Juju devel 1.23-beta2 is released

[Stuart Bishop]

On 26 March 2015 at 23:58, Curtis Hovey-Canonical <curtis at canonical.com> wrote:

> ### Service Leader Elections
>
> Services running in an environment bootstrapped with the
> "leader-election" feature flag now have access to three new hook tools:
>
>     is-leader - returns true only if the executing unit is guaranteed
>         service leadership for the next 30s
>     leader-get - as relation-get; accessible only within the service
>     leader-set - as relation-set; will fail if not executed on leader
>
> ...and two new hooks:
>
>     leader-elected - runs when the unit takes service leadership
>     leader-settings-changed - runs when another unit runs leader-set
>
> When a unit starts up, it will always run either leader-elected or
> leader-settings-changed as soon as possible, delaying only doing so only
> to run the install hook; complete any queued or in-flight operation; or
> resolve a hook or upgrade error.

Looking forward to this.

Looking at the specifics, I'm interested in how the leader unit
performs long running operations with the 30s lease. Lets say unit 0
is the leader, and decides that it is an appropriate time for unit 0
to run a repair operation that might take a few hours.

As the wording currently stands, no hooks are triggered on the leader
when the leader calls leader-set. This gives the leader no alternative
but to perform the long running operation in the same hook, and risk
another leader being elected and making a conflicting decision.

I think that the leader-settings-changed hook needs to be called
whenever *any* unit runs leader-set (including the current unit if it
is the leader), rather than only when a different unit runs
leader-set. This way, the leader can make its decisions and exit
whatever hook triggered it within its 30s lease, and all units can
perform their long running tasks in the leader-settings-changed hook.

Alternatively, it could kick of an asynchronous task but they don't exist yet.

Or would I need to run 'is-leader' in a thread every 30s to keep the
lease renewed?

-- 
Stuart Bishop <stuart.bishop at canonical.com>