backward compatibility

[James Hebden]

On Mon, Oct 30, 2017 at 01:33:40PM +0400, John Meinel wrote:
> I think saying that we offer "we will allow clients that are <1yr old to
> stay compatible with current controllers", and vice versa seems ok, and
> doesn't seem like a significant maintenance burden. (we could at least
> release a new 2.X if we broke compatibility with 2.(X-2).)

As someone who's been upgrading a fair few Juju environments lately, I
would say my expectation has always been interoperability with minor
releases, I wouldn't expect different major version upgrades and
controllers on different major versions to maintain backward
compatibility.

I think it's also important that wherever the line is drawn, 
`juju upgrade-juju` without specifying version is clever enough to not
get a user into a state where their environment is broken due to making
an unsupported jump - i.e. upgrading the model to 2.4 whilst the agents
in the environment expected to take the upgrade are still 2.1 agents.
Perhaps different rules as to what is supported from an upgrade/model
migration perspective to what is generally supported makes sense?

James


> 
> John
> =:->
> 
> 
> On Mon, Oct 30, 2017 at 8:22 AM, Anastasia Macmood <
> anastasia.macmood at canonical.com> wrote:
> 
> > Hi
> >
> > Now that we are settled on Juju 2, going forward we need to have a way
> > to retire older minor versions in a user-friendly manner.
> >
> > We propose to use client/server version comparison to flag retiring
> > versions in 3 distinct steps - deprecated, obsolete and unsupported.
> >
> > For example, we can determine that if your client version differs from
> > your controller version by:
> >
> >   * 2 minor versions, you are running a deprecated back-end;
> >   * 3 minor versions, you are running an obsolete back-end;
> >   * 4+ minor versions, you are running an unsupported backend.
> >
> > In this world, it means that when you are running a 2.4 client, you will
> > be told that the controller on:
> >
> >   * 2.2 is deprecated;
> >   * 2.1 is obsolete;
> >   * 2.0 is unsupported.
> >
> > This will be surfaced as a warning on 'juju status'.
> >
> > This approach will allow us to not just retire certain API versions, but
> > also help triage bugs and set clear user expectations. Additional
> > benefits for maintenance and support - we will not be carrying around
> > huge amount of backward compatible code and craft... For example, does
> > it really makes sense for us to carry around and cater for backward
> > compatibility with Juju 2.0 when we are developing 2.6?
> >
> > Thoughts?
> >
> > Sincerely Yours,
> >
> > Anastasia
> >
> >
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> >

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-- 
James Hebden
Cloud Reliability Engineer
BootStack Squad @ Canonical Ltd.
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