How can a Smart Contract ask informations to a node?

Question

I need to code something quite complex compared to my experience. Just for academic purpose, I'm supposed to have a lightweight blockchain that connects a group of nodes; in this blockchain every node ask and send informations to each other through a smart contract. By ignoring the problem of how to set up this kind of blockchain, my only duty is to program a test code for the smart contract and the Client (using Python) that will be run by every node.

My problems start when i have to ask informations to a node from the smart contract: Let's suppose that our contract has a function "getDataFrom(address)" which can be called by a node when he needs to get specific data from another node. How will this code be structured? The only thing that comes to my mind is to make a transaction from the smart contract to the address, but even in this case how much ETH should be used? Then, if this approach is correct, i think that in the Client code there will be a function that handles a received transaction and then calls a "returnData(data)" from the smart contract, right?

I don't know if that's correct or not, so please i'd be very grateful if someone can help me. Another thing i wanted to ask, is if exists some way to make the Client functions usable from the Smart Contract without sending a generic transaction, i think that this would stop all of my problems.

Answer 1

A smart-contract is a passive software object that, on its own initiative, cannot request anything from anyone. A smart-contract is activated by a transaction sent to its address. The smart-contract cannot directly request anything on the Node, but it can emit an Event containing the request required for the smart-contract. The corresponding Node catch the Event and, having processed it, sends a transaction with the necessary data to the smart-contract.

For detail see https://stackoverflow.com/a/65528527/6459192 (а more complex case is described there, when an additional proxy contract is used for the request, but the principle, I think, is clear)