Why there is stickiness in the browser. Why the request from the same browser goes to the same pid ?

The "stickiness" you're observing in your browser — where requests from the same browser keep going to the same process ID (PID) or server instance — is typically due to session stickiness, also known as sticky sessions or session affinity.

Let’s break it down:

Why it happens:

1. Load Balancer Behavior (Sticky Sessions)

If you're running multiple backend server instances behind a load balancer, some load balancers are configured to route the same client (browser) to the same backend server for the duration of the session. This is to maintain session consistency.

• Mechanisms:
  • IP hash: Client IP is hashed to always hit the same server.
  • Cookie-based stickiness: Load balancer sets a cookie in the user's browser that pins them to a particular backend instance.
  • Header-based routing or custom rules.

So if a user hits your service, the load balancer remembers which backend PID served it and routes subsequent requests to that same one.

2. Browser Reuse of HTTP Connections (Keep-Alive)

Browsers reuse persistent HTTP connections (enabled by default via Connection: keep-alive) for requests to the same origin. This means:

• The same TCP connection can serve multiple HTTP requests.
• These go to the same backend server process if connection is not closed.

This makes it seem like all requests go to the same PID, but it's a browser + networking optimization.

3. Caching and Localhost Development

If you're testing locally:

• You may only have one backend process running (e.g., localhost:3000), so it must always be the same PID.
• If you’re running in a clustered Node.js environment (e.g., using cluster or PM2 with multiple instances), your system may still route requests based on session stickiness.

4. WebSockets or Sessions