Sometimes you can use the SFlow or Netflow to extra add intelligence to your network. The collectors available on internet are usually there just to collect and store data used for accounting or nice graphics. But the collectors are either not allowing you to execute your own code in case of certain rules/thresholds reached, or do not react in real time (in general, the protocols delays you too. You cannot expect NetFlow accounting to be used in real time at all, while SFlow has modes that are bit more fast to react, by design, it is still not considered to be real-time sampling/accounting).
Just imagine you have a simple goal - you want to automatically detect floods and notify the operators or you can even automatically apply filters.
If you have an algorithm that can distinguish the incorrect traffic from the normal traffic from NetFlow/SFlow sampling you may like to execute an operation immediately when that happens.
The modern DoS attacks and floods may be complex and hard to detect. But mainly it is hard to make the currently available NetFlow/SFlow collector software to do that for you and then trigger/execute external application.
However, it is very easy to program it yourself.
I am giving you a simple example that uses the node-sflow module to collect packet samples, measure how many of them match a certain destination ip address and if they are above certain pps thresholds to execute an external program (that is supposed to block that traffic). Then after a period of time it will execute another program (that is supposed to unblock the traffic).
This program is very small - about 120 lines of code and allows you to use complex configuration file where you can define a list of rules that can match optionally vlans and networks for the sampled packet and then count how many samples you have per destination for that rule. The rule list is executed until first match in the configured order within the array, so that allows you to create black and white lists and different thresholds per networks and vlans, or to have different rules per overlapped ip addresses as long as they belong to different vlans.
Keep in mind this is just an example software there just for your example, showing you how to use node-sflow and pcap modules together! It is not supposed to be used in production, unless you definitely know what you are doing!
The goal of this example it here just to show you how easy is to add extra logic within your network.
The code is available on git-hub here https://github.com/delian/sflow-collector/