[Ethereum] How to watch for 12th confirmation with web3 filters

filtersweb3js

It doesn't seem possible to update the options in web3.eth.filter() dynamically, so how would one watch for a transaction involving a particular account appearing on the 12th latest block?

Best Answer

Answering own question.

const Web3 = require('web3')
let web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:8545"))

let account = '0x05430f5201585c2601bb75d9658007202864c993'
let filter = web3.eth.filter('latest')
filter.watch(function(error, result) {
  if (!error) {
    let confirmedBlock = web3.eth.getBlock(web3.eth.blockNumber - 11)
    if (confirmedBlock.transactions.length > 0) {
      confirmedBlock.transactions.forEach(function(txId) {
        let transaction = web3.eth.getTransaction(txId)
        if (transaction.to == account) {
          // Do something useful.
        }
      })
    }
  }
})

Related Solutions

Web3.js – Parsing Transaction Receipt Logs

Do this: You'll need to pull code from web3, and it works best if your frontend is bundled using something like webpack or browserify:

var SolidityCoder = require("web3/lib/solidity/coder.js");
var log = receipt.logs[0];
var data = SolidityCoder.decodeParams(["string", "uint"], log.data.replace("0x", ""));

In this case, we're decoding log data that contains two variables, one a string type and one a uint.

EDIT:

If you have the ABI available, you can detect which event it is related to that ABI:

var SolidityCoder = require("web3/lib/solidity/coder.js");

// You might want to put the following in a loop to handle all logs in this receipt.
var log = receipt.logs[0];
var event = null;

for (var i = 0; i < abi.length; i++) {
  var item = abi[i];
  if (item.type != "event") continue;
  var signature = item.name + "(" + item.inputs.map(function(input) {return input.type;}).join(",") + ")";
  var hash = web3.sha3(signature);
  if (hash == log.topics[0]) {
    event = item;
    break;
  }
}

if (event != null) {
  var inputs = event.inputs.map(function(input) {return input.type;});
  var data = SolidityCoder.decodeParams(inputs, log.data.replace("0x", ""));
  // Do something with the data. Depends on the log and what you're using the data for.
}

Web3js – How to Asynchronously Query the Blockchain Using Filters

Unfortunately, there is no filter provided to parse the confirmed transactions without going through a block first i.e., you can trigger a filter (and execute call_async_method()) whenever there is a latest block: eth.getBlock("latest"), but there is no such function for a transaction.

That said, if you're operating on a private blockchain (you trust all the nodes) and believe that all the initiated transactions will successfully be added to the blockchain, you can create filters to react upon eth.pendingTransactions so that the actions like call_async_method() will be performed immediately without waiting for the transactions to get confirmed and added to the blockchain. However, at a later time, you should always check if these transactions are indeed confirmed.

This is not an ideal or recommended solution and contradicts with the fundamental principles of blockchain, but you can test and check if it's suitable for your use case.

Edit: Fast way to scan blocks for transactions on an account. This is an asynchronous scanner with 200 threads: https://ethereum.stackexchange.com/a/7184/2460

Source code here.

Best Answer

Related Solutions

Web3.js – Parsing Transaction Receipt Logs

Web3js – How to Asynchronously Query the Blockchain Using Filters

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