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<LI><A href="node5.html#SECTION005100000000000000000">4.1 Introduction</a>
<LI><A href="node5.html#SECTION005200000000000000000">4.2 Generic load-balancing method</a>
<LI><A href="node5.html#SECTION005300000000000000000">4.3 Multi-processor, unix-based machine</a>
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<H1><A NAME="SECTION005000000000000000000">
4. How to use load-balancing for your web site</A>
</H1>
<H1><A NAME="SECTION005100000000000000000">
4.1 Introduction</A>
</H1>
Having a lot of traffic on their web site is what most webmasters dream about ...
<P>
Unfortunately, it also comes with its share of problems.
Basically, if requests come in faster than your server can handle them, you're screwed :-)
<P>
One way to deal with that is to use load-balancing. This basically means that several threads or processes will
be serving the pages. It is only efficient if you have several machines or if your machine has several processors.
(otherwise, the same processor will just run several threads or processes, but the overall speed will be the same).
<P>
Of course, this means that you have to take care of the data sharing between the threads/processes. One way to
do that is to use a database where you store all the data that needs to be shared amongst the processes. All processes
read and write to the same database, insuring that they all have the same informations.
<P>
There are two ways to do load-balancing with CherryPy. One of them is easier to set up, but only applies to
multi-processor machines running a Unix-based OS.
<P>
<H1><A NAME="SECTION005200000000000000000">
4.2 Generic load-balancing method</A>
</H1>
Since a CherryPy server is a self-contained process that includes everything to run the web site, it is easy to
start as many processes as you want (on the same machine or on different machines). If you start several
processes on the same machine, you just have to use a different configuration file for each of them to specify
a different port each time.
<P>
Then all you have to do is use a simple load-balancer (there are many of those out there) to redirect the requests
to your CherryPy servers.
<P>
Let's take an example:
<UL>
<LI>You have 3 machines, called host1, host2 and host3
</LI>
<LI>host2 and host3 are single-proc, and host1 is dual-proc.
</LI>
<LI>For the load-balancer, we'll use <b>balance</b>, an easy to use, lightweight, open source load balancer available from
<a class="url" href="http://balance.sourceforge.net">http://balance.sourceforge.net</a>.
</LI>
<LI>On host1, we'll run 2 CherryPy servers (since it's dual-proc) plus the load-balancer
</LI>
<LI>On host2 and host3, we'll only run one CherryPy server.
</LI>
<LI><b>balance</b> will listen on port 80. The CherryPy servers on host1 will listen on ports 8080 and 8081. The CherryPy servers on
host2 and host3 will listen on port 8080.
</LI>
<LI>Let's assume that your CherryPy server file is called <span class="file">MyServer.py</span>
</LI>
</UL>
<P>
Here is what we have to do:
<UL>
<LI>Copy <span class="file">MyServer.py</span> on host1, host2 and host3. (alternatively, you can export the file system from one machine to the others, to deal with only one copy of the file)
</LI>
<LI>On host1, host2 and host3, create a configuration file called <span class="file">MyServer8080.cfg</span> containing the following:
<div class="verbatim"><pre>
[server]
socketPort=8080
</pre></div>
On host1, create another configuration file called <span class="file">MyServer8081.cfg</span> containing the following:
<div class="verbatim"><pre>
[server]
socketPort=8081
</pre></div>
</LI>
<LI>On host1, host2 and host3, start the servers that listen on port 8080:
<div class="verbatim"><pre>
[host1] % python MyServer.py -C MyServer8080.cfg
[host2] % python MyServer.py -C MyServer8080.cfg
[host3] % python MyServer.py -C MyServer8080.cfg
</pre></div>
</LI>
<LI>On host1, start the second server that listens on port 8081:
<div class="verbatim"><pre>
[host1] % python MyServer.py -C MyServer8081.cfg
</pre></div>
</LI>
<LI>Now all you have to do is start the load balancer and indicate what the available CherryPy servers are:
<div class="verbatim"><pre>
[host1] % /usr/sbin/balance 80 host1:8080 host1:8081 host2:8080 host3:8080
</pre></div>
</LI>
</UL>
And voila !
<P>
<H1><A NAME="SECTION005300000000000000000">
4.3 Multi-processor, unix-based machine</A>
</H1>
The method described in the previous section works in all cases, but if you have a unix-based machine with multiple
processors, there is another method for load-balancing with CherryPy.
<P>
The trick is to create the socket where the CherryPy server will listen, and then do a <b>fork()</b>. This way, we'll
have multiple processes listening on the same socket. When one process is busy building a page, the next one will be
listening on the socket and thus serving a request that might come in.
<P>
This feature is built in. All you have to do to use it is to use the <b>fixedNumberOfProcesses</b> option in
the configuration file (in the <var>[server]</var> section):
<div class="verbatim"><pre>
[server]
socketPort=80
fixedNumberOfProcesses=3
</pre></div>
And that's it.
<P>
Note: This method is only useful if your machine has several processors. If not, then the processor will run multiple
CherryPy processes, but the overall speed won't be improved.
<P>
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