How to Make a Syncer based on Moonscan API | Moonbeam Branch 0x02 - Web3dAppDevCamp

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MoonbeamScan ：

https://moonbeam.moonscan.io/

Web3DevNFT Contract in this article：

https://moonbeam.moonscan.io/address/0xb6FC950C4bC9D1e4652CbEDaB748E8Cdcfe5655F

Basic Concept

What is a Syncer? It's a scheduled job to retrieve transactions from one contract regularly and persist them locally.

The Syncer has two main parts:

Syncer: responsible for retrieving transactions from the contract and persistence handling.
Syncer Server: repsonsible for job scheduling and monitoring.
Parser：Parse the transactions.

Syncer is useful, because it formats and caches on-chain data, allowing various dApps and services to read on-chain data more quickly, improving the experience.

The traditional Syncer is implemented by connecting blockchain nodes, which has the advantage of being more stable, but we also have a lighter option -- to implement Syncer by calling the API of the Etherscan-like browser! Etherscan provides an interface to get all transactions of a specific contract, so we can do Syncer with lower complexity. This doesn't sound so Web3 because Syncer relies on the browser service to function properly, but in most scenarios, it works well!

Implementation in Elixir

Repo：

https://github.com/WeLightProject/tai_shang_nft_gallery

Syncer

The processing logic of Syncer contain below three main steps:

Retrieves the latest block number of the blockchain.
Based on the last locally persited block number, retrieves transactions from the chain using Etherscan API and persist locally.
Update last block number of the contract locally.

# https://github.com/WeLightProject/tai_shang_nft_gallery/blob/main/lib/tai_shang_nft_gallery/nft/syncer.ex

defmodule TaiShangNftGallery.Nft.Syncer do
  alias TaiShangNftGallery.NftContract
  alias TaiShangNftGallery.TxHandler
  alias TaiShangNftGallery.Chain.Fetcher
  alias TaiShangNftGallery.ScanInteractor
  require Logger

  @api_keys %{
    "Moonbeam" => System.get_env("MOONBEAM_API_KEY")
  }
  def sync(chain, %{last_block: last_block} = nft_contract) do

    # Step 1
    best_block = Fetcher.get_block_number(chain)
    # Step 2
    do_sync(chain, nft_contract, last_block, best_block)
    # Step 3
    NftContract.update(
      nft_contract,
      %{last_block: best_block + 1}
    )
  end

  def do_sync(%{name: name} = chain, %{addr: addr} = nft_contract, last_block, best_block) do
    # Step 2.1 - Retrieves Txns by Etherscan API
    {:ok, %{"result" => txs}}=
      ScanInteractor.get_txs_by_contract_addr(
        chain,
        addr,
        last_block,
        best_block,
        @api_keys[name]
      )

    # Step 2.2 - Persist transactions locally
    handle_txs(chain, nft_contract, txs)
  end

  def handle_txs(chain, nft_contract, txs) do
    Enum.each(txs, fn tx ->
      Logger.info("Handling tx: #{inspect(tx)}")
      tx_atom_map = ExStructTranslator.to_atom_struct(tx)
      TxHandler.handle_tx(chain, nft_contract, tx_atom_map)
    end)
  end
end

Syncer Server

Syncer Server is implemented by GenServer. For those who do not know about it, you can imagine that it's some kind of process that runs forever. If there is any error, it can automatically restart.

The GenServer operates by handling the messages received, from itself or others. There are two main functions here:

init: The initialization function. It loads the Contract locally, store it in the process state and sends itself a :sync message.
handle_info: The :sync message handler that calls the API of the Syncer to do the actual work.

After each processing, the Process.send_after/3 function sends itself a :sync message after 10 minutes.

# https://github.com/WeLightProject/tai_shang_nft_gallery/blob/main/lib/tai_shang_nft_gallery/syncer_server.ex
defmodule TaiShangNftGallery.SyncerServer do
  @moduledoc """
    Genserver as Syncer
  """
  alias TaiShangNftGallery.Nft.Syncer
  alias TaiShangNftGallery.NftContract
  use GenServer
  require Logger

  @sync_interval 600_000 # 10 minutes
  # +-----------+
  # | GenServer |
  # +-----------+
  def start_link(state) do
    GenServer.start_link(__MODULE__, state, name: :"#nft_syncer")
  end

  def init([nft_contract_id: nft_contract_id]) do
    Logger.info("SyncerServer started yet.")
    nft_contract =
      nft_contract_id
      |> NftContract.get_by_id()
      |> NftContract.preload()
      state =
        [nft_contract: nft_contract]
    send(self(), :sync)
    {:ok, state}
  end

  def handle_info(:sync, [nft_contract: nft_contract] = state) do
    Syncer.sync(nft_contract.chain, nft_contract)
    sync_after_interval()
    {:noreply, state}
  end

  def sync_after_interval() do
    Process.send_after(self(), :sync, @sync_interval)
  end
end

Decode Transaction

Different transactions actually represent different contract method call. In order to identify what exactly the transaction is doing, we have to decode the transaction input because the input is a hex value like 0x379607f5000000000000000000000000000000000000000000000000000000000000026d. TxHandler.handle_tx/3 uses ABI related APIs encapsulated in ABIHandler.find_and_decode/2 to accomplish that.

# https://github.com/WeLightProject/tai_shang_nft_gallery/blob/main/lib/tai_shang_nft_gallery/tx_handler.ex
defmodule TaiShangNftGallery.TxHandler do
  @moduledoc """
      Handle Ethereum Tx.
  """

  alias TaiShangNftGallery.ABIHandler
  alias TaiShangNftGallery.NftContract

  def handle_tx(chain, nft_contract, tx) do
    %{
      from: from,
      to: to,
      value: value,
      input: input
    } = tx

    do_handle_tx(chain, from, to, value, input, nft_contract)
  end

  def do_handle_tx(chain, from, to, value, input, %{type: type} = nft_contract) do
    input_handled =
      nft_contract
      |> NftContract.preload(:deep)
      |> Map.get(:contract_abi)
      |> Map.get(:abi)
      |> ABIHandler.find_and_decode(input)

    "Elixir.TaiShangNftGallery.TxHandler.#{type}"
    |> String.to_atom()
    |> apply(:handle_tx, [chain, nft_contract, from, to, value, input_handled])
  end
end

# https://github.com/WeLightProject/tai_shang_nft_gallery/blob/main/lib/tai_shang_nft_gallery/abi_handler.ex
defmodule TaiShangNftGallery.ABIHandler do
  alias Utils.TypeTranslator
  def find_and_decode(abi, input_hex) do
    abi
    |> ABI.parse_specification
    |> ABI.find_and_decode(TypeTranslator.hex_to_bin(input_hex))
  end
end

In the exact contract handler (type Web3Dev), we can see that only below methods are handled. Other than those, it returns {:ok, "pass"} directly.

safeTransferFrom / trasnferFrom: Used to transfer the ownership of the NFT
claim: Used to initialize the NFT token
setTokenInfo: Used to set NFT token info

# https://github.com/WeLightProject/tai_shang_nft_gallery/blob/main/lib/tai_shang_nft_gallery/tx_handler/web_3_dev.ex
defmodule TaiShangNftGallery.TxHandler.Web3Dev do
  alias TaiShangNftGallery.Nft
  alias TaiShangNftGallery.Nft.Interactor
  alias Utils.TypeTranslator

  require Logger
  def handle_tx(chain, nft_contract, from, to, value,
    {%{function: func_name}, data})  do
      do_handle_tx(
        func_name,
        nft_contract, from, to, value, data, chain
      )
  end

  def handle_tx(_chain, _nft_contract, _from, _to, _value, _others) do
    :pass
  end

  def do_handle_tx(func, _nft_contract, from, _to, _value,
    [_from_bin, to_bin, token_id], _chain) when
    func in ["safeTransferFrom", "transferFrom"] do
    # Change Owner
    to_str = TypeTranslator.bin_to_addr(to_bin)
    Logger.info("Transfer NFT from #{from} to #{to_str}")
    nft = Nft.get_by_token_id(token_id)
    Nft.update(nft, %{token_id: token_id, owner: to_str})
  end

  def do_handle_tx("claim", %{id: nft_c_id, addr: addr}, from, _to, _value, [token_id], chain) do
    # INIT Token
    uri = Interactor.get_token_uri(chain, addr, token_id)
    Nft.create(
      %{
        uri: uri,
        token_id: token_id,
        owner: from,
        nft_contract_id: nft_c_id
    })
  end

  def do_handle_tx(
    "setTokenInfo",
    %{id: nft_c_id}, _from, _to, _value,
    [token_id, badges_raw], _chain) do
    # UPDATE TokenInfo

    token_id
    |> Nft.get_by_token_id_and_nft_contract_id(nft_c_id)
    |> Nft.update(%{
        badges: Poison.decode!(badges_raw), token_id: token_id
    }, :with_badges)
  end

  def do_handle_tx(_others, _, _, _, _, _, _) do
    {:ok, "pass"}
  end
end

Implementation in Node.js

See code in:

https://github.com/WeLightProject/Web3-dApp-Camp/tree/main/blockchain-server/syncer/syncer_demo_js

Syncer

Let's see how do we implement the Syncer in Node.js. Using APIs provided by Web3 and Moonbeam, it's very easy to interact with Moonbeam chain.

getBlockNumber

web3.eth.getBlockNumber([callback])Returns the current block number.

Returns

Promise returns Number - The number of the most recent block.

Get a list of 'Normal' Transactions By Address

[Optional Parameters] startblock: starting blockNo to retrieve results, endblock: ending blockNo to retrieve results

https://api-moonbeam.moonscan.io/api?module=account&action=txlist&address=0x0000000000000000000000000000000000001004&startblock=1&endblock=99999999&sort=asc&apikey=YourApiKeyToken(Returned 'isError' values: 0=No Error, 1=Got Error)(Returns up to a maximum of the last 10000 transactions only)

or

https://api-moonbeam.moonscan.io/api?module=account&action=txlist&address=0x0000000000000000000000000000000000001004&startblock=1&endblock=99999999&page=1&offset=10&sort=asc&apikey=YourApiKeyToken(To get paginated results use page= and offset=)

Below is Syncer implemented in JavaScript. In this example, I persist the data using sqlite but you can choose anything you are familiar with. And I simply use setTimeout and Catch-All-Error approach to simulate a forever running process.

import Web3 from 'web3';
import got from 'got';
import sqlite3 from 'sqlite3';
import abiDecoder from 'abi-decoder';

const db = new sqlite3.Database('./sqlite.db');

const web3 = new Web3(new Web3.providers.HttpProvider('https://rpc.api.moonbeam.network'));

const INTERVAL = 1 * 60 * 1000; // 10 minutes
const API_KEY = 'Y6AIFQQVAJ3H38CC11QFDUDJWAWNCWE3U8';

const CONTRACT_ADDRESS = '0xb6FC950C4bC9D1e4652CbEDaB748E8Cdcfe5655F';
const CONTRACT_ABI = []; // ABI can be retrieved from https://moonbeam.moonscan.io/address/0xb6FC950C4bC9D1e4652CbEDaB748E8Cdcfe5655F#code

abiDecoder.addABI(CONTRACT_ABI);

async function sync() {
  try {
    const contract = await getContractInfo(CONTRACT_ADDRESS);

    // Step 1 - Retrieves the latest block number of the blockchain
    const latestBlock = await web3.eth.getBlockNumber();
    console.log(`Latest blockNumber is: ${latestBlock}`);

    const lastBlockNumber = contract.lastBlockNumber || latestBlock - 100000;

    // Step 2.1 - Retrieves Txns by Etherscan API
    const transactions = await getTransactionsOfContract(contract.address, lastBlockNumber, latestBlock);
    console.log(`${transactions.length} transactions retrieved from Block ${lastBlockNumber} to ${latestBlock}.`);

    if (transactions.length > 0) {
      // Step 2.2 - Handle transactions
      await handleTransactions(contract, transactions);
    }

    // Step 3 - Persist the latest block number
    contract.lastBlockNumber = latestBlock + 1;
    await updateContractLastBlockNumber(contract);
    console.log(`lastBlockNumber of contract ${contract.id} is updated to ${contract.lastBlockNumber}.`);
  } catch (error) {
    console.error('Failed to sync transactions.', error);
  }

  nextRun();
}

function nextRun() {
  setTimeout(sync, INTERVAL);
}

async function getTransactionsOfContract(contractAddress, fromBlock, toBlock) {
  const url = `https://api-moonbeam.moonscan.io/api?module=account&action=txlist&address=${contractAddress}&startblock=${fromBlock}&endblock=${toBlock}&sort=asc&apikey=${API_KEY}`;
  const response = await got(url).json();

  return response.result;
}

async function _getContract(address) {
  return new Promise(function (resolve, reject) {
    db.get('SELECT id, lastBlockNumber, address FROM contract WHERE address = $address', { $address: address }, function (err, result) {
      if (err) {
        return reject(err);
      }

      return resolve(result);
    });
  });
}

async function getContractInfo(address) {
  let contract = await _getContract(address);
  if (contract) {
    return contract;
  }

  contract = {
    lastBlockNumber: 0,
    address
  };
  const contractId = await _runSQL(
    'INSERT INTO contract (lastBlockNumber, address) VALUES ($lastBlockNumber, $address)',
    {
      $lastBlockNumber: contract.lastBlockNumber,
      $address: contract.address
    }
  );
  contract.id = contractId;
  return contract;
}

async function updateContractLastBlockNumber(contract) {
  return _runSQL(
    'UPDATE contract SET lastBlockNumber = $lastBlockNumber WHERE id = $id',
    {
      $lastBlockNumber: contract.lastBlockNumber,
      $id: contract.id
    }
  );
}

async function execute() {
  // SQLite DB tables should be initialized first.

  await sync();
}

execute();

Decode Transactions

In order to decode the transaction, we need to unitilize the APIs provided by Web3.

encodeFunctionCall

web3.eth.abi.encodeFunctionCall(jsonInterface, parameters);Encodes a function call using its JSON interface object and given parameters.

Parameters

jsonInterface - Object: The JSON interface object of a function.

parameters - Array: The parameters to encode.

Function - (optional) Optional callback, returns an error object as first parameter and the result as second.

Returns

String - The ABI encoded function call. Means function signature + parameters.

Example
web3.eth.abi.encodeFunctionCall({
  name: 'myMethod',
  type: 'function',
  inputs: [{
      type: 'uint256',
      name: 'myNumber'
  },{
      type: 'string',
      name: 'myString'
  }]
}, ['2345675643', 'Hello!%']);
"0x24ee0097000000000000000000000000000000000000000000000000000000008bd02b7b0000000000000000000000000000000000000000000000000000000000000040000000000000000000000000000000000000000000000000000000000000000748656c6c6f212500000000000000000000000000000000000000000000000000"

call

web3.eth.call(callObject [, defaultBlock] [, callback])Executes a message call transaction, which is directly executed in the VM of the node, but never mined into the blockchain.

Parameters

Object - A transaction object, see web3.eth.sendTransaction. For calls the from property is optional however it is highly recommended to explicitly set it or it may default to address(0) depending on your node or provider.

Number|String|BN|BigNumber - (optional) If you pass this parameter it will not use the default block set with web3.eth.defaultBlock. Pre-defined block numbers as "earliest", "latest" and "pending" can also be used.

Function - (optional) Optional callback, returns an error object as first parameter and the result as second.

Returns

Promise returns String - The returned data of the call, e.g. a smart contract functions return value.

Example
web3.eth.call({
  to: "0x11f4d0A3c12e86B4b5F39B213F7E19D048276DAe", // contract address
  data: "0xc6888fa10000000000000000000000000000000000000000000000000000000000000003"
})
.then(console.log);
"0x000000000000000000000000000000000000000000000000000000000000000a"

hexToUtf8

web3.utils.hexToUtf8(hex)Returns the UTF-8 string representation of a given HEX value.

Parameters

hex - String: A HEX string to convert to a UTF-8 string.

Returns

String: The UTF-8 string.

Example
web3.utils.hexToUtf8('0x49206861766520313030e282ac');
"I have 100€"

const TRANSACTION_HANDLER = {
  safeTransferFrom: transferHandler,
  transferFrom: transferHandler,
  claim: claimHandler
}

async function handleTransactions(contract, transactions) {
  for (let i = 0; i < transactions.length; i++) {
    const transaction = transactions[i];
    const decodedMethod = abiDecoder.decodeMethod(transaction.input);

    const handler = TRANSACTION_HANDLER[decodedMethod.name];
    if (handler) {
      await handler(contract, transaction, decodedMethod.params);
    }
  }
}

/**
 * Transfer the ownership of the NTF token
 * @param {*} contract The NFT contract
 * @param {*} transaction The transaction executing the claim method
 * @param {*} params Decoded parameters provided to the method.  Sample:
 *  [
 *    {
 *      name: 'from',
 *      value: '0xc994b5384c0d0611de2ece7d6ff1ad16c34a812f',
 *      type: 'address'
 *    },
 *    {
 *      name: 'to',
 *      value: '0x9c88a415f6a8043d7eaf14db721efbd8309e7365',
 *      type: 'address'
 *    },
 *    { name: 'tokenId', value: '888888', type: 'uint256' }
 *  ]
 */
async function transferHandler(contract, transaction, params) {
  const to = _getParam(params, 'to').value;
  const tokenId = parseInt(_getParam(params, 'tokenId').value);

  return _runSQL(
    `UPDATE nft SET owner = $owner WHERE contractId = $contractId and tokenId = $tokenId`,
    { $owner: to, $contractId: contract.id, $tokenId: tokenId }
  );
}

/**
 * Initialize the NFT token
 * @param {*} contract The NFT contract
 * @param {*} transaction The transaction executing the claim method
 * @param {*} params Decoded parameters provided to the method.  Sample: [{ name: 'tokenId', value: '199398', type: 'uint256' }]
 */
async function claimHandler(contract, transaction, params) {
  const tokenId = parseInt(_getParam(params, 'tokenId').value);
  const uri = await getTokenUri(contract.address, tokenId);

  const nft = {
    $contractId: contract.id,
    $tokenId: tokenId,
    $uri: uri,
    $owner: transaction.from,
  };

  return _runSQL(
    `INSERT INTO nft (contractId, tokenId, uri, owner) VALUES ($contractId, $tokenId, $uri, $owner)`,
    nft
  );
}

async function getTokenUri(contractAddress, tokenId) {
  const data = web3.eth.abi.encodeFunctionCall({
    name: 'tokenURI',
    type: 'function',
    inputs: [{
      type: 'uint256',
      name: 'tokenId'
    }]
  }, [tokenId]);

  const uriHex = await web3.eth.call({
    to: contractAddress, // contract address
    data
  });

  return web3.utils.hexToUtf8(uriHex);
}

Authors:

thinkingincrowd

leeduckgo