Have you ever wondered how the Fidelity platform (and many other stock brokers) allows its users to make limit orders for purchasing a particular commodity? My best guess is that each user when submitting a limit order receives a corresponding cron job. The cron job runs all the time and keeps checking if the conditions set by the user have become True. If they have the cron job executes the order.
My guess could be completely wrong. But how does one create cron jobs on the fly, through a medium such as a user interface? Furthermore, lets say that the cron job needs to make calls to an API. 10 cron jobs will make approximately 10 more calls to the API than a single job which really becomes adds up if the API is not free :/ .