Ethereum: Issues with Multiple Contract Method Calls
When working with Web3.js, using multiple contracts concurrently 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 go into the details of how Web3.js handles concurrent method calls and provide guidance on how to mitigate potential issues.
Issue
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 exactly concurrent.
When you make three method calls to mint on different contracts (TokenContract, MintTokenContract, and MintToAddressContract), you may notice an exception. This problem occurs because 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 the contract “mint”, Web3j returns a callback object. This callback is used to process the result of the method call.
- Promises: The first two calls (
contract.mint(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 method calls to be called at the same time.
Sample Code
Here is a sample code snippet that illustrates using Web3j with callbacks:
`javascript
const Web3 = require('web3');
const web3 = new Web3(new Web3.providers.HttpProvider('
// Define contract and method parameters
const TokenContract = {
mint: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Created token ${result.tokenId});
});
}
};
// Define three contracts and their methods
const MintTokenContract = {
mintToken: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Selected token in MintToken contract);
});
}
};
const MintToAddressContract = {
mintToAddress: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Selected token in address ${result.address}`);
});
}
};
// Make three method calls simultaneously
web3.eth.getAccounts((err, accounts) => {
if (err) throws an error;
// Call the MintToken contract first
const mintTokenPromise = TokenContract.mintToken({ from: accounts[0] })
.then(() => console.log(‘Token made in MintToken contract’));
mintTokenPromise.then(() => {
// Call the MintToAddress contract second
const mintToAddressPromise = MintToAddressContract.mintToAddress({ from: accounts[1] })
.then(() => console.log(‘Token cast to address in MintToAddress contract’));
mintToAddressPromise.then(() => {
// Call the TokenContract a third time (this is not needed)
const mintTokenPromise3 = TokenContract.mintToken({ from: accounts[2] })
.then(() => console.log(‘Selected token in TokenContract’));
return mintTokenPromise3;
});
}).catch((err) => {
console.