Each Resin instance can serve many virtual hosts. The virtual host will have its own servlets and documents. For greater isolation, you can configure each virtual host to have its own JVM and you can have all the virtual hosts controlled by a single web server.

Virtual Host Concepts

A "virtual host" has a unique domain name, but the same ip address as other domain names. For example, www.gryffindor.com and www.slytherin.com can have the same ip address 192.168.0.13 and share the same web server.

In this scenario, both www.gryffindor.com and www.slytherin.com are registered with the standard domain name service registry as having the IP address 192.168.0.13. A user types in the url http://www.gryffindor.com/hello.jsp in their browser. The user's computer resolves the name www.gryffindor.com to the IP address 192.168.0.13. Resin is the webserver on the machine that has the IP address 192.168.0.13, so it receives the request. Resin determines which virtual host to use by looking at the request URL. Resin get's the from a HTTP header submitted by the browser.

Note: IIS, just to be different, uses the term "virtual sites" instead of "virtual hosts".
Testing virtual hosts

During development and testing, it is often inconvenient or impossible to use real virtual host names that are registered as internet sites, and resolve to an internet-available IP address. OS-level features on the test client machine can be used to map a virtual host name to an IP address.

Standalone Virtual Hosting

Configuring the standalone server is the easiest and best way of testing a virtual host configuration. The resin.conf is identical for Resin standalone and for Resin as a servlet runner. So even when using an external web server like Apache or IIS, it's a good idea to test configuring with Resin standalone.

Each virtual host has its own host block. At the very least, each host will define the id specifying the host name and a root web-app. A <root-directory> is often used to provide a host specific root for logfiles.

Virtual Hosts with Apache or IIS

A common configuration uses virtual hosts with Apache or IIS. As usual, Apache or IIS will pass matching requests to Resin.
Apache

The Resin JVM configuration with Apache is identical to the standalone configuration. That similarity makes it easy to debug the Apache configuration by retreating to Resin standalone if needed.

The ServerName directive in Apache is vital to make Resin's virtual hosting work. When Apache passes the request to Resin, it tells Resin the ServerName. Without the ServerName, Resin can get very confused which host to serve.

Dynamic virtual hosts

Resin can be configured to deploy virtual hosts dynamically. Instead of adding a <host> tag to resin.conf, a directory on the file-system is established, and subdirectories become virtual hosts.

JVM per virtual host

In some ISP setups, it may make sense to assign a JVM for each virtual host. The isolation of web-apps may not be sufficient; each host needs a separate JVM. In this configuration, each JVM needs its own srun-port and possibly its own srun-host.

The setup is similar to load-balancing . A front-end web server receives all requests, and is configured to dispath to back-end Resin JVM's that correspond to the host name.
Back-end JVM's

In the most straightforward configuration, each host specific backend JVM gets its own resin.conf. The resin.conf can use resin:include to share common configuration.

In this example, the virtual hosts www.gryffindor.com and www.slytherin.com each get their own JVM. The first step is the configuration and startup of the back-end, host specific instances of Resin. The second step is the by the configuration of the front-end server that dispatches requests to the appropriate back-end JVM.

Resin front-end

The host-specific back-end JVM's are ready to receive requests on their srun ports. A third instance of Resin in a separate JVM can be used as the front-end server. It receives all requests and dispatches to the back-end servers.

The Resin web server is configured using the LoadBalanceServlet to dispatch to the back-end JVM's. A cluster is defined for each back-end JVM, so that the LoadBalanceServlet knows how to find them.

IP-Based Virtual Hosting

While Resin's virtual hosting is primarily aimed at named-based virtual hosts, it's possible to run Resin with IP-Based virtual hosts.

With IP virtual hosting, each <http> block is configured with the virtual host name. This configuration will override any virtual host supplied by the browser.

Internationalization

Resin's virtual hosting understands host names encoded using rfc3490 (Internationalizing Domain Names in Applications). This support should be transparent. Just specify the virtual host as usual, and Resin will translate the brower's encoded host name the unicode string.