Avoid This Silent Timeout Bug with Axios in Lambda as proxy, ALB, and Content-Length

#lambda #loadbalancer #axios #http

Have you ever encountered a 504 Gateway Timeout from your AWS Application Load Balancer (ALB) when proxying requests via Axios, even though everything looks right on the surface?

Here’s a tricky issue I ran into — and how to avoid it in the future.

💥 The Setup

You're using AWS Lambda using a Node.js proxy to forward a POST request to a backend behind ALB:

await axios.post('http://my-backend', {
  timestamp: '2025-06-10 10:49:46',
  token: 'fEjck23HJkLlrRkDKtNWVI:APA91bHphS5as...'
}, {
  headers: {
    'Content-Type': <keep it from original request>,
    'Content-Length': <get from original request>
  }
});

Looks harmless, but suddenly:

504 Gateway Timeout from ALB

And the backend logs show:

SyntaxError: Unterminated string in JSON at position xxx

🧠 Root Cause
The error comes from a mismatch between the Content-Length and the actual body size.

What actually happens:

Axios serializes the data object to JSON.
You manually set Content-Length: xxx or forward it from orignial request, but the serialized body is longer.
The backend receives only the first xxx bytes.
It tries to parse, but the JSON is incomplete.
It waits for the rest of the body, but it never comes.
Eventually, the connection times out → ALB returns a 504.

❌ Why Not a 400?

Normally, a malformed JSON gives a 400 Bad Request.
But here:
- The backend doesn't know the full body has been sent.
- It thinks the rest is still coming.
- So it never sends a response → ALB gives up and returns 504.

✅ The Fix

Just don’t manually set Content-Length. Let Axios compute it.

await axios.post('http://my-backend', {
  timestamp: '2025-06-10 10:49:46',
  token: 'fEjck23HJkLlrRkDKtNWVI:APA91b...'
}, {
  headers: {
    'Content-Type': <keep it from original request>
    // ✅ Do not manually set Content-Length
  }
});