Help with chaining of sub-requests

[Ashish S]

Hi,

I have 3 existing http backend web-services  S1, S2 and S3.  I am
trying to use nginx for a use-case where  i need to first get a "key"
from service S1, use this key along with some input data, to query S2,
and based on S2's response, query S3.    My question is, would
sub-request chaining be the best method to go about this?  I can use
the upstream method to connect to S1.   Am i doing something wrong, if
I then fire a sub-request (request to S2) from within the content
handler (process header) method registered for this upstream?.  How do
i then initiate the sub-request to S3 ?

Any pointers or example that i might be able to follow, for something
like the above?

Thanks,
Ashish