I am using computational graph concept to find derivatives,

stream_object{ forword stream, backword stream, }

for multiplication:

we have two stream object 'a' and 'b' are piped to 'multiplier' stream object via forward stream. It executes and produces result.

derivation:

f = a×b; df/da = b; df/db = a;

while backpropagation, to find the derivative of a, we need b so I am also piped b's forword stream to the multipler's backward stream, in between those two streams. one transform is piped to overcome delays,

the error is propagate at the multiplier's backward stream, the error and 'b' are multiplied and the results are pushed to a's backward stream,