If it _needs_ to be in RAM, then either you got a big enough machine (then by definition it's not Big Data) or it's impossible. If you manage to come by with RA

If it _needs_ to be in RAM, then either you got a big enough machine (then by definition it's not Big Data) or it's impossible. If you manage to come by with RAM using smart algorithms, although the full dataset would never fit in RAM, then it's Big Data. So I'd argue, stream processing is Big Data, exactly because it doesn't rely too much on RAM.