Extract data

Overview

Warden encodes responses from HTTP requests in CBOR (Concise Binary Object Representation): a binary data format similar to JSON but more compact. You could see an example of such output in the previous guide, when testing the CoinGecko API.

This tutorial will guide you through extracting specific data from CBOR-encoded responses. You'll create a contract that can extract the Bitcoin price from the CoinGecko API responses.

Step 1. Create a contract

Create a new file DataExtraction.sol:

warden-http-examples/src/DataExtraction.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.25;

import "./interfaces/IAsync.sol";
import "./interfaces/Http.sol";

contract DataExtraction {
    uint64 public lastFutureId;
    bytes public responseBody;
    uint256 public statusCode;
    
    // Make a request to the CoinGecko API: fetch the current Bitcoin price
    function getBitcoinPrice() public returns (Http.Request memory request) {
        request.url = "https://api.coingecko.com/api/v3/simple/price?ids=bitcoin&vs_currencies=usd";
        request.method = "GET";
        request.body = "";
        
        lastFutureId = IASYNC_CONTRACT.addFuture("http", abi.encode(request), address(this));
    }
    
    // Process the response
    function processResponse() public returns (bool) {
        FutureByIdResponse memory future = IASYNC_CONTRACT.futureById(lastFutureId);
        if (future.futureResponse.result.id == 0) {
            return false; // Not ready yet
        }
        
        // Decode the response
        Http.Response memory response = abi.decode(future.futureResponse.result.output, (Http.Response));
        
        // Store the response data
        statusCode = response.status;
        responseBody = response.body;
        
        return true;
    }
    
    // A callback function: the Warden node calls it automatically
    function cb() external {
    }
    
    // Get the raw bytes at a specific position in the response
    function getBytesAt(uint256 start, uint256 length) public view returns (bytes memory) {
        bytes memory result = new bytes(length);
        for (uint256 i = 0; i < length && i + start < responseBody.length; i++) {
            result[i] = responseBody[i + start];
        }
        return result;
    }
    
    // Dump the response bytes as a hex value for debugging
    function dumpResponseHex() public view returns (bytes memory) {
        return responseBody;
    }
    
    // Extract the Bitcoin price using a simple approach
    function getBitcoinPriceSimple() public view returns (uint256) {
        bytes memory data = responseBody;
        
        // Look for the pattern that indicates the price
        for (uint256 i = 0; i < data.length - 10; i++) {
            if (data[i] == 0xFB) {
                // Found the float64 marker, extract the next 8 bytes
                uint64 priceBits;
                for (uint256 j = 0; j < 8; j++) {
                    priceBits = (priceBits << 8) | uint8(data[i + 1 + j]);
                }
                
                // Convert to an integer (simplified, assumes price < 2^53)
                uint256 price = uint256(priceBits >> 12);
                return price;
            }
        }
        
        return 0;
    }
}

This contract adds:

• Functions to inspect the raw response bytes
• A simple function to extract the Bitcoin price from the CBOR data

Step 2. Deploy and test

1. Deploy the contract:

   forge create --rpc-url $RPC_URL --private-key $PRIVATE_KEY \
     src/DataExtraction.sol:DataExtraction --broadcast

2. Note down the value returned as Deployed to and set it as an environment variable:

   export CONTRACT_ADDRESS=my-contract-address

3. Make an HTTP request by calling the getBitcoinPrice() function:

   cast send $CONTRACT_ADDRESS "getBitcoinPrice()" \
     --private-key $PRIVATE_KEY \
     --rpc-url $RPC_URL

4. Wait a few seconds, then use the processResponse() function to process the result:

   cast send $CONTRACT_ADDRESS "processResponse()" \
     --private-key $PRIVATE_KEY \
     --rpc-url $RPC_URL

5. Get the raw response bytes by calling dumpResponseHex():

   cast call $CONTRACT_ADDRESS "dumpResponseHex()(bytes)" --rpc-url $RPC_URL

   In the output, you'll see a HEX string:

   0xa1...

6. Extract the Bitcoin price by calling getBitcoinPriceSimple()(uint256) :

   cast call $CONTRACT_ADDRESS "getBitcoinPriceSimple()(uint256)" --rpc-url $RPC_URL

   In the output, you'll see the Bitcoin price represented as an integer.

Next steps

Now you can implement a more advanced example: Fetch multiple prices.