balance.xtp

解压在c盘

<example>
c:\resin1.2> bin/httpd -install-as ResinA -server a
c:\resin1.2> bin/httpd -install-as ResinB -server b
</example>

<p>On Unix, the <var/-pid/> is used to keep track of the live srun so
a later <var/httpd.sh stop/> will work.  It just names a file that will
contain the process id (pid) of the started process.</p>

<p>To make sure that your web server understands the configuration,
look at <var/http://host/caucho-status/>.  caucho-status will
show the current state of all the JVMs.</p>

<s3 title="Summary">

<ul>
<li>The webserver plugin (mod_caucho, or isapi_srun) looks in the resin.conf
for &lt;srun> and &lt;srun-backup> elements.
<li><var//caucho-status/> shows all the servlet runners and backups.
<li><var/httpd.sh -server a/> start the srun with id="a".
<li>The plugin will always send a session to the same backend srun.
</ul>

</s3>

</s2>

<s2 title='Single Web Server Load Balancing'>

<p>Once you start using multiple computers, you can start distributing
the load between a single web server and multiple JVMs.
This is a cheap alternative to a router-based load balancer.  Also, by
using Resin's load balancing, you can make sure that sessions stay on
the same machine.</p>

<figure src="sticky-sessions.gif"/>

<example>
&lt;caucho.com>
&lt;http-server>

  &lt;srun id="1" host='host1' port='6802' srun-index='1'/>
  &lt;srun id="2" host='host2' port='6802' srun-index='2'/>
  &lt;srun id="3" host='host3' port='6802' srun-index='3'/>

  ...
  
&lt;/http-server>
&lt;/caucho.com>
</example>

<p>Each host and the plugin can share the same resin.conf.  Sharing the
resin.conf makes maintenance easier, since the files won't get out of sync.</p>

<p>However, you could use a different resin.conf for plugin and
for each of the JVMs.  The resin.conf for the plugin, e.g. mod_caucho,
just needs enough information to list the &lt;srun> hosts and enough
servlet-mappings to tell the plugin where to send the requests.</p>

<s3 title="Summary">

<ul>
<li>The webserver plugin (mod_caucho, or isapi_srun) looks in the resin.conf
for &lt;srun> and &lt;srun-backup> elements.
<li>The webserver plugin can share the same resin.conf as the JVMs.
<li><var/httpd.sh -server a/> starts the srun with id="a".
<li>The plugin sends a session to the same backend srun.
<li>New sessions or non-session requests balance the load with a round-robin
policy.
</ul>

</s3>

</s2>

<s2 title='What about sessions?'>

<p>A session needs to stay on the same JVM that started it.
Otherwise, each JVM would only see every second or third request and
get confused.</p>

<p>To make sure that sessions stay on the same JVM, Resin encodes the
cookie with the host number.  In the previous example, the hosts would
generate cookies like:</p>

<deftable>
<tr><th>srun-index<th>cookie prefix
<tr><td>1<td><var/a/>xxx
<tr><td>2<td><var/b/>xxx
<tr><td>3<td><var/c/>xxx
</deftable>

<note>srun-index only exists in Resin 1.2.3.  If unspecified, it defaults
to the index of the srun in the srun list.</note>

<p>On the web server, mod_caucho will decode the cookie and send it
to the appropriate host.  So <var/bX8ZwooOz/> would go to host2.</p>

<p>In the infrequent case that host2 fails, Resin will send the
request to host3.  The user will lose the session but that's a minor
problem compared to showing a connection failure error.  To save sessions,
you'll need to use <a href="sessions.xtp">distributed sessions</a>.
Also take a look at
<a href="../java_tut/tcp-sessions.xtp">tcp sessions</a>.</p>

<p>The following example is a typical configuration for a distributed
server using an external hardware load-balancer, i.e. where each Resin is
acting as the HTTP server.  Each server will be started
as <var/-server a/> or <var/-server b/> to grab its specific configuration.</p>

<p>In this example, sessions will only be stored when the server shuts down,
either for maintenance or with a new version of the server.  This is the most
lightweight configuration, and doesn't affect performance significantly.
If the hardware or the JVM crashes, however, the sessions will be lost.
(If you want to save sessions for hardware or JVM crashes,
remove the &lt;save-on-shutdown/> flag.)</p>

<example title="resin.conf">
&lt;caucho.com>
&lt;http-server>
  &lt;http id='a' port='80'/>
  &lt;srun id='a' port='6802' host='192.168.0.1' srun-index='1'/>

  &lt;http id='b' port='80'/>
  &lt;srun id='b' port='6802' host='192.168.0.2' srun-index='2'/>

  &lt;http id='c' port='80'/>
  &lt;srun id='c' port='6802' host='192.168.0.3' srun-index='3'/>

  &lt;!-- enable tcp-store for all hosts/web-apps -->
  &lt;session-config>
    &lt;tcp-store/>
    &lt;save-on-shutdown/>
  &lt;/session-config>

  ...
&lt;/http-server>
&lt;/caucho.com>
</example>

</s2>

<s2 title='Multiple Web Servers'>

<p>Many larger sites like to use multiple web servers with a JVM and a
web server on each machine.  A router will distribute the load between
the machines.</p>

<p>In this configuration, the site needs to take control of its own
sessions.  Because the router will distribute the load randomly, any
persistent session state needs to be handled by a centralized server
like a database or use Resin's TCP-ring storage.
</p>

<p>Even in this configuration, you can use Resin's load balancing to
increase reliability.  Each web server should choose its own JVM
first, but use another machine as a backup.
</p>

<p>In this case, you can use the trick that <var/localhost/> refers
to the preferred host.  The configuration would look like:</p>

<example>
&lt;caucho.com>
&lt;http-server>

  &lt;srun id="a" host='localhost' port='6802' srun-index='1'/>
  &lt;srun-backup id="b" host='host1' port='6802' srun-index='2'/>
  &lt;srun-backup id="c" host='host2' port='6802' srun-index='3'/>
  &lt;srun-backup id="d" host='host3' port='6802' srun-index='4'/>
  ...
  
&lt;/http-server>
&lt;/caucho.com>
</example>

<p>Alternately, if you're using Apache, you could configure the sruns
in the httpd.conf.</p>

<example title='host1 httpd.conf'>
CauchoConfigFile /home/www/resin/conf/resin.conf
CauchoHost host1 6802
CauchoBackup host2 6802
</example>

<example title='host2 httpd.conf'>
CauchoConfigFile /home/www/resin/conf/resin.conf
CauchoBackup host1 6802
CauchoHost host2 6802
</example>

<p><b>The order must be consistent for all servers so sessions will always
go to the correct machine.</b>  <var/bXXX/> must always go to host2.</p>

</s2>

<s2 title='Multiple Web Servers, Single JVM'>

<p>Multiple web servers can use the same JVM.  For example, a fast
plain webserver and an SSL web server may only need a single JVM.
(Although a backup would be good.)  Since the JVM doesn't care where
the request comes from, it can treat each request identically.</p>

<p>This simplifies SSL development.  A servlet just needs to check
the <var/request.isSecure()/> method to see if the request is SSL or
not.  Other than that, all requests are handled identically.</p>

</s2>

<s2 title='When everything fails'>

<p>By default, if <var/mod_caucho/> can't connect to any of the JVMs,
it will return a basic "can't connect" page to the user.  Sites which
want a more professional response can redirect the user to an error
page.</p>

<p>In the resin.conf, you'll use:</p>

<example>
&lt;caucho.com>
&lt;http-server>
  &lt;error-page exception-type='connection'
               location='/error.html'/>
&lt;/http-server>
&lt;/caucho.com>
</example>

<p>The error page must be an absolute path because it could be called
from any url.  Of course, it can't refer to a servlet or to a JSP file.</p>

</s2>
</s1>