Constraints ignored when using add-unit

[Simon Richardson]

When doing the following we ignore the constraints:

    $ juju bootstrap aws test
    $ juju deploy redis
    $ juju add-machine --series=trusty --constraints="arch=arm64"
    $ juju add-unit redis

Redis will land on the arm64 even though it's implied to work on amd64. This becomes more of a problem with charmhub architecture where you won't have a charm for arm64 and will fall on it's face.

[Ian Booth (wallyworld)]

It's not so much that we ignore constraints, but that there aren't any. Juju's algorithm to assign a unit to a machine is to pick the first unused machine, which here will be arm64. The disconnect is that if no constraints are specified and Juju needs to create a new machine, it will create an amd64 one. The implies there's an amd64 constraint by default, but no constraint is actually created. We should look at making selection of unused machines assume amd64 unless there's an actual constraint, to give behaviour consistent with create machine.

[Simon Richardson (simonrichardson)]

Marking this as wontifx. If you're playing around with heterogeneous architectural deployments, then it's up to you as an operator to select the right architecture for your deployment to land. Juju will just select an available machine for the redis to use, in this case, the new machine meets all the requirements.