Mesos can run in fault-tolerant mode, in which multiple Mesos masters run simultaneously, with one of them being the active master, and the others acting as stand-bys ready to take over if the active master fails. Mesos uses Apache ZooKeeper in to elect a new active master. 

Fault-tolerant mode requires Mesos to be built with ZooKeeper. This can be done with the configure option @--with-included-zookeeper@, which will ensure that ZooKeeper (which resides in the @third_party@ directory) gets compiled. It is also possible to run Mesos with an external ZooKeeper by using the configure option @--with-zookeeper=DIR@, setting @DIR@ to the directory of the external ZooKeeper.

To run Mesos in fault-tolerant mode, ZooKeeper has to be up and running. The script @third_party/zookeeper-*/bin/zkServer.sh@ can be used to launch ZooKeeper (see the ZooKeeper documentation for more information). Once ZooKeeper is running, the master daemon, slave daemon(s), and the framework schedulers have to be passed a URL to the running ZooKeeper instance. The URL is of the form @zoo://host1:port1,host2:port2/znode@, where the @host:port@ pairs are ZooKeeper servers and @znode@ is a path to a znode (ZooKeeper's equivalent of a directory) for use by Mesos. It is also possible to use the URL @zoofile://filename/znode@, in which case @filename@ should contain one @host:port@ pair per line. This URL replaces the Mesos master URL (i.e. @mesos://@) which is passed when Mesos is not running in fault-tolerant mode. Multiple Mesos masters can be executed this way. Mesos will ensure, through ZooKeeper, that only one of them is the active master at any given time.