Extract data

Overview

Warden returns responses from HTTP requests as JSON data. To extract specific values from these responses, you can use the read() function of the JSON precompile.

This tutorial will guide you through extracting the Bitcoin price from the CoinGecko API responses.

1. Create a contract

Create a DataExtraction.sol file:

warden-http-examples/src/DataExtraction.sol

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

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

contract DataExtraction {
    uint64 public lastTaskId;
    bytes public responseBody;
    uint256 public statusCode;
    int256 public bitcoinPrice;
    
    // 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 = "";
        
        lastTaskId = IASYNC_CONTRACT.addTask("http", abi.encode(request), address(this));
    }
    
    // Process the response
    function processResponse() public returns (bool) {
        TaskByIdResponse memory task = IASYNC_CONTRACT.taskById(lastTaskId);
        if (task.taskResponse.result.id == 0) {
            return false; // Not ready yet
        }
        
        // Decode the response
        Http.Response memory response = abi.decode(task.taskResponse.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 {
        TaskByIdResponse memory task = IASYNC_CONTRACT.taskById(lastTaskId);
        if (task.taskResponse.result.id == 0) {
            revert("Not ready yet");
        }
        
        // Decode the response
        Http.Response memory response = abi.decode(task.taskResponse.result.output, (Http.Response));
        
        // Store the response data
        statusCode = response.status;
        responseBody = response.body;
        
        // Extract the Bitcoin price using the JSON precompile
        IJson.ReadKeyValue[] memory keyValuePairs = new IJson.ReadKeyValue[](1);
        keyValuePairs[0] = IJson.ReadKeyValue("bitcoin.usd", "float", 2);
        
        bytes[] memory readResult = IJSON_CONTRACT.read(responseBody, keyValuePairs);
        bitcoinPrice = abi.decode(readResult[0], (int256));
    }
    
    // Get the raw response bytes for debugging
    function dumpResponse() public view returns (bytes memory) {
        return responseBody;
    }
    
    // Get the response as a string for easier reading
    function getResponseAsString() public view returns (string memory) {
        return string(responseBody);
    }
}

This contract adds:

• Functions to inspect the raw response bytes
• A function to extract the Bitcoin price from the JSON data using the JSON precompile

2. Deploy

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

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

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

2. 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

3. Get the raw response by calling dumpResponse():

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

   In the output, you'll see the JSON response:

   {"bitcoin":{"usd":50000.0}}

4. Extract the Bitcoin price by calling bitcoinPrice():

   cast call $CONTRACT_ADDRESS "bitcoinPrice()(int256)" --rpc-url $RPC_URL

   In the output, you'll see the Bitcoin price represented as an integer with 2 decimal places.

Next steps

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