如何在负载均衡服务器上实现会话保持？

http {
    upstream backend_servers {
        ip_hash;
        server backend1.example.com;
        server backend2.example.com;
        server backend3.example.com;
    }

    ...
}

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass         http://backend_servers;
        proxy_http_version 1.1;

        # 设置Header信息
        proxy_set_header   Host $host;
        proxy_set_header   X-real-ip $remote_addr;
        proxy_set_header   X-forwarded-for $proxy_add_x_forwarded_for;
        proxy_set_header   X-Forwarded-Proto $scheme;

        # 配置会话保持
        proxy_cookie_domain backend1.example.com example.com;
        proxy_cookie_path / /;
        proxy_cookie_secure on;
        proxy_cookie_httponly on;
    }
}

sudo service nginx restart