How to sign a transaction automatically? [duplicate]

Question

An api will call the smart contract function. It should sign the transaction on its own instead of human intervention.

I think raw transaction can work here. Are there any other options?

Any help is much appreciated. Thanks!

--Edit-- The question is updated!

Answer 1

Try this (using web3.js v1):

let Web3 = require("web3");
let web3 = new Web3(NODE_ADDRESS);

async function send(transaction, reestimate = function(gas) {return gas;}) {
    let gas = await transaction.estimateGas({from: PUBLIC_KEY});
    let options = {
        to  : transaction._parent._address,
        data: transaction.encodeABI(),
        gas : reestimate(gas)
    };
    let signedTransaction = await web3.eth.accounts.signTransaction(options, PRIVATE_KEY);
    return await web3.eth.sendSignedTransaction(signedTransaction.rawTransaction);
}

Usage example #1:

let receipt = await send(myContract.methods.myFunc(arg1, arg2, arg3));
...

Usage example #2:

let reestimate = function(gas) {return gas * 2;};
let receipt = await send(myContract.methods.myFunc(arg1, arg2, arg3), reestimate);
...

Answer 2

A contract can not sign transaction by its own. The transaction must be signed by externally owned accounts. If a transaction is not signed by an externally owned account then it won't be included in the blockchain.