Sometimes, you just want some code to run on only one machine in a cluster at any particular time. Perhaps you only need one copy running, and you'd like to have something ready to failover, or maybe you want to make sure you don't take down all your machines simultaneously for a code upgrade.

Either way, Consul::Mutex has got you covered.


It's a gem:

gem install consul-mutex

There's also the wonders of the Gemfile:

gem 'consul-mutex'

If you're the sturdy type that likes to run from git:

rake install

Or, if you've eschewed the convenience of Rubygems entirely, then you presumably know what to do already.


Simply instantiate a new Consul::Mutex, giving it the key you want to use as the "lock":

require 'consul/mutex'

mutex ='/my/something/weird')

Then, whenever you want to only have one thing running at once, run the code inside a block passed to #synchronize:

mutex.synchronize { print "There can be"; sleep 5; puts " only one." }

If your consul server is not accessable via http://localhost:8500, you'll need to tell Consul::Mutex where to find it:

mutex ='/some/key', consul_url: 'http://consul:8500')

By default, the "value" of the lock resource will be the hostname of the machine that it's running on (so you know who has the lock). If, for some reason, you'd like to set the value to something else, you can do that, too:

mutex ='/some/key', value: "It is now #{}")

Failure Is An Option

One thing that is a bit unsettling about Consul-mediated mutexes is that the lock can be "lost" for a variety of reasons. The most common one, of course, is simple communications failure -- if you're on the wrong side of a split-brain, the rest of the cluster can recover and continue on, and you no longer have the lock, because your half of the cluster is dead. Consul also allows locks to be force-unlocked by an operator, because otherwise, if something died without unlocking, the lock would be held forever.

As a result of all this, the block of code that you pass to #synchronize runs on a separate thread, and can be killed without warning if the mutex determines that it no longer holds the lock. That means you want to be a bit extra-careful about idempotence and releasing resources you acquire (via ensure blocks).


Bug reports should be sent to the Github issue tracker, or e-mailed. Patches can be sent as a Github pull request, or e-mailed.


Unless otherwise stated, everything in this repo is covered by the following copyright notice:

Copyright (C) 2015 Civilized Discourse Construction Kit Inc.

This program is free software: you can redistribute it and/or modify it
under the terms of the GNU General Public License version 3, as
published by the Free Software Foundation.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <>.