Hi. I've put up my Leadership layer on http://interfaces.juju.solutions/. This work was broken out from my PostgreSQL charm refactorings and will also be used by the Cassandra charm when I get onto that. It is obviously small and focused on leadership; I had been considering consolidating a number of similar parts into a swiss army knife 'hookenv' layer, but decided that smaller bricks are more in the spirit of layered charms and avoids most feature creep.
# Leadership Layer for Juju Charms The Leadership layer is for charm-tools and 'charm build', making it easier for layered charms to deal with Juju leadership. This layer will initialize charms.reactive states, allowing you to write handlers that will be activated by these states. It allows you to completely avoid writing leader-elected and leader-settings-changed hooks. As a simple example, these two handlers are all that is required to make the leader unit generate a password if it is not already set, and have the shared password stored in a file on all units: ```python from reactive.leadership import leader_get, leader_set from charmhelpers.core.host import pwgen @when('leadership.is_leader') @when_not('leadership.set.admin_password') def generate_secret(): leader_set(admin_password=pwgen()) @when('leadership.changed.admin_password') def store_secret(): write_file('/etc/foopass', leader_get('admin_password')) ``` ## States The following states are set appropriately on startup, before any @hook decorated methods are invoked: * `leadership.is_leader` This state is set when the unit is the leader. The unit will remain the leader for the remainder of the hook, but may not be leader in future hooks. * `leadership.set.{varname}` This state is set for each leadership setting (ie. the `leadership.set.foo` state will be set if the leader has set the foo leadership setting to any value). It will remain set for the remainder of the hook, unless the unit is the leader and calls `reactive.leadership.leader_set()` and resets the value to None. * `leadership.changed.{varname}` This state is set for each leadership setting that has changed since the last hook. It will remain set for the remainder of the hook. It will not be set in the next hook, unless the leader has changed the leadership setting yet again. ## Methods The `reactive.leadership` module exposes the `leader_set()` and `leader_get()` methods, which match the methods found in the `charmhelpers.core.hookenv` module. `reactive.leadership.leader_set()` should be used instead of the charmhelpers function to ensure that the reactive state is updated when the leadership settings are. If you do not do this, then you risk handlers waiting on these states to not be run on the leader (because when the leader changes settings, it triggers leader-settings-changed hooks on the follower units but no hooks on itself). ## Support This layer is maintained on Launchpad by Stuart Bishop (stuart.bis...@canonical.com). Code is available using git at git+ssh://git.launchpad.net/leadership-layer. Bug reports can be made at https://bugs.launchpad.net/leadership-layer. Queries and comments can be made on the Juju mailing list, Juju IRC channels, or at https://answers.launchpad.net/leadership-layer. -- Stuart Bishop <stuart.bis...@canonical.com> -- Juju mailing list Juju@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/juju