Leadership layer released

[Cory Johns]

This is awesome.  A very clean interface for dealing with leadership in
layered charms.

I also very much like the pattern of having states that notify of changes
to the leadership settings and think this pattern could apply well to
config settings.  One addition I would suggest to that pattern would be to
also have a blanket `leadership.changed` state that would be set when *any*
value was changed.  This is mostly to handle the case where you need to
react to any of a set of multiple values changing.  Thoughts?

Thanks for creating this!

On Wed, Jan 27, 2016 at 11:35 PM, Stuart Bishop <stuart.bishop at canonical.com
> wrote:

> 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.bishop at 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.bishop at canonical.com>
>
> --
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> Juju at lists.ubuntu.com
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>
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