trace_block | Polygon

Polygon API method that returns traces for all the transactions within a specific block. Developers can use the trace_block method to gain insight into the behavior of smart contracts within a block, analyze gas usage and optimize their contracts accordingly. This method is only available on an Erigon instance.

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Parameters

• quantity or tag — the integer of a block encoded as hexadecimal or the string with:

  • latest — the most recent block in the blockchain and the current state of the blockchain at the most recent block
  • earliest — the earliest available or genesis block
  • pending — the pending state and transactions block. The current state of transactions that have been broadcast to the network but have not yet been included in a block.

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Response

• result — an object containing the traces of all of the transactions in the block:
  • action — an object that describes the action taken by the transaction:
    • from — the address of the sender who initiated the transaction.
    • callType — the type of call, call or delegatecall, two ways to invoke a function in a smart contract. call creates a new environment for the function to work in, so changes made in that function won’t affect the environment where the function was called. delegatecall doesn’t create a new environment. Instead, it runs the function within the environment of the caller, so changes made in that function will affect the caller’s environment.
    • gas — the units of gas included in the transaction by the sender.
    • input — the optional input data sent with the transaction, usually used to interact with smart contracts.
    • to — the address of the recipient of the transaction if it was a transaction to an address. For contract creation transactions, this field is null.
    • value — the value of the native token transferred along with the transaction, in Wei.
    • blockHash — the hash of the block in which the transaction was included.
    • blockNumber — the number of the block in which the transaction was included.
    • error — a string that indicates whether the transaction was successful or not. null if successful, Reverted if not.
    • result — an object that contains additional data about the execution of the transaction:
      • gasUsed — the total used gas by the call, encoded as hexadecimal.
      • output — the return value of the call, encoded as a hexadecimal string.
    • subtraces — the number of sub-traces created during execution. When a transaction is executed on the EVM, it may trigger additional sub-executions, such as when a smart contract calls another smart contract or when an external account is accessed.
    • traceAddress — an array that indicates the position of the transaction in the trace.
    • transactionHash — the hash of the transaction.
    • transactionPosition — the position of the transaction in the block.
    • type — the type of action taken by the transaction, call or create. call is the most common type of trace and occurs when a smart contract invokes another contract’s function. create represents the creation of a new smart contract. This type of trace occurs when a smart contract is deployed to the blockchain.

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trace_block code examples

const Web3 = require("web3");
const NODE_URL = "CHAINSTACK_NODE_URL";
const web3 = new Web3(NODE_URL);

// Define the trace_block custom method
web3.extend({
  property: 'debug',
  methods: [{
    name: 'traceBlock',
    call: 'trace_block',
    params: 1,
    inputFormatter: [web3.extend.formatters.inputHashFormatter],
    outputFormatter: web3.extend.formatters.outputCallFormatter
  }]
});

async function traceBlock(block) {
    const result = await web3.debug.traceBlock(block);
    console.log(result);
}

traceBlock("latest");

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Use case

A practical use case for the trace_block method on Polygon can be to conduct an analysis of the most recent block on the network. For example, by examining the amount of gas supplied by all the transactions and the actual amount of gas utilized, developers can gain valuable insights into the block’s behavior. This can be the first step to developing a chain explorer.

The following code is an implementation of this idea using web3.js:

const Web3 = require("web3");
const NODE_URL = "CHAINSTACK_NODE_URL";
const web3 = new Web3(NODE_URL);

// Extend the web3 object with a custom method for tracing a block's transactions
web3.extend({
  property: 'debug',
  methods: [{
    name: 'traceBlock',
    call: 'trace_block',
    params: 1,
    inputFormatter: [web3.extend.formatters.inputHashFormatter], // Format the input parameter as a hash
    outputFormatter: web3.extend.formatters.outputCallFormatter, // Format the output as a call object
  }],
});

// A function that returns an array of result and gas fields for each transaction in a block
async function traceBlock(block) {
  try {
    const traces = await web3.debug.traceBlock(block);
    const resultFields = [];
    const gasFields = [];

    for (const { result, action } of traces) {
      resultFields.push(result);
      gasFields.push(action.gas);
    }

    return [resultFields, gasFields];
  } catch (error) {
    console.error(`Error while tracing block: ${error}`);
    throw error;
  }
}

// A function that calculates the total gas used by all transactions in a block
function returnGasUsed(results) {
  const totalGasUsed = results.reduce((total, result) => {
    if (result !== null && result.gasUsed !== undefined) {
      const decimalValue = web3.utils.hexToNumberString(result.gasUsed);
      total += Number(decimalValue);
    }
    return total;
  }, 0);
  return totalGasUsed;
}

// A function that calculates the total gas supplied to a block
function returnTotalGas(gasArray) {
  const totalGas = gasArray.reduce((total, gas) => {
    if (gas !== undefined) {
      const decimalValue = web3.utils.hexToNumberString(gas);
      total += Number(decimalValue);
    }
    return total;
  }, 0);
  return totalGas;
}

// The main function that runs the gas analysis
async function main() {
  try {
    const block = 'latest';
    const [results, gas] = await traceBlock(block);

    const gasUsed = returnGasUsed(results);
    const totalGas = returnTotalGas(gas);
    console.log('===== Gas analysis =====');
    console.log(`Total gas supplied: ${totalGas}`);
    console.log(`Gas used: ${gasUsed}`);
    console.log(`Unused gas: ${totalGas - gasUsed}`);

    const gasUsedPercentage = (gasUsed / totalGas) * 100;
    console.log(`Percentage of gas used: ${gasUsedPercentage.toFixed(2)} %`);
  } catch (error) {
    console.error(`Error while running gas analysis: ${error}`);
  }
}

// Call the main function to start the program
main();

The traceBlock function takes a block number or tag as an input parameter and returns an array of two elements. The first element is an array of the results of each transaction in the block, which contains the gasUsed field, and the second element is an array of the gas field used by each call.

The returnGasUsed function calculates the total gas used by all transactions in a block by iterating over the results array and summing up the gasUsed fields of each non-null element.

The returnTotalGas function calculates the total gas supplied to a block by iterating over the gasArray array and summing up the non-undefined elements.

The main function calls the traceBlock function with the block number or tag calculates the gas used and total gas supplied using the returnGasUsed and returnTotalGas functions, and outputs the results to the console.

The main function is wrapped in a try-catch block to handle any errors that may occur during execution.

Finally, the main function is called at the end of the script to run the gas analysis.