num_units is redundant when charm declared with placement in a bundle

[Frode Nordahl]

In a live fast-paced deployment scenario all kinds of things can go wrong which requires last minute changes to the scale of bundles to match the reality of available functional physical machines and other factors.

When making changes to a bundle that has placement declared with the `to` key for all charms, the requirement for also specifying `num_units` becomes a nuisance and kills some of the elastic modeling feeling of the tool.

It would be of great value to users of Juju if the requirement for specifying `num_units` could be removed when `to` is present for a charm, meaning that Juju would deduce the value of `num_units` from the contents of `to`.

[John A Meinel (jameinel)]

The caveat there is that some values you can supply in "to:" imply
elasticity in themselves. eg "-to: lxd:" means that all the units of this
application end up in containers.

On Fri, Jan 8, 2021 at 5:55 AM Frode Nordahl <email address hidden>
wrote:

> Public bug reported:
>
> In a live fast-paced deployment scenario all kinds of things can go
> wrong which requires last minute changes to the scale of bundles to
> match the reality of available functional physical machines and other
> factors.
>
> When making changes to a bundle that has placement declared with the
> `to` key for all charms, the requirement for also specifying `num_units`
> becomes a nuisance and kills some of the elastic modeling feeling of the
> tool.
>
> It would be of great value to users of Juju if the requirement for
> specifying `num_units` could be removed when `to` is present for a
> charm, meaning that Juju would deduce the value of `num_units` from the
> contents of `to`.
>
> ** Affects: juju
> Importance: Undecided
> Status: New
>
>
> ** Tags: ps5
>
> --
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> https://bugs.launchpad.net/bugs/1910733
>
> Title:
> num_units is redundant when charm declared with placement in a bundle
>
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[Pen Gale (pengale)]

Agree that this would be a useful feature. Adding it to the wishlist for now, but we're definitely open to bumping this in priority!

(Adding the "bitesize" tag, just in case there is a simple implementation -- feel free to remove if you dig into the code, and determine that the fix would require more extensive work.)