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smart-contract-event-listener
A smart contract event listener implemented in Node.js, using Web3.js, and deployed on AWS using AWS CDK.
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Before we start, make sure to check or clone the sample repository, which contains all the necessary code and files for the smart contract event listener. We'll refer to this repository throughout the guide.
import 'dotenv/config'; import * as erc20abi from './abi.json'; import Web3, { Contract, WebSocketProvider } from 'web3'; /* Workaround for JSON.stringify() event logs with BigInt values. We need to stringify event logs for more readable logging in CloudWatch. https://github.com/GoogleChromeLabs/jsbi/issues/30 */ (BigInt.prototype as any).toJSON = function () { return this.toString(); }; /** * Starts the smart contract event listener. * Websocket Provider config: https://docs.web3js.org/api/web3-providers-ws/class/WebSocketProvider * @param chain - Name of the blockchain network for logging purposes. * @param wssEndpoint - Websocket endpoint for the blockchain network. * @param contractAddress - Smart contract address. */ const startEventListener = async (chain: string, wssEndpoint: string, contractAddress: string) => { const provider = new WebSocketProvider( wssEndpoint, {}, { autoReconnect: true, delay: 10000, // Default: 5000 ms maxAttempts: 10, // Default: 5 }, ); provider.on('connect', () => { console.log(`Connected to ${chain} websocket provider`); }); provider.on('disconnect', error => { console.error(`Closed ${chain} webSocket connection`, error); }); const web3 = new Web3(provider); /* Smart contract event listeners Listening to events: - Transfer - Approval */ const contract = new web3.eth.Contract(erc20abi, contractAddress); await subscribeToEvent(chain, contract, 'Transfer'); await subscribeToEvent(chain, contract, 'Approval'); }; /** * Subscribes to a smart contract event. * @param chain - Name of the blockchain network for logging purposes. * @param contract - Smart contract address. * @param eventName - Name of the event to subscribe to. */ const subscribeToEvent = async (chain: string, contract: Contract, eventName: string) => { const subscription = await contract.events[eventName](); subscription.on('connected', subscriptionId => { console.log(`${chain} USDT '${eventName}' SubID:`, subscriptionId); }); subscription.on('data', event => { console.log(`${chain} USDT '${eventName}'`, JSON.stringify({ event })); // cannot json.stringify BigInt... }); subscription.on('changed', event => { // Remove event from local database }); subscription.on('error', error => { console.error(`${chain} USDT '${eventName}' error:`, error); }); }; /* Start smart contract event listeners Chains: - Ethereum */ startEventListener('Ethereum', process.env.ETH_WSS_ENDPOINT!, process.env.ETH_SMART_CONTRACT_ADDRESS!);
JavaScript, TypeScript, and Node.js
Docker
Now that you have a working smart contract event listener, we'll deploy the resources to AWS using AWS CDK, which is an Infrastructure as Code (IaC) tool. AWS CDK allows you to configure, deploy, and manage AWS cloud resources using popular programming languages such as TypeScript.