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Right, you don't want to read it until after cfn-signal is called. That's why "Fn::GetAtt" adds a dependency, so any instance referencing the WaitCondition's Data won't be created until after the WaitCondition is complete (i.e. cfn-signal has been called). So the method I mentioned above is guaranteed to work by design - in fact, that's the entire point of orchestration.

The cfn-get-metadata method, on the other hand, is not portable (won't work on CloudFormation) and is extremely likely to stop working on Heat in the future, possibly the very near future. It's making use of a hack (WaitCondition data is stored in the metadata of the handle) in Heat that we are already taking steps to eliminate.