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![]() Sites can increase reliability using the web server to distribute the load to more than one JVM. If something happens to the first JVM, Resin will send the request to the backup without the browser even being aware of the redirection.
The cheapest backup strategy just uses a single machine for the web server and two JVMs. One JVM is designated as primary and the other is a backup. If the first fails for some reason, the second will take over. Because the backup is normally not used, it doesn't really take up much system resources.
You will, of course, start the two srun processes separately. Here's how on unix:
selects names a pid file so can tell which srun to stop. select which srun to start. Alternately, you can specify the port directly like:
To make sure that your web server understands the configuration, look at . will show the current state of all the JVMs.
Once you start using multiple servers, 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.
Each host will have an identical configuration.
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. 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:
On the web server, mod_caucho will decode the cookie and send it to the appropriate host. So would go to host2.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.
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. A central database handles session state
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. 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. In this case, you can use the trick that refers to the preferred host. The configuration would look like:
Alternately, if you're using Apache, you could configure the sruns in the httpd.conf.
I've made the order consistent so sessions will always go to the correct machine. will always go to host2. Normally, of course, sites using this configuration will handle their own cookies.
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. This simplifies SSL development. A servlet just needs to check the method to see if the request is SSL or not. Other than that, all requests are handled identically.
By default, if 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.In the resin.conf, you'll use:
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.
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