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Callback and SOAP requests
SOAP requests sometimes take a very long time before receiving an answer. Downloading is done immediately if possible (there is a queue and a limit per user) and the callback only responds once the downloading is over (there is a timeout which may return an answer before). But the downloading of big files (several hundreds of MB or even GB) can take a few minutes depending upon which distant server is called.
Depending upon the client SOAP settings, you may not need to wait for the answer of the SOAP server when the functions are called. This prevents the call from blocking and therefore allows to make more successive calls quite quickly without waiting.