How does the large-scale crypto trade platform , just like Binance or Bittrex , track ETH and Token transfer in their every account over time?

Question

I wrote some simple code with web3js to track an address/account's transaction in real-time. It works, but I think it is in poor performance or could consume too much cpu resources.

sample code

 let filter = web3.eth.filter("latest");
 filter.watch(function(error, result) {
    if (!error) {
      //tracking start in 12 blocks earlier .
      var earlierNumber = web3.eth.getBlock(result).number - 12;
      var confirmedBlock = web3.eth.getBlock(earlierNumber ) ;

      if (confirmedBlock.transactions.length > 0) {
          confirmedBlock.transactions.forEach(function(tx) {
             if ( tx.to == address ) {                     
                var depositAmount =  web3.fromWei(tx.value.toNumber(), 
                "ether" )
                console.log("address: " +address+ " deposit amount:  "+depositAmount+ " ETH." ) ;                     
              }     
          }) 
       }
     }  
  })  

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How could those large scale trade or exchange platform track huge amount of account' transaction accurately ? I find much good project such as MetaMask-token-tracker ,on Git. How about it? Or any open source to recommend？

Answer 1

The exchanges don't make the transactions between blockchain accounts. Furthermore, the transactions aren't even visible in the blockchain. If you buy coin A with coin B, the exchange just shuffles numbers around inside their own databases and nothing is published to the blockchain. This makes perfect sense resource-wise: changing a few database values is a lot cheaper and faster than publishing transactions to a blockchain.

In theory, ERC20 token balances are tracked only within the token contract. But when you exchange tokens in an exchange, it's also just shuffling numbers in their databases and according to the blockchain the tokens never move anywhere.

Correct transactions are published only when you withdraw/deposit assets from the exchange. Otherwise everything the exchange owns (coins & tokens) are just stored in their own wallets.