Ethereum: Multiple Contract Method Calls Having Issues
When working with Web3.js, interacting with multiple contracts in parallel can be a common challenge. One such issue arises when trying to mint tokens using different contract methods at the same time. In this article, we will explore how Web3.js handles concurrent method calls and provide guidance on how to mitigate potential issues.
Problem
Imagine that you have three contracts: TokenContract, MintTokenContract, and MintToAddressContract. You want to mint a token using these different methods at the same time. However, due to the asynchronous nature of Web3.js, each contract call is executed in a separate thread or process, which may not be entirely concurrent.
When you call the three mint methods on different contracts (TokenContract, MintTokenContract, and MintToAddressContract), you may notice that an exception is thrown. This problem occurs because the methods are called one after the other without ensuring that they do not interfere with each other or cause concurrency issues.
Web3j Solution
In Web3j, multiple method calls to the same contract are handled using a combination of callbacks and promises. Here’s how it works:
- Callbacks
: When you call mint on a contract, Web3j returns a callback object. This callback is used to process the result of the method call.
- Promises: The first two calls (“contract.mint(mintParams).sendAsync()”) return promises that resolve when the mint method completes. The third call also returns a promise that resolves when the mint method completes.
By using callbacks for the first two calls and promises for the third call, Web3j ensures that each contract method call is handled independently of the others. This approach prevents concurrency issues and allows multiple methods to be called at the same time.
Code example
Here is an example code snippet showing how to use Web3j with callbacks:
“javascript
const Web3 = require('web3');
const web3 = new Web3(new Web3.providers.HttpProvider('
// Define the contract and method parameters
const TokenContract = {
mint: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Minted token ${result.tokenId});
});
}
};
// Define the three contracts and their methods
const MintTokenContract = {
mintToken: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Minted token in MintToken contract);
});
}
};
const MintToAddressContract = {
mintToAddress: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Password created at ${result.address}`);
});
}
};
// Make three method calls at once
web3.eth.getAccounts((err, accounts) => {
if (err) throw error;
// Call the MintToken contract first
const mintTokenPromise = TokenContract.mintToken({ from: accounts[0] })
.then(() => console.log(‘Minted key in “MintToken” contract’));
mintTokenPromise.then(() => {
// Call the MintToAddress contract second
const mintToAddressPromise = MintToAddressContract.mintToAddress({ from: accounts[1] })
.then(() => console.log(‘Minted access key for address MintToAddress in contract’));
mintToAddressPromise.then(() => {
// Call the TokenContract a third time (this is not needed)
const mintTokenPromise3 = TokenContract.mintToken({ from: accounts[2] })
.then(() => console.log(‘Minted token in TokenContract’));
return mintTokenPromise3;
});
}).catch((err) => {
console.