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.

As traffic increases, web sites need to add additional web servers and servlet engines. Distributing the traffic across the servers and coping when a server restarts is the challenge of load balancing.

Sessions can be persistent across server restarts, including application restarts when classes change. During development, for example, using file-based persistent sessions will let you work with a single session even while you're modifying servlet classes.