Mql4 Guide: Integrating External Data with the WebRequest() Function

To fetch data from a website or a JSON API in MQL4, you must use the built-in WebRequest function. Since MQL4 does not have a native JSON parser, you must also implement a method to parse the returned string to extract the specific data you need.

Prerequisites

Before the code will work, you must enable HTTP requests in your MetaTrader 4 terminal:

1. Go to Tools -> Options -> Expert Advisors.
2. Check "Allow WebRequest for listed URL".
3. Add the specific URL of your API to the list (e.g., https://api.binance.com or https://jsonplaceholder.typicode.com).

Implementation Strategy

1. WebRequest: Sends a GET request to the API.
2. CharArrayToString: Converts the raw byte response into a readable string.
3. Custom Parsing: A helper function searches the string for a specific JSON key (e.g., "price":) and extracts the value associated with it.

MQL4 Code Example

This Expert Advisor fetches data from a public testing API. It sends a request, receives a JSON response, and parses a specific value.

//+------------------------------------------------------------------+
//|                                               APIFetcherBot.mq4  |
//|                                     Copyright 2023, Quantitative |
//|                                             https://www.mql5.com |
//+------------------------------------------------------------------+
#property copyright "Copyright 2023, Quantitative"
#property link      "https://www.mql5.com"
#property version   "1.00"
#property strict

//--- Input Parameters
input string InpUrl = "https://jsonplaceholder.typicode.com/todos/1"; // Example API URL
input int    InpTimeout = 5000; // Timeout in milliseconds

//+------------------------------------------------------------------+
//| Expert initialization function                                   |
//+------------------------------------------------------------------+
int OnInit()
{
   // Check if WebRequest is allowed
   if(!TerminalInfoInteger(TERMINAL_DLLS_ALLOWED))
   {
      Print("Error: DLLs must be allowed for some network functions (optional check).");
   }
   
   // Note: There is no direct programmatic check for "Allow WebRequest" 
   // specifically, but the function will return error 4060 if not allowed.
   
   return(INIT_SUCCEEDED);
}

//+------------------------------------------------------------------+
//| Expert tick function                                             |
//+------------------------------------------------------------------+
void OnTick()
{
   // We use a static variable to run this only once per bar or specific interval
   // to avoid spamming the API and getting banned.
   static datetime lastCheck = 0;
   
   if(TimeCurrent() - lastCheck < 10) return; // Wait 10 seconds between requests
   lastCheck = TimeCurrent();

   // --- 1. Prepare Request Data ---
   char postData[];           // Array for POST data (empty for GET)
   char resultData[];         // Array to store the API response
   string resultHeaders;      // To store response headers
   
   // --- 2. Send the WebRequest ---
   // Reset error state
   ResetLastError();
   
   int res = WebRequest(
      "GET",              // HTTP Method
      InpUrl,             // URL
      NULL,               // Cookie (usually NULL)
      NULL,               // Referer (usually NULL)
      InpTimeout,         // Timeout
      postData,           // POST data array
      0,                  // Data size (0 for GET)
      resultData,         // Result array
      resultHeaders       // Result headers
   );

   // --- 3. Handle the Response ---
   if(res == -1)
   {
      int error = GetLastError();
      if(error == 4060)
      {
         Print("Error 4060: WebRequest not allowed. Go to Tools->Options->Expert Advisors and add the URL.");
      }
      else
      {
         Print("WebRequest failed. Error code: ", error);
      }
   }
   else if(res == 200) // HTTP 200 OK
   {
      // Convert the character array to a string
      string jsonResponse = CharArrayToString(resultData);
      
      Print("Raw JSON: ", jsonResponse);
      
      // --- 4. Parse Specific Data ---
      // Example: Extracting the value of the "title" key from the JSON
      string titleValue = GetJsonValue(jsonResponse, "title");
      string completedValue = GetJsonValue(jsonResponse, "completed");
      
      Print("Parsed Title: ", titleValue);
      Print("Parsed Completed Status: ", completedValue);
      
      // Logic Example: If 'completed' is false, maybe do something
      if(completedValue == "false")
      {
         // Trading logic here...
      }
   }
   else
   {
      Print("API returned unexpected HTTP code: ", res);
   }
}

//+------------------------------------------------------------------+
//| Helper: Simple JSON Value Extractor                              |
//| Note: This is a basic string parser. For complex nested JSON,    |
//| a dedicated library is recommended.                              |
//+------------------------------------------------------------------+
string GetJsonValue(string json, string key)
{
   string searchKey = "\"" + key + "\"";
   int startPos = StringFind(json, searchKey);
   
   if(startPos == -1) return(""); // Key not found
   
   // Move position to the end of the key
   startPos += StringLen(searchKey);
   
   // Find the colon
   int colonPos = StringFind(json, ":", startPos);
   if(colonPos == -1) return("");
   
   // Start reading after the colon
   int valueStart = colonPos + 1;
   
   // Skip whitespace
   while(StringSubstr(json, valueStart, 1) == " " && valueStart < StringLen(json))
   {
      valueStart++;
   }
   
   // Check if value is a string (starts with quote)
   bool isString = (StringSubstr(json, valueStart, 1) == "\"");
   
   int valueEnd;
   if(isString)
   {
      valueStart++; // Move past opening quote
      valueEnd = StringFind(json, "\"", valueStart);
   }
   else
   {
      // Find end of value (comma or closing brace)
      int commaPos = StringFind(json, ",", valueStart);
      int bracePos = StringFind(json, "}", valueStart);
      
      if(commaPos == -1) valueEnd = bracePos;
      else if(bracePos == -1) valueEnd = commaPos;
      else valueEnd = MathMin(commaPos, bracePos);
   }
   
   if(valueEnd == -1) return("");
   
   return(StringSubstr(json, valueStart, valueEnd - valueStart));
}
//+------------------------------------------------------------------+

Key Considerations for MQL4 Networking

1. Synchronous Execution: WebRequest is synchronous. This means the entire trading robot pauses and waits for the server to respond. If the API takes 2 seconds to respond, your EA stops processing ticks for 2 seconds. Do not use this in high-frequency scalping strategies inside OnTick without careful timer management.
2. Indicators: You cannot use WebRequest inside Custom Indicators. It only works in Expert Advisors (EAs) and Scripts.
3. JSON Complexity: The GetJsonValue function provided above is a lightweight text parser. It works for flat JSON structures (e.g., {"price": 1.12, "vol": 100}). If you are dealing with complex, nested arrays (e.g., {"data": [{"id": 1}, {"id": 2}]}), you should look for a dedicated MQL4 JSON library (such as JAson.mqh) rather than writing your own parser.
4. Error 4060: This is the most common error. It means you forgot to add the specific URL to the "Allowed URLs" list in the terminal settings.

Q&A

Q: Can I use Python to get the data and send it to MT4?
A: Yes. You can write a Python script to fetch the data and write it to a text file in the MQL4/Files folder (common folder). The MT4 EA can then read that file using FileOpen. This avoids the freezing issue of WebRequest.

Q: How do I send data (POST) instead of getting it?
A: You need to populate the postData array.

string headers = "Content-Type: application/json\r\n";
string jsonBody = "{\"signal\": \"buy\"}";
char postData[];
StringToCharArray(jsonBody, postData);
int res = WebRequest("POST", url, headers, NULL, timeout, postData, ArraySize(postData), resultData, resultHeaders);

Q: Why does my EA freeze when fetching data?
A: As mentioned, WebRequest halts execution until a response is received. To mitigate this, run the request in a separate "Timer" event (OnTimer) rather than OnTick, or use a separate "Worker EA" just for data fetching that writes to Global Variables, which your main trading EA reads.