How to track incoming to smart contract transactions using web3js?

Question

I've got one Ethereum smart contract which calls functions of the other one. How could I track these calls using web3js or some other tool if it is possible at all? I need it because I want to execute some JavaScript (or other non-solidity language) code after receiving such call and then call other function of first smart contract so the sequence of calls should be following:

1. Smart contract N1 calls N2 smart contract function.
2. JavaScript (or other non-solidity language) code catches this call and execute some code.
3. Smart contract N2 calls back N1 using data from previous step.

Could anybody help with it?

Answer 1

What you are trying to archive is contrary to the design principles of Solidity and the blockchain. The answer to the question Why can't contracts make API calls? will pretty much explain why.

In short words: If you would open up Solidity to call arbritrary non Solidity code it would become non-deterministic as that non Solidity code would not exist outside of the blockchain and could be modified independently.

Answer 2

Contract N1 interfaces method A in contract N2.

Once method A in contract N2 successfully executes, it logs an event on the blockchain and it passes as argument the output of the method.

You then use a service which monitors your contracts events (such as Openzeppelin Defender Sentinels) to catch the events of your contract, send you a webhook, parse the json of the webhook extract the parameter that was passed with the event that you want to pass to N1 and use the output of method N2 to then call using web3.py contract N1.

Alternatively, you can simply interface method 1 N2 in method 1 N1, and return the value as a normal return variable without any inbetween off chain stuff happening and this would all be on chain which is better and cheaper.