Redis is a key-value database in a similar vein to memcache but the
dataset is non-volatile. Redis additionally provides native support
for atomically manipulating and querying data structures such as lists
and sets. The dataset is stored entirely in memory and periodically
flushed to disk.
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:
from charmhelpers.core.host import pwgen
The following states are set appropriately on startup, before any @hook
decorated methods are invoked:
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
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
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.
One or more leadership settings has changed since the last hook.
This state will remain set for the remainder of the hook. It will
not be set in the next hook, unless the leader has made further
The charms.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).
This layer is maintained on Launchpad by
Stuart Bishop (firstname.lastname@example.org).
Code is available using git at git+ssh://git.launchpad.net/layer-leadership.
Bug reports can be made at https://bugs.launchpad.net/layer-leadership.
Queries and comments can be made on the Juju mailing list, Juju IRC
channels, or at https://answers.launchpad.net/layer-leadership.