evm-watcher

EVM Watcher

This NPM Module is a thin wrapper around ethersjs module.

This is only used to talk to ethereum nodes, and follow along in the blockchain watching for specific events.

Then install it into your project

npm install evm-watcher

Then import it with it's types and everything.

import { EvmWatcher } from "evm-watcher";

Examples

See the src/example.ts file for a working example. Try it out by cloning this repo locally, and running:

npm install

ts-node ./src/example.ts

How It Works

IMPORTANT: This module requires that you bring your own state. If you run a stateless application, it will not be able to persist important data between subsequent invocations (page-refresh, restarts, etc.) You will need a place for this module to keep track of the "last block processed", but should not need to worry about the minutia of using/refreshing this state.

Create some initial params to bound our ethereum "worker".

const initialParams = {
  // startBlock (required) must be an integer > -1. No decimals, or abstract numerical values (e.g.: Infinity).
  startBlock: 12508210,
  // endBlock (optional) default = undefined, meaning: run forever.
  endBlock: undefined,
  // network (optional) default = ETHERUM_MAINNET
  network: SupportedNetwork.ETHERUM_MAINNET,
  // maxLogBatchSize (optional) default = 10
  maxLogBatchSize: 100,
};

Create a Data Access Object matching the expected interface, capable of reading/writing to state.

const dao = {
  getWorkerState: async ({
    startBlock,
    endBlock,
  }: {
    startBlock: number;
    endBlock: number;
  }) => {
    // ... Do anything! :D
    // ... e.g.: query postgres, or read form S3, call your grandmother...
    // This just needs to return an object matching the interface of the WorkerState.
    return testState; // < -- replace with your data
  },
  setLastBlockProcessed: async (lastBlock: number) => {
    // Again, do whatever you need to do here, but this must write lastBlock to state, such that it is retrieved when `getWorkerState` is called.
    testState.lastBlockProcessed = lastBlock;
    return undefined;
  },
};

Choose an event filter.

// This is an example of an event filter that the Aggregator uses.
const TOPIC_TRANSFER_721_OR_20 =
  "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef";
const TOPIC_TRANSFER_1155 =
  "0xc3d58168c5ae7397731d063d5bbf3d657854427343f4c083240f7aacaa2d0f62";
const TOPIC_TRANSFER_BATCH_1155 =
  "0x4a39dc06d4c0dbc64b70af90fd698a233a518aa5d07e595d983b8c0526c8f7fb";

const eventFilter = {
  topics: [
    [TOPIC_TRANSFER_BATCH_1155, TOPIC_TRANSFER_1155, TOPIC_TRANSFER_721_OR_20],
    null,
    null,
    null,
  ],
};

// This is an example of an event filter looking for events around a single address.
const eventFilter = { address: "0xD5525D397898e5502075Ea5E830d8914f6F0affe" };

"Subscribe" to a stateful event stream, which will ensure you fully process a block. This is not truly pub/sub, as it's operating on promises, not Nodejs Events... but it does give you that pubby-subby feel you love so much.

const watcher = new EvmWatcher({ dao, initialParams });

watcher.onLogEvent(eventFilter, async (log, isNewBlock) => {
  if (isNewBlock)
    console.log(
      "I should now delete transactional data for block " +
        log.blockNumber +
        "incase this is a restart."
    );
  console.log(log.transactionHash); // ... do stuff.
  return undefined;
});

What if there is a failure!?

If/when your log processing function, or anything within this module fails for any reason in the middle of processing logs for a block, Errors will bubble-up, and that block will be considered "unprocessed." This means that in the event of a restart of your failed process, your onLogEvent handler may be invoked with logs from a block it has already partially processed. To ensure that you don't double-record, it's encouraged that you delete any transfer events you have recorded before recording anything for a new block (Note the second param in your callback function).