</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#sharding">URL-based sharding accross multiple backends</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#on-the-fly-content">On-the-fly Content-Regeneration</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#load-balancing">Load Balancing</a></li>
</ul><h3>See also</h3><ul class="seealso"><li><a href="../mod/mod_rewrite.html">Module documentation</a></li><li><a href="intro.html">mod_rewrite introduction</a></li><li><a href="remapping.html">Redirection and remapping</a></li><li><a href="access.html">Controlling access</a></li><li><a href="avoid.html">When not to use mod_rewrite</a></li></ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
</dd>
</dl>
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="load-balancing" id="load-balancing">Load Balancing</a></h2>
+
+
+
+ <dl>
+ <dt>Description:</dt>
+
+ <dd>
+ <p>We wish to randomly distribute load across several servers
+ using mod_rewrite.</p>
+ </dd>
+
+ <dt>Solution:</dt>
+
+ <dd>
+ <p>We'll use <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code> and a list of servers
+ to accomplish this.</p>
+
+<div class="example"><pre>
+RewriteEngine on
+RewriteMap lb rnd:/path/to/serverlist.txt
+
+RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]
+</pre></div>
+
+<p><code>serverlist.txt</code> will contain a list of the servers:</p>
+
+<div class="example"><pre>
+## serverlist.txt
+
+servers one.example.com|two.example.com|three.example.com
+</pre></div>
+
+<p>If you want one particular server to get more of the load than the
+others, add it more times to the list.</p>
+
+ </dd>
+
+ <dt>Discussion</dt>
+ <dd>
+<p>Apache comes with a load-balancing module -
+<code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> - which is far more flexible and
+featureful than anything you can cobble together using mod_rewrite.</p>
+ </dd>
+ </dl>
+
</div></div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/rewrite/avoid.html" title="English"> en </a></p>
</section>
+<section id="load-balancing">
+
+ <title>Load Balancing</title>
+
+ <dl>
+ <dt>Description:</dt>
+
+ <dd>
+ <p>We wish to randomly distribute load across several servers
+ using mod_rewrite.</p>
+ </dd>
+
+ <dt>Solution:</dt>
+
+ <dd>
+ <p>We'll use <directive
+ module="mod_rewrite">RewriteMap</directive> and a list of servers
+ to accomplish this.</p>
+
+<example><pre>
+RewriteEngine on
+RewriteMap lb rnd:/path/to/serverlist.txt
+
+RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]
+</pre></example>
+
+<p><code>serverlist.txt</code> will contain a list of the servers:</p>
+
+<example><pre>
+## serverlist.txt
+
+servers one.example.com|two.example.com|three.example.com
+</pre></example>
+
+<p>If you want one particular server to get more of the load than the
+others, add it more times to the list.</p>
+
+ </dd>
+
+ <dt>Discussion</dt>
+ <dd>
+<p>Apache comes with a load-balancing module -
+<module>mod_proxy_balancer</module> - which is far more flexible and
+featureful than anything you can cobble together using mod_rewrite.</p>
+ </dd>
+ </dl>
+
+</section>
+
+
+
</manualpage>
<li><img alt="" src="../images/down.gif" /> <a href="#structuredhomedirs">Structured Homedirs</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#dynamic-mirror">Dynamic Mirror</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#retrieve-missing-data">Retrieve Missing Data from Intranet</a></li>
-<li><img alt="" src="../images/down.gif" /> <a href="#load-balancing">Load Balancing</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#new-mime-type">New MIME-type, New Service</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#autorefresh">Document With Autorefresh</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#mass-virtual-hosting">Mass Virtual Hosting</a></li>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
-<h2><a name="load-balancing" id="load-balancing">Load Balancing</a></h2>
-
-
-
- <dl>
- <dt>Description:</dt>
-
- <dd>
- <p>Suppose we want to load balance the traffic to
- <code>www.example.com</code> over <code>www[0-5].example.com</code>
- (a total of 6 servers). How can this be done?</p>
- </dd>
-
- <dt>Solution:</dt>
-
- <dd>
- <p>There are many possible solutions for this problem.
- We will first discuss a common DNS-based method,
- and then one based on <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>:</p>
-
- <ol>
- <li>
- <strong>DNS Round-Robin</strong>
-
- <p>The simplest method for load-balancing is to use
- DNS round-robin.
- Here you just configure <code>www[0-9].example.com</code>
- as usual in your DNS with A (address) records, e.g.,</p>
-
-<div class="example"><pre>
-www0 IN A 1.2.3.1
-www1 IN A 1.2.3.2
-www2 IN A 1.2.3.3
-www3 IN A 1.2.3.4
-www4 IN A 1.2.3.5
-www5 IN A 1.2.3.6
-</pre></div>
-
- <p>Then you additionally add the following entries:</p>
-
-<div class="example"><pre>
-www IN A 1.2.3.1
-www IN A 1.2.3.2
-www IN A 1.2.3.3
-www IN A 1.2.3.4
-www IN A 1.2.3.5
-</pre></div>
-
- <p>Now when <code>www.example.com</code> gets
- resolved, <code>BIND</code> gives out <code>www0-www5</code>
- - but in a permutated (rotated) order every time.
- This way the clients are spread over the various
- servers. But notice that this is not a perfect load
- balancing scheme, because DNS resolutions are
- cached by clients and other nameservers, so
- once a client has resolved <code>www.example.com</code>
- to a particular <code>wwwN.example.com</code>, all its
- subsequent requests will continue to go to the same
- IP (and thus a single server), rather than being
- distributed across the other available servers. But the
- overall result is
- okay because the requests are collectively
- spread over the various web servers.</p>
- </li>
-
- <li>
- <strong>DNS Load-Balancing</strong>
-
- <p>A sophisticated DNS-based method for
- load-balancing is to use the program
- <code>lbnamed</code> which can be found at <a href="http://www.stanford.edu/~riepel/lbnamed/">
- http://www.stanford.edu/~riepel/lbnamed/</a>.
- It is a Perl 5 program which, in conjunction with auxiliary
- tools, provides real load-balancing via
- DNS.</p>
- </li>
-
- <li>
- <strong>Proxy Throughput Round-Robin</strong>
-
- <p>In this variant we use <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
- and its proxy throughput feature. First we dedicate
- <code>www0.example.com</code> to be actually
- <code>www.example.com</code> by using a single</p>
-
-<div class="example"><pre>
-www IN CNAME www0.example.com.
-</pre></div>
-
- <p>entry in the DNS. Then we convert
- <code>www0.example.com</code> to a proxy-only server,
- i.e., we configure this machine so all arriving URLs
- are simply passed through its internal proxy to one of
- the 5 other servers (<code>www1-www5</code>). To
- accomplish this we first establish a ruleset which
- contacts a load balancing script <code>lb.pl</code>
- for all URLs.</p>
-
-<div class="example"><pre>
-RewriteEngine on
-RewriteMap lb prg:/path/to/lb.pl
-RewriteRule ^/(.+)$ ${lb:$1} [P,L]
-</pre></div>
-
- <p>Then we write <code>lb.pl</code>:</p>
-
-<div class="example"><pre>
-#!/path/to/perl
-##
-## lb.pl -- load balancing script
-##
-
-$| = 1;
-
-$name = "www"; # the hostname base
-$first = 1; # the first server (not 0 here, because 0 is myself)
-$last = 5; # the last server in the round-robin
-$domain = "foo.dom"; # the domainname
-
-$cnt = 0;
-while (<STDIN>) {
- $cnt = (($cnt+1) % ($last+1-$first));
- $server = sprintf("%s%d.%s", $name, $cnt+$first, $domain);
- print "http://$server/$_";
-}
-
-##EOF##
-</pre></div>
-
- <div class="note">A last notice: Why is this useful? Seems like
- <code>www0.example.com</code> still is overloaded? The
- answer is yes, it is overloaded, but with plain proxy
- throughput requests, only! All SSI, CGI, ePerl, etc.
- processing is handled done on the other machines.
- For a complicated site, this may work well. The biggest
- risk here is that www0 is now a single point of failure --
- if it crashes, the other servers are inaccessible.</div>
- </li>
-
- <li>
- <strong>Dedicated Load Balancers</strong>
-
- <p>There are more sophisticated solutions, as well. Cisco,
- F5, and several other companies sell hardware load
- balancers (typically used in pairs for redundancy), which
- offer sophisticated load balancing and auto-failover
- features. There are software packages which offer similar
- features on commodity hardware, as well. If you have
- enough money or need, check these out. The <a href="http://vegan.net/lb/">lb-l mailing list</a> is a
- good place to research.</p>
- </li>
- </ol>
- </dd>
- </dl>
-
- </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
-<div class="section">
<h2><a name="new-mime-type" id="new-mime-type">New MIME-type, New Service</a></h2>
</section>
- <section id="load-balancing">
-
- <title>Load Balancing</title>
-
- <dl>
- <dt>Description:</dt>
-
- <dd>
- <p>Suppose we want to load balance the traffic to
- <code>www.example.com</code> over <code>www[0-5].example.com</code>
- (a total of 6 servers). How can this be done?</p>
- </dd>
-
- <dt>Solution:</dt>
-
- <dd>
- <p>There are many possible solutions for this problem.
- We will first discuss a common DNS-based method,
- and then one based on <module>mod_rewrite</module>:</p>
-
- <ol>
- <li>
- <strong>DNS Round-Robin</strong>
-
- <p>The simplest method for load-balancing is to use
- DNS round-robin.
- Here you just configure <code>www[0-9].example.com</code>
- as usual in your DNS with A (address) records, e.g.,</p>
-
-<example><pre>
-www0 IN A 1.2.3.1
-www1 IN A 1.2.3.2
-www2 IN A 1.2.3.3
-www3 IN A 1.2.3.4
-www4 IN A 1.2.3.5
-www5 IN A 1.2.3.6
-</pre></example>
-
- <p>Then you additionally add the following entries:</p>
-
-<example><pre>
-www IN A 1.2.3.1
-www IN A 1.2.3.2
-www IN A 1.2.3.3
-www IN A 1.2.3.4
-www IN A 1.2.3.5
-</pre></example>
-
- <p>Now when <code>www.example.com</code> gets
- resolved, <code>BIND</code> gives out <code>www0-www5</code>
- - but in a permutated (rotated) order every time.
- This way the clients are spread over the various
- servers. But notice that this is not a perfect load
- balancing scheme, because DNS resolutions are
- cached by clients and other nameservers, so
- once a client has resolved <code>www.example.com</code>
- to a particular <code>wwwN.example.com</code>, all its
- subsequent requests will continue to go to the same
- IP (and thus a single server), rather than being
- distributed across the other available servers. But the
- overall result is
- okay because the requests are collectively
- spread over the various web servers.</p>
- </li>
-
- <li>
- <strong>DNS Load-Balancing</strong>
-
- <p>A sophisticated DNS-based method for
- load-balancing is to use the program
- <code>lbnamed</code> which can be found at <a
- href="http://www.stanford.edu/~riepel/lbnamed/">
- http://www.stanford.edu/~riepel/lbnamed/</a>.
- It is a Perl 5 program which, in conjunction with auxiliary
- tools, provides real load-balancing via
- DNS.</p>
- </li>
-
- <li>
- <strong>Proxy Throughput Round-Robin</strong>
-
- <p>In this variant we use <module>mod_rewrite</module>
- and its proxy throughput feature. First we dedicate
- <code>www0.example.com</code> to be actually
- <code>www.example.com</code> by using a single</p>
-
-<example><pre>
-www IN CNAME www0.example.com.
-</pre></example>
-
- <p>entry in the DNS. Then we convert
- <code>www0.example.com</code> to a proxy-only server,
- i.e., we configure this machine so all arriving URLs
- are simply passed through its internal proxy to one of
- the 5 other servers (<code>www1-www5</code>). To
- accomplish this we first establish a ruleset which
- contacts a load balancing script <code>lb.pl</code>
- for all URLs.</p>
-
-<example><pre>
-RewriteEngine on
-RewriteMap lb prg:/path/to/lb.pl
-RewriteRule ^/(.+)$ ${lb:$1} [P,L]
-</pre></example>
-
- <p>Then we write <code>lb.pl</code>:</p>
-
-<example><pre>
-#!/path/to/perl
-##
-## lb.pl -- load balancing script
-##
-
-$| = 1;
-
-$name = "www"; # the hostname base
-$first = 1; # the first server (not 0 here, because 0 is myself)
-$last = 5; # the last server in the round-robin
-$domain = "foo.dom"; # the domainname
-
-$cnt = 0;
-while (<STDIN>) {
- $cnt = (($cnt+1) % ($last+1-$first));
- $server = sprintf("%s%d.%s", $name, $cnt+$first, $domain);
- print "http://$server/$_";
-}
-
-##EOF##
-</pre></example>
-
- <note>A last notice: Why is this useful? Seems like
- <code>www0.example.com</code> still is overloaded? The
- answer is yes, it is overloaded, but with plain proxy
- throughput requests, only! All SSI, CGI, ePerl, etc.
- processing is handled done on the other machines.
- For a complicated site, this may work well. The biggest
- risk here is that www0 is now a single point of failure --
- if it crashes, the other servers are inaccessible.</note>
- </li>
-
- <li>
- <strong>Dedicated Load Balancers</strong>
-
- <p>There are more sophisticated solutions, as well. Cisco,
- F5, and several other companies sell hardware load
- balancers (typically used in pairs for redundancy), which
- offer sophisticated load balancing and auto-failover
- features. There are software packages which offer similar
- features on commodity hardware, as well. If you have
- enough money or need, check these out. The <a
- href="http://vegan.net/lb/">lb-l mailing list</a> is a
- good place to research.</p>
- </li>
- </ol>
- </dd>
- </dl>
-
- </section>
-
<section id="new-mime-type">
<title>New MIME-type, New Service</title>
projects / apache / commitdiff