# Upstream Groups and Balancing Methods — Nginx

Source: https://www.skillbyai.com/en/nginx/l-upstream

> Distribute traffic across servers with the right balancing method.

## Spreading requests across servers

An **`upstream`** block names a group of servers that `proxy_pass` can target. The default method is **round robin**, optionally **weighted** (`weight=3` sends three times as many requests to that server). **`least_conn`** sends each request to the server with the fewest active connections, better when request durations vary. **`ip_hash`** pins clients to servers by IP address (a simple form of stickiness, unreliable when many users share an IP). **`hash $key consistent`** routes by any key, such as `$request_uri` for cache-friendly distribution, using consistent hashing so adding a server moves few keys. NGINX Open Source also offers **`random two least_conn`** (the power-of-two-choices method). Server parameters include `max_fails` and `fail_timeout` (passive health checking), **`backup`** (used only when primaries are down) and **`down`** (temporarily removed). Prefer **stateless** application servers over session stickiness, so any server can handle any request.

## Balancing methods

Round robin rotates, least connections picks the least busy, hashing pins a key to a server.

![Three small diagrams: arrows rotating evenly across three boxes; arrows going to the emptiest of three boxes; arrows from labelled sources always going to the same box.](assets/figures/nginx/section-4-map.svg) — Figure 4.1 — Round robin, least connections and hashing.

## Weighted, least-connections and hashed upstreams

Choose a method per backend type.

```nginx
upstream web_app {
    least_conn;
    server 10.0.1.11:8080 weight=2;       # bigger machine
    server 10.0.1.12:8080;
    server 10.0.1.13:8080;
    server 10.0.1.20:8080 backup;         # only when the others are unavailable
    keepalive 64;
}

upstream image_cache {
    hash $request_uri consistent;          # same image -> same cache node
    server 10.0.2.21:8080;
    server 10.0.2.22:8080;
    server 10.0.2.23:8080;
}
```

## Stickiness hides statefulness

If your app only works with `ip_hash`, sessions are stored in server memory. Move them to a shared store (Redis, a database, signed cookies) so deploys and failures do not log users out.

**Quiz:** Requests to an API vary from 5 ms to several seconds. Which method usually balances better than plain round robin?

- [ ] ip_hash
- [ ] backup
- [x] least_conn
- [ ] down

*Answer:* least_conn. least_conn accounts for in-flight work, avoiding piling requests on a busy server.