Discussion on triggers

I was asked this in an interview where the interviewer after talking about basic stock and inventory classes stopped me and wanted to talk to me about triggers . He gave me this scenario .
He said that suppose you are on a stock brokerage system where you are allowed to make decisions in programmatic manner . for example

  1. Sell all Microsoft stocks as soon as average of Microsoft and Facebook stock comes bellow 100
  2. Buy 10 Google stocks as soon as microsoft stock rises above 161 dollar

How user enters this info in UI is immaterial . Assume you know rule and the list stocks symbols on a rule give to you in a vector . 1 user will have just 1 rule .

He said that such a system would need to be extremely fast . As soon as a stock changes all programmatic rules are executed . How will you design this triggering system given that a stock brokerage system will have 1 milion active users .