Pen

The Pen project allows several servers to appear as one to the outside.

It 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. The client table has a number of slots (default 2048, settable through command-line arguments). When the table is full, the least recently used one will be thrown out to make room for the new one.

This is superior to a simple round-robin algorithm, which sends a client that connects repeatedly to different servers. Doing so breaks 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.

What is new in this release:

• This version adds support for GeoIP access lists.
• It is now possible to restrict access to the load balancer using syntax such as "acl 0 permit country CA" and "acl 0 deny country SE".
• These access control entries can be mixed with the usual IP-based ones.

What is new in version 0.20.2:

• The penctl and penlog utilities are now IPv6-compatible.
• The Automake configuration was updated.
• A clause was added to the license to explicitly permit distributing binaries linked with the openssl library.