Cases

Order Book case

Users send bid and ask limit orders into the order book. Every order in the order book should be executed when counter order with the same price will be received. Speaking other word it should happen when bid meets ask at the same price. In this case limit order should be notified that there is a matching order and swap should be executed with resulting funds sent to the owners of both orders. Looks like this notification is reasonable to implement using triggers.

Challenges
But the challenge here is that there might be really a lot of orders in the order book and every order should have a trigger and every trigger should be checked somehow if price counter order exists or not. Also