Doing some research on smart contract systems. I want to create a self executing smart contract that can monitor external data like say the weather from an api. Currently ethereum and other smart contract systems requires a separate entity to execute these smart contracts, otherwise they just sit there dormant. I can run a cron job every minute to pull down weather api data and if its sunny fire the smart contract, but that requires me to constantly have a server on. Are there other systems that doesn't require something like this or have a built in smart contract system that is "self executing" by detecting external data?