Gearman is a queuing system that has been in Debian for a long time and is quite reliable.

I ran into problems however when upgrading a server from Debian squeeze to wheezy however. Here's how I debugged my Gearman setup.

Log verbosity

First of all, I started by increasing the verbosity level of the daemon by adding --verbose=INFO to /etc/default/gearman-job-server (the possible values of the verbose option are in the libgearman documentation) and restarting the daemon:

/etc/init.d/gearman-job-server restart

I opened a second terminal to keep an eye on the logs:

tail -f /var/log/gearman-job-server/gearman.log

Listing available workers

Next, I registered a very simple worker:

gearman -w -f mytest cat

and made sure it was connected properly by telneting into the Gearman process:

telnet localhost 4730

and listing all currently connected workers using the workers command (one of the commands available in the Gearman TEXT protocol).

There should be an entry similar to this one:

30 - : mytest

Because there are no exit or quit commands in the TEXT protocol, you need to terminate the telnet connection like this:

  1. Press Return
  2. Press Ctrl + ]
  3. Press Return
  4. Type quit at the telnet prompt and press Return.

Finally, I sent some input to the simple worker I setup earlier:

echo "hi there" | gearman -f mytest

and got my input repeated on the terminal:

hi there

Gearman bug

I traced my problems down to this error message when I sent input to the worker:

gearman: gearman_client_run_tasks : connect_poll(Connection refused)
getsockopt() failed -> libgearman/

It turns out that it is a known bug that was fixed upstream but still affects Debian wheezy and some versions of Ubuntu. The bug report is pretty unhelpful since the work-around is hidden away in the comments of this "invalid" answer: be explicit about the hostname and port number in both gearman calls.

So I was able to make it work like this:

gearman -w -h -p 4730 -f mytest cat    
echo "hi there" | gearman -h -p 4730 -f mytest

where the hostname matches exactly what's in /etc/default/gearman-job-server.