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absolutely changed. A cluster-enabled persistent CGI program does not stay
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in the CGI job during a session; it is invoked each time a request comes in.
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However, the concept of persistency is kept due to CGI’s state (and the state
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of the Web server regarding the CGI) stored in the CHT and always passed to
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the CGI program with subsequent requests.</p>
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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="rzaieparcgi.htm" title="This topic provides information about Common Gateway Interfaces (CGI).">Common Gateway Interface</a></div>
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</div>
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</div>
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</body>
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</html>
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