Github user aledsage commented on the issue:
https://github.com/apache/brooklyn-server/pull/565
@neykov you beat me to replying - agreed :-)
@ahgittin to give some more background, the original use-case for
"immediate" stemmed from a failure in clocker:
https://github.com/brooklyncentral/clocker/blob/v2.1.0/swarm/catalog/swarm/swarm.bom#L260-L270
In a `Transformer` enricher, the target value is evaluated each time the
"trigger sensor" changes, but its value may not yet be resolvable. It uses the
idea of "immediately" to avoid leaving a bunch of threads running - see
https://github.com/apache/brooklyn-server/blob/rel/apache-brooklyn-0.10.0/core/src/main/java/org/apache/brooklyn/enricher/stock/Transformer.java#L81-L98.
Originally this was purely time-based (cancel after 200ms).
However, sometimes on a heavily loaded machine, the "immediately" would
time out after 200ms even though the value really was available. The trigger
sensor would never change again, so the target sensor would never be set.
By adding `ImmediateSupplier` we were able to call something that would
only return "absent" if it really could not be resolved yet.
It is that style of use-case / usage that makes me want to let
`$brooklyn:external` do IO operations (rather than aborting them by
interrupting the thread).
I believe this model would be simpler for a user to understand, rather than
explaining to tell them that their config resolution will fail in certain
situations, such as the `Transformer`, if it requires IO operations (e.g. if it
tries to contact vault, or read from a file, etc).
Longer term, I definitely want to get rid of the ugly/brittle "wait for up
to 500ms", but I'd prefer we leave that in for a while longer rather than
remove support for the above use-case.
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