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Following the previous paragraph,  the natural way to group requests would be:
all parameters, all levels, all members, all time-steps for 1 hindcast date.

Note the following:

1. 'all' means 'all' that the user wants. It doesn't have to be all parameters.
2. If a user is interested only on z500,  he may request more hindcast dates in one go, since the overall request will not be so big.

for hindcastDate in hindcastDateList (eg, 2010-03-01 to 2010-03-31)
     S2S-request(hindcastDate)

You may wish to have a look on some CMA re-forecast examples or to visit the ECMWF Web API Home