[Tutorial] ERC20 Token Watcher

ERC20 Token Watcher‌

The ERC20 token watcher is a simple dashboard for subscribing events from a ERC20 smart contract. You are able to‌ show events of the Tether ERC20 smart contract on the dashboard.

Project Structure

├── /config
├── /contracts # ERC20 contract files
├── /public
├── /src # frontend code
├── .env # configuration file
├── henesis.yaml # configuration file for integration
├── index.js # API server code
├── jsconfig.json
├── package.json
  • The API server has the following interface.

    • GET /api/events : Returns all events information.

  • Frontend

    • keep polling the end point, GET /api/events , to subscribe to all events information.

Step by Step

  1. Install the Henesis CLI

  2. Clone the sample repository

    git clone https://github.com/HAECHI-LABS/sample-erc20-watcher
  3. Install dependencies

    npm install
  4. Login (If you did not login yet)

    henesis login
  5. Deploy Integration

    henesis integration:deploy
  6. Check the integration-Id and client-Id

    • integration-Id: henesis integration:status

    • client-Id: henesis account:describe

  7. Modify the CLIENT_ID and INTEGRATION_ID in the .env

    CLIENT_ID=<your clien id>
    INTEGRATION_ID=<your integration id>
  8. Build source codes.

    npm run build:standalone
  9. Run the API server and browse http://locahost:3000

    node index.js

How does it works?

There are two main parts in this tutorial: Deployment integration and subscription to events through the Henesis SDK.

Deploy Integration

When you type henesis integration:deploy command at step 5, the Henesis CLI reads henesis.yaml and deploys an integration based on this configuration. Therefore, it is important to correctly set the henesis.yaml file.

version: v1
name: tether-tutorial
- address: '0xdac17f958d2ee523a2206206994597c13d831ec7'
name: TetherToken
files: # The events of the contracts listed below can be combined together at this address.
- path: ./contracts/TetherToken.sol
contractName: TetherToken
compilerVersion: 0.4.18
platform: ethereum
network: mainnet
threshold: 5
type: webSocket

Subscription to Events through the Henesis SDK

index.js is also important in this tutorial as well. This subscribes to events by using the Henesis SDK.

import { EventStreamer } from '@haechi-labs/henesis-sdk-js'
async function henesis() {
// create a new Henesis instance
const eventStreamer = new EventStreamer(CLIENT_ID);
  • It is easy to initiate a webSocket subscription by using the subscribe method.‌ Don't forget the subscriptionId should be unique per subscription.

import { EventStreamer } from '@haechi-labs/henesis-sdk-js'
async function henesis( ) {
const subscription = await eventStreamer.subscribe(
subscriptionId: "your-subscription-id",
ackTimeout: 30 * 1000 // optional. (default: 10000, unit: ms)
  • And you can subscribe to events and parse the data through the following codes.

import Henesis from '@haechi-labs/henesis-sdk-js'
async function henesis( ) {
subscription.on('message', async message => {
// In case of disconnection due to network abnormalities (such as Wi-Fi problem), up to one duplicated message can be delivered.
// You can check message duplication with messageId or block number.
if (getBlockNumber(message) > processedBlockNumber) {
const events = messageToEvents(message);
// processing events
// For example, you can save events to your database.
events.forEach(event => model.push(event));
//console.log(JSON.stringify(events, undefined, 2));
console.log(`data received, event:${events}`);
// You need to remember the processed index(messageId or block number) of the message you received.
message.ack(); // (MUST) Send an ACK message even if duplicated message!!
subscription.on('error', err => {
subscription.on('close', err => {