IP Hash Load Balancing in Nginx确保同一客户端IP始终路由到同一后端服务器，1. 通过客户端IP哈希值选择服务器，实现简单粘性会话；2. 仅当服务器宕机或上游列表变更时重新分配；3. 适用于无共享会话存储的场景；4. 缺点是不感知负载且扩缩容会破坏现有映射；5. 建议结合健康检查并监控流量分布以优化效果，适合中小规模静态后端环境使用。

IP Hash Load Balancing in Nginx is a method used to distribute client requests across multiple backend servers — but with a key twist: it ensures that a given client IP address always gets routed to the same backend server, as long as that server is available.

This is especially useful when you need session persistence (sticky sessions) — for example, in web apps where user data or sessions are stored locally on a specific server rather than in a shared database or cache like Redis.

? How It Works

Nginx calculates a hash from the client’s IP address and uses that hash to pick a server from the defined upstream block. The same IP → same server mapping persists unless:

• The selected server goes down
• You modify the upstream list (e.g., add/remove servers)

Here’s a basic config example:

upstream backend {
    ip_hash;
    server 192.168.0.10:8080;
    server 192.168.0.11:8080;
    server 192.168.0.12:8080;
}

server {
    location / {
        proxy_pass http://backend;
    }
}

In this setup:

• Nginx hashes the client’s IPv4 address (entire octet, e.g., 192.168.1.100)
• Uses the hash to choose one of the three servers — consistently
• If a server fails, Nginx re-hashes and picks another (maintaining balance among healthy nodes)

✅ When to Use IP Hash

Use IP hash when:

• You’re not using shared session storage (like Redis or a database)
• Your app stores session data locally (e.g., in memory or on disk per server)
• You want simple sticky sessions without extra modules or cookies
• You’re okay with some imbalance — because not all IPs generate equal traffic

⚠️ Note: With IPv6, only the first /64 prefix is used for hashing — to avoid too granular distribution from unique client addresses.

⚖️ Pros & Cons

? Tips for Better Results

• Combine with health checks (max_fails, fail_timeout) to avoid sending traffic to dead servers
• Avoid frequent changes to your upstream list — it invalidates existing client→server mappings
• For high-scale apps, prefer shared session storage round-robin/load-aware balancing over IP hash
• Monitor traffic distribution — use tools like nginx-plus or custom logging to detect imbalances

Basically, IP hash is a quick and effective way to get session stickiness in Nginx without complex setups — just remember it’s not perfect for every scenario. If your traffic is uneven or your backend scales dynamically, consider more advanced options like least_conn or using a sticky cookie module instead.

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