If I remember it correctly there is a functionality in IRIS to make this possible.
I've got two incoming Business Services of files from different sources.
BS1: An Ens.StreamContainer with some info and metadata which point to a specific file. Ens.StreamContainer will be send to a BP.
BS2: The files where the metadata points to. It's in the Ens.StreamContainer
But dependent on the flow the file could be later available on BS2. So I want the BPL to wait for the file to arrive at BS2.