Aled Sage commented on BROOKLYN-356:

An easy way to reproduce this (when debugging) is to change 
{{DependentConfiguration.$WaitInTaskForAttributeReady.call()}} to add a sleep 
at the start (e.g. {{Time.sleep(500);}}).

Then run 
It fails first time because the transformer's attempt to retrieve the value 
times out after 200ms (whereas the task will take a minimum of 500ms).

> The sensor Transformer enricher fails to resolve its targetValue 
> indeterministically.
> -------------------------------------------------------------------------------------
>                 Key: BROOKLYN-356
>                 URL: https://issues.apache.org/jira/browse/BROOKLYN-356
>             Project: Brooklyn
>          Issue Type: Bug
>            Reporter: Svetoslav Neykov
> The sensor {{Transformer}} enricher fails to resolve its {{targetValue}} 
> indeterministically, especially so on loaded systems. This is especially 
> problematic if sourceSensor/triggerSensors change infrequently or do not 
> change ever (when using default values).
> Bueprints using the {{Transformer}} behave perfectly fine during development 
> and on test setups, but will fail on production machines (i.e. under load).
> The code [doing the value 
> resolving|https://github.com/apache/brooklyn-server/blob/b59e7463a9b337c2d0e7931cd420d5bac68d8549/core/src/main/java/org/apache/brooklyn/enricher/stock/Transformer.java#L90-L94]
>  tries to do so in a non-blocking fashion by spinning a thread to try to 
> resolve and cancelling it after a short while without guarantees it ever got 
> scheduled to run. It's more likely to fail when nesting DSLs, for example 
> nesting several levels of {{$brooklyn:formatString}} and ending with a 
> {{$brooklyn:attributeWhenRready}}. It's a common pattern in moderately 
> complex blueprints. It needs to schedule a thread for each nesting level thus 
> maximizing the chance that the value will not be resolved in the allotted 
> time even if resolvabe. This is especially bad for sensors which don't get 
> updated, for example {{PortAttributeSensorAndConfigKey} set only when 
> initializing the entity or any config values which are always resolvable.

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