What are upstream servers in Nginx and how do you configure them?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Declare the upstream block

   In the http context, add upstream backend { server 10.0.0.1:8080; server 10.0.0.2:8080; } listing each backend.

2. Step 2

   Reference it in proxy_pass

   Inside a location block, use proxy_pass http://backend; so Nginx forwards matched requests to the group.

3. Step 3

   Pick a balancing method

   Keep round-robin, or add least_conn, ip_hash, or hash $request_uri for the routing behaviour you need.

4. Step 4

   Tune per-server parameters

   Set weight, max_fails, fail_timeout, backup, and down on each server line to control share and failover.

5. Step 5

   Set proxy headers and reload

   Forward Host and X-Forwarded-For, then run nginx -t and nginx -s reload to apply without downtime.

Code Example

upstream app_backend {
    least_conn;
    server 10.0.0.1:8080 weight=3 max_fails=2 fail_timeout=30s;
    server 10.0.0.2:8080;
    server 10.0.0.3:8080 backup;
}

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://app_backend;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

Follow-up Questions

• What load-balancing methods does Nginx support and when would you use each?
• How do max_fails and fail_timeout provide passive health checks?
• What does a backup server do in an upstream block?
• How does ip_hash achieve session persistence?
• How do active health checks differ, and where are they available?