Photo by on Lukas Unsplash How about a Twitter bot tweeting about significant ERC-20 token transfers? This information can be interesting for traders or to g tive real-time alerts about hacks and exit scams. In this guide we will see how to track token transfers, how to tweet about them and how to host the bot on Heroku for free. The bot runs at and the source code is available on . @erc20events Github Prerequisites To follow this guide you will need a Twitter account for your bot, a free account and on your development machine. Heroku node.js Getting a Twitter developer account After you’ve created a fresh Twitter account, to the developers platform. Fill all the necessary details and wait for Twitter to approve you. It normally takes 24–48 hours. Once approved, in order to generate API keys and tokens. apply for access create a new app Detecting ERC-20 Token Transfers For this project I’ve chosen 12 biggest ERC-20 tokens by market cap and saved them in a Javascript array: Since the list was short, I just copypasted their contract address from . Etherscan So how are we going to follow the transfers of these tokens? Each ERC-20 token transfer emits a Transfer() event. The natural way would be to use the web3.js library to to these events and get notified directly by the blockchain (via Infura’s infrastructure) each time an event is emitted by a tracked ERC-20 contract, like so: subscribe There is only one problem with this approach. First we have to connect to Infura via a web socket: const web3 = new Web3(new Web3.providers.WebsocketProvider(‘wss://mainnet.infura.io/ws’)); And in my experience, this connection is not the most reliable for getting frequent notifications. Since our bot is going to be hosted on Heroku (which restarts its instances every day), we need a more reliable solution. This solution is the free provided by . Specifically the endpoint returns an array of any token’s operations. It can filter by a type of operation (transfer, in our case) and by time. API Ethplorer.io getTokenHistory For example, this call will return the most recent five BNB token transfers. This is exactly what we need. http://api.ethplorer.io/getTokenHistory/0xB8c77482e45F1F44dE1745F52C74426C631bDD52?apiKey=freekey&type=transfer&limit=5 Calculating the value of ERC-20 token transfer We have seen how to poll the Ethplorer API and get the latest ERC-20 token transfer per token contract address. But we are only interested in significant transfers, say transfers of tokens valued above $25,000. To calculate the value of a transfer, we need multiply the amount of tokens transferred by the token’s current going rate. Conveniently, Ethplorer’s API returns a object in the response to API call. This object contains updated price information for the token. But there is a catch. For some interesting tokens, like REP (Augur) and HOT (Holo), the price information is not returned. tokenInfo getTokenHistory To work around this issue, we can use the . The free tier gives you 100,000 calls per month, which is more than enough for retrieving our tokens’ current price daily. CryptoCompare API On initializing our bot, we will simply call CryptoCompare API’s endpoint for each token in our token array and save the price that we get. We only need to do it once, because Heroku reboots the instance daily anyway, so the price will be updated every 24 hours. /data/price Now that we have all token rates, we can easily calculate the values of each token transfer that we detect and tweet only about the significant ones. Continous transfer tracking with setInterval() Knowing how to ask about token transfers and calculate their value, we can now tie it all together: This code does all the work of continuously tracking specific ERC-20 token transfers. We call the Ethplorer API and ask about all the transfers that happened in the last minute, then we send each received to the provided callback. operation The call to Ethplorer API is wrapped in to execute the call repeated with a fixed time delay. The interval is randomized, so that we don’t bombard the API with call bursts. setInterval Filtering the important transfers We have seen that for each detected token transfer, a callback is invoked. This callback function needs to calculate the value of a transfer and tweet about it only if the value exceeds our threshold. Here we convert the amount of transferred tokens into a normalized form, taking into account the decimals that each token defines. Most ERC-20 tokens have 18 decimals (as Ethereum itself), but some are not: USDC stable coin defines only 6 and Zilliqa has 12, so we have to take this into account. Next, we multuply the transferred tokens amount by the price and get the value. If it exceeds the threshold (currently $50000$), we construct a message string and tweet it. We also save the transaction hash so that we won’t tweet about the same transfer more than once. The simple act of tweeting Please see this informative on how to connect to the Twitter API with node.js. article It succintly describes how to generate Twitter API keys and use them in our bot. Once we have those, we can simply tweet any message (and log it to the console too for a good measure): Bringing it all together To connect all these pieces, all we need to do is to read our API secrets from the .env file, initialize the tokens with their prices and start tracking them. Deploying the Twitter bot on Heroku I’ve deployed the bot by signing up on , creating a new app and connecting it to the bot’s Github repo. Heroku There are two modifications needed for Heroku: Add a `Procfile` to the repo’s root. This lets Heroku know that this is a worker process and not a web application, and it only has a single line of code: file worker: node app.js 2. We cannot use the module anymore to read our API secrets from the .env file. Naturally, the .env file is not uploaded to Github and doesn’t get deployed to Heroku. The simple solution is to manually define the secrets as config vars on Heroku’s app settings page: dotenv Results The ERC-20 transfer tracker has been running for a few days now, making 301 tweets and already has 10 followers. bot There are a few things that can be done to improve it, such as make the tweets more visually attractive and filter out stablecoin tokens transfers. Please let me know in the comments if you have any quesitons or suggestions.
