state, mstate: implement UnitWatcher

state, mstate: implement UnitWatcher



https://code.launchpad.net/~rogpeppe/juju-core/060-unit-watcher/+merge/124078

(do not edit description out of merge proposal)

[rog, 2012-09-12 23:25:35]

Please take a look.

[fwereade, 2012-09-13 10:43:53]

On 2012/09/12 23:25:35, rog wrote:
> Please take a look.

Where's mstate? :p

[fwereade, 2012-09-13 12:11:12]

Fairly strongly -1 on this change. It sounds as though the only current use case
is for testing (in which case, IMO, we should submit to the indignity of polling
rather than dirtying up the public API); and the only projected use case is for
major version upgrade, which STM like it would be better handled by a
ToolsVersionWatcher that can be shared by unit and machine. Apparently niemeyer
is -1 on this; I'd be keen to know why :).

Also, this looks like it will respond to unit status changes, but it won't
(unless it also watches for unit life).

[niemeyer, 2012-09-17 13:14:52]

We've discussed William's concern live in the sprint, and agreed that we'll
migrate trivial watchers (watchers that in general monitor a field in an
entity's document) to generic entity watchers (e.g. service exposed watcher =>
service watcher).

So, we'll indeed need a unit watcher, and should obsolete the other watchers.

That said, we're pushing forward the state > mstate transition at the moment, so
I suggest we have that watcher straight into mstate instead.