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On 03/04/14 03:21, Thomas Herve wrote:
>> Speaking of offering options for selection, there is another proposal on
>> >adding conditional creation of resources [3], whose use case to enable
>> >or disable a resource creation (among others).  My perception is that
>> >these are all relevant enhancements to the reusability of HOT templates,
>> >though I don't think we really need very sophisticated combinatory
>> >conditionals.
> I think that's interesting that you mentioned that, because Zane talked about a "variables" section, which would encompass what "conditions" and "mappings" mean. That's why we're discussing extensively about those design points, to see where we can be a bit more generic to handle more use cases.

There was some discussion in the review[1] of having an if/then function 
(equivalent of the ternary ?: operator in C) for calculating variable... 
on reflection that is nothing more than a dumbed down version of 
Fn::Select in CloudFormation (which we have no equivalent to in HOT) in 
which the only possible index values are "true" and "false".

The differences between Fn::Select and Fn::FindInMap are:

1) The bizarre double-indirect lookup, of course; and
2) The actual mappings are defined once in a single place, rather than 
everywhere you need to access them.

I think we're all agreed that (1) is undesirable in itself. It occurs to 
me that the existence of a variables section could render (2) moot also 
(since you could calculate the result in one place, and just reference 
it from there on).

So if we had the variables section, we probably no longer need to 
consider a mapping section and a replacement for Fn::FindInMap, just a 
replacement for Fn::Select that could also cover the if/then use case.

Thoughts?

cheers,
Zane.

[1] https://review.openstack.org/84468