Pen is a highly scalable, highly available, robust load balancer for tcp and udp based protocols such as dns, 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.
Pen is known to scale to hundreds of thousands of connections on low-end hardware.
Recent(-ish) changelog excerpt
150818 Released 0.30.0. 150803 Added UDP mode for Direct Server Return. 150803 Updated configure.ac for compatibility with CentOS 6. 150725 Added #ifdef around SSLv3 initialization code in ssl, as suggested by email@example.com. 150608 Released 0.29.0. 150528 Transparent reverse proxy support for Linux, FreeBSD and OpenBSD. 150527 Allow the client table size to be updated on the fly. Default size still 2048. Allow the connection table size to be updated in the fly. Default still 500. See penctl.1, options clients_max and conn_max. 150526 Introduced the macro NO_SERVER to be used instead of -1 to signify error conditions and such. Removed the fixed server table size along with the -S option. 150525 Fixed cosmetic bug in startup code which required port to be specified on backend servers even if it was the same as the listening port.
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.
Description of redundancy using vrrpd on Linux
The Ultimate Cheapskate Cluster
A side-effect of the load-balancing is that several logfiles are produced, and all accesses seem to come from the load balancer. The program penlogd solves this problem by merging pen’s log file with the ones produced by the web servers. See penlogd(1) and penlog(1) for details.
Pen emits statistics when it receives a USR1 signal, but the output can be hard to interpret. The cgi script webstats can be used in conjunction with the -w option to pen to get statistics in HTML format instead.
New: Take a look here to see what the latest Pen status is here at siag.nu. This is not “live” data, but generated from this cron job every 10 minutes:
2,12,22,32,42,52 * * * * kill -USR1 `cat /var/run/pen.pid`
Note that there are usually few active connections. This is because of the way the web works: the client connects, the server sends data, the client disconnects. The whole transaction is over in seconds.
This load balancer is known to work on FreeBSD, Linux, HP-UX and Solaris. Other Unixes should work as well, possibly requiring trivial changes. Success stories or problem reports are welcome.
Actually, the easiest way to install Pen nowadays is to get it from one of the distributions that package it. On e.g. Debian or Ubuntu the entire process is:
apt-get install pen
If you still want to install from source because you need features not built into the packages, here’s how. Type:
./configure make make install
By default the programs are installed in /usr/local/bin. This can be changed like this example:
to install into /usr/bin instead.
Penbw, Pen backend watcher, monitors and blacklists backends automatically in case of a failure
Zen load balancer, a load balancer appliance based on Pen.