Solving 502 Errors in Microservices Node.js and AWS ALB

#aws #node #502 #javascript

Understanding 502 Errors
A 502 Bad Gateway error occurs when a server acting as a gateway or proxy (like AWS Application Load Balancer or ALB) receives an invalid response from the upstream server (e.g., your Node.js application server). In microservices architectures, where multiple servers communicate through a load balancer, this error is common when server-to-server communication is interrupted.

The Problem: Timeout Mismatch
In my setup, AWS ALB was configured with a 30-second timeout, and the Node.js servers were handling requests using their default timeout settings. However, these default server timeout settings were not aligned with the ALB configuration, leading to unexpected connection terminations and resulting in 502 errors.

To resolve this, the timeout settings on the Node.js servers must be configured to complement ALB’s timeout.

Key Timeout Settings in Node.js
There are two critical timeout settings in Node.js that affect its interaction with AWS ALB:

KeepAliveTimeout
This setting determines how long the server keeps a connection open to allow additional requests from the same client. By default, Node.js sets this to 5 seconds, which is too short compared to ALB’s 30 seconds.
HeadersTimeout
This setting specifies the maximum time the server waits for complete request headers before closing the connection. By default, it is 60 seconds, which can be optimized for consistency.

To prevent 502 errors, the KeepAliveTimeout should be set slightly longer than the ALB timeout, and the HeadersTimeout should exceed the KeepAliveTimeout.

Adjust Node.js Server Timeout Settings
Configure the timeout settings in your Node.js application server. Here’s an example of how to do this:

const http = require("http");
const app = require("./app"); // Your Express or custom app setup
const server = http.createServer(app);
// Configure timeouts
server.keepAliveTimeout = 35000; // 35 seconds
server.headersTimeout = 40000; // 40 seconds
server.listen(3000, () => {
console.log("Server running on port 3000");
});

Apply these timeout settings consistently across all Node.js servers involved in the microservices architecture to prevent unexpected close connections.

Why These Settings Work
The KeepAliveTimeout ensures the Node.js server keeps the connection alive long enough to match or slightly exceed ALB’s idle timeout.
The HeadersTimeout ensures the server has ample time to process and respond to headers, preventing premature connection termination.