Implementing leadership

Leadership provides a mechanism whereby multiple units of an application can make use of a single, shared, authoritative source for charm-driven configuration settings.

Every application deployed by Juju is guaranteed to have at most one leader at any time. This is true independent of what the charm author does; whether or not you implement the hooks or use the tools, the unit agents will each seek to acquire leadership, and maintain it while they have it or wait for the current leader to drop out.

Leadership hooks

If you wish to be notified when your unit's leadership status changes, you should implement the following hooks:

leader-elected
which will run at least once, when the unit is known to be leader and guaranteed to remain so for at least 30 seconds.

leader-settings-changed
will run at least once when the unit is not guaranteed continued leadership for the next 30 seconds; and also whenever some other unit writes leader settings.

No particular guarantees can be made regarding the timeliness of the leader-settings-changed hook; it's always possible for the Juju agent itself to be taken out of commission at the wrong moment and not restart for a long time.

Leadership tools

Every unit can discover whether it's the leader, independent of the hook that's running. There is deliberately no mechanism for discovering which other unit is the leader; such data always risks staleness and opens the door to a lot of race scenarios.

is-leader
will write "True" or "False" to stdout, and return 0, if the unit is currently leader and can be guaranteed to remain so for 30 seconds. Output can be expressed as --format json or --format yaml if desired. If it returns a non-zero exit code, no inferences regarding true leadership status can be made, but you should generally fail safe and refrain from acting as leader when you cannot be sure.

Truth is independent of hook sequence. If a unit has been designated leader while mid-hook, it will start to return true; and if a unit were to, for example, lose its connection to the state-server mid-hook, and be unable to verify continued leadership past lease expiry time, it would start to return false.

Every application deployed by Juju also has access to a pseudo-relation over which leader settings can be communicated with the following tools:

leader-set
acts much like relation-set, in that it lets you write string key/value pairs (in which an empty value removes the key), but with the following differences:

there's only one leader-settings bucket per application (not one per unit)
only the leader can write to the bucket
only minions are informed of changes to the bucket
changes are propagated instantly, bypassing the sandbox

The instant propagation exists to satisfy the use case where shared data can be chosen by the leader at the very beginning of, for example, the install hook; by propagating it instantly, any running minions can make use of the data and progress immediately, without having to wait for the leader to finish its hook.

It also means that you can guarantee that a successful leader-set call has been reflected in the database, and that all minions will converge towards seeing that value, even if an unexpected error takes down the current hook.

For both these reasons we strongly recommend that leader settings are always written as a self-consistent group (leader-set foo=bar baz=qux ping=pong, rather than leader-set foo=bar; leader-set baz=qux etc, in which minions may end up seeing a sandbox in which only foo is set to the "correct" value).

leader-get
acts much like relation-get, in that it lets you read string values by key (and expose them in helpful formats), but with the following difference:

it reads only from the single leader-settings bucket

...and the following key similarity:

it presents a sandboxed view of leader-settings data.

This is necessary, as it is for relation data, because a hook context needs to present consistent data; but it means that there's a small extra burden on users of leader-set.

Leadership howtos

See Leadership howtos for examples.