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Ethereum: Problems with Multiple Contract Method Calls
When working with Web3.js, interacting with multiple contracts in parallel can be a common challenge. One such problem is trying to mint tokens using multiple contract methods at the same time. In this article, we’ll dive into how Web3.js handles concurrent method calls and provide tips on how to mitigate potential issues.
Problem
Imagine you have three contracts: “TokenContract”, “MintTokenContract”, and “MintToAddressContract”. You want to mint the token using these different methods at the same time. However, due to the asynchronous nature of Web3.js, each contract invocation runs on a separate thread or process, which may not be exactly concurrent.
When you make three calls to the mint method in different contracts (TokenContract, MintTokenContract, and MintToAddressContract), you may see an exception. This issue occurs because methods are called one after another without ensuring that they do not conflict 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
mintin the contract, Web3j returns a callback object. This callback is used to handle the result of the method call.
- Promises
: The first two calls (
contract.mint(mintParams).sendAsync()) return promises that resolve after themintmethod completes. The third call also returns a promise that resolves when the “mint” method completes.
By using callbacks for the first two invocations and promises for the third invocation, Web3j ensures that each contract method invocation is handled independently of the others. This approach prevents concurrency issues and allows multiple method calls to be made at the same time.
Sample Code
Here is a sample code snippet that shows how to use Web3j with callbacks:
“javascript
const Web3 = require('web3');
const web3 = new Web3(new Web3.providers.HttpProvider('
// Defines contract and method parameters
const ContractToken = {
mint: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Generated token ${result.tokenId});
});
}
};
// Defines three contracts and their methods
const MintTokenContract = {
mintToken: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Token generated in MintToken contract);
});
}
};
const MintToAddressContract = {
mintToAddress: (mintParams) => {
return web3.eth.mint(mintParams).on('completed', (error, result) => {
console.log(Token minted in ${result.address}`);
});
}
};
// Make three calls to the method at once
web3.eth.getAccounts((err, account) => {
if (err) throws err;
// Call the MintToken contract first
const mintTokenPromise = TokenContract.mintToken({ from: account[0] })
.then(() => console.log(‘Token generated in MintToken contract’));
mintTokenPromise.then(() => {
// Call the MintToAddress contract second
const mintToAddressPromise = MintToAddressContract.mintToAddress({ from: account[1] })
.then(() => console.log(‘Token generated for address in MintToAddress contract’));
mintToAddressPromise.then(() => {
//Call TokenContract a third time (this one is not necessary)
const mintTokenPromise3 = TokenContract.mintToken({ from: account[2] })
.then(() => console.log(‘Token minted in TokenContract’));
return mintTokenPromise3;
});
}).catch((err) => {
console.
