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I've recently been writing a tool which uses Nova's external API. This
is my first time consuming this API, so it has involved a certain amount
of discovery. The tool is here for the curious:

  https://gist.github.com/mdbooth/163f5fdf47ab45d7addd

I have felt hamstrung by the general inability to distinguish between
different types of error. For example, if a live migration failed is it
because:

1. The compute driver doesn't support support it.

2. This instance requires block storage migration.

3. Something ephemeral.

These 3 errors all require different responses:

1. Quit and don't try again.

2. Try again immediately with the block migration argument.[1]

3. Try again in a bit.

However, all I have is that I made a BadRequest. I could potentially
grep the human readable error message, but the text of that message
doesn't form part of the API, and it may be translated in any case. As
an API consumer, it seems I can't really tell anything other than 'it
didn't work'. More than that requires guesswork, heuristics and inference.

I don't think I've missed some source of additional wisdom, but it would
obviously be great if I have. Has there ever been any effort to define
some contract around more fine-grained error reporting?

Thanks,

Matt

[1] Incidentally, this suggests to me that live migrate should just do
this anyway.
-- 
Matthew Booth
Red Hat Engineering, Virtualisation Team

Phone: +442070094448 (UK)
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