Datasource Fallbacks (simplestreams)

[John Arbash Meinel]

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Andrew has a patch up today that allows simplestreams to keep
searching if it hasn't found what it wanted in a given stream:
 https://code.launchpad.net/~axwalk/juju-core/lp1233924-simplestreams-getmetadata-fallback/+merge/188758

There are 2 thoughts on this, so I wanted to bring it up for discussion.

We came to this because the Azure provider was adding the "daily"
stream to the search path. Which meant that we would find the daily
stream, parse it, not find anything and then stop. Without falling
back to the releases stream which has the actual entries we are
looking for. (We "solved" this by just removing the daily stream,
which was also for other reasons.)


I believe the current behavior is for people that set up their own
images. Then when we ask for "precise" we will see the one they set up
(say in their private-bucket, or at a custom imagemetadata-url).

If they don't set up a "saucy" image, then when asked for one, rather
than giving them a *stock* saucy we just give them nothing.

I can understand a bit the idea of "if I ask for something
incorrectly, don't give me something I don't want". But I think most
users would actually be served best with fallbacks enabled.

So if you set up a custom "precise" image, we'll use it when asked,
but if you ask for anything else, you still get the stock images.

I can see a user wanting the existing behavior (I'd rather it fail
early than start something that isn't actually what I want). But I
don't think it should be our default behavior.

I feel strongly enough that I think we can just land his patch and
then file a bug about "if users want this, we can do it in this way".

Thoughts, concerns?

John
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