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Pen

This is pen, 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.

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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

Change log

There is a mailing list: pen@siag.nu. Send mail to majordomo@siag.nu with the message body:

subscribe pen

Mailing list archive

Penlogd

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.

Webstats

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.

Portability

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.

It runs on Windows, too.
And on MacOS X.

Installation

Type:

	./configure
        make
        make install

By default the programs are installed in /usr/local/bin. This can be changed like this example:

        ./configure --prefix=/usr

to install into /usr/bin instead.

Download source

Contributed start scripts

Links

Hercules Load Balancer Virtual Appliance, a VMware virtual machine based on Pen

Penbw, Pen backend watcher, monitors and blacklists backends automatically in case of a failure

Pre-compiled packages for Solaris

A redundant load-balancing firewall system, using FreeBSD

Red Hat RPMs courtesy of Dag Wieers

Zen load balancer, a load balancer appliance based on Pen.

Freshmeat project page

More stuff

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Ulric Eriksson - May 2008 - ulric@siag.nu