locksmith is a reboot manager for the CoreOS update engine which uses etcd to ensure that only a subset of a cluster of machines are rebooting at any given time. locksmithd runs as a daemon on CoreOS machines and is responsible for controlling the reboot behaviour after updates.

There are three different strategies that locksmithd can use after the update engine has successfully applied an update:

• etcd-lock - reboot after first taking a lock in etcd.
• reboot - reboot immediately without taking a lock.
• best-effort - if etcd is running, then do etcd-lock; otherwise, reboot.

These strategies can be configured via /etc/coreos/update.conf with a line that looks like:

REBOOT_STRATEGY=reboot

The reboot strategy can also be configured through cloud-config.

The default strategy is best-effort.

locksmithctl is a simple client that can be use to introspect and control the lock used by locksmith. It is installed by default on CoreOS.

$ locksmithctl status
Available: 0
Max: 1

MACHINE ID
69d27b356a94476da859461d3a3bc6fd

In some cases a machine may go away permanently or semi-permanently while holding a reboot lock. A system administrator can clear the lock of a specific machine using the unlock command:

$ locksmithctl unlock 69d27b356a94476da859461d3a3bc6fd

By default the reboot lock only allows a single holder. However, a user may want more than a single machine to be upgrading at a time. This can be done by increasing the semaphore count.

$ locksmithctl set-max 4
Old: 1
New: 4

The following section describes how locksmith works under the hood.

locksmith uses a semaphore in etcd (located at the key coreos.com/updateengine/rebootlock/semaphore) to coordinate the reboot lock.

The semaphore is a JSON document, describing a simple semaphore, that clients swap to take the lock.

When it is first created it will be initialized like so:

{
	"semaphore": 1,
	"max": 1,
	"holders": []
}

For a client to take the lock, the document is swapped with this:

{
	"semaphore": 0,
	"max": 1,
	"holders": [
		"69d27b356a94476da859461d3a3bc6fd"
	]
}