Comment 0 for bug 1243768

For the discussion below , "update" refers to upgrade-charm or config-change.

To add reliability to mission critical services, we should be able to do rolling updates ("update ok -> next"), and/or being able to partially update a subset of units.
E.g. use-case: ~1000 units cassandra restarting at the same time (think also: shared storage layer thundering effect).

E.g suggested usage:
$ juju deploy --max-in-flight=2 ...

Then to properly support resuming above (if interrupted)
$ juju deploy --num-units-to-update=10

Then to complete either rollback:
$ juju deploy --roll-back ...
$ juju deploy --roll-forward ...

FYI webops currently has workarounded this at some charms by:
juju set units-to-update=0,2,5
juju set units-to-update=all