Pen is a load balancer for "simple" tcp based protocols such as http or smtp. It allows several servers to appear as one to the outside and automatically detects servers that are down and distributes clients among the available servers. This gives high availability and scalable performance.

The load balancing algorithm keeps track of clients and will try to send them back to the server they visited the last time. This is useful for applications that maintain state between connections in the server, including most modern web applications.

When pen detects that a server is unavailable, it scans for another starting with the server after the most recently used one. That way we get load balancing and "fair" failover for free.

Correctly configured, pen can ensure that a server farm is always available, even when individual servers are brought down for maintenance or reconfiguration.

The final single point of failure, pen itself, can be eliminated by running pen on several servers, using vrrp to decide which is active.