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.