# ============================================================================
# NGINX.CONF - Load Balancer Configuration
# ============================================================================

events {
    worker_connections 2048;
    use epoll;
    multi_accept on;
}

http {
    # ==================== UPSTREAM BACKEND SERVERS ====================
    upstream backend {
        least_conn;  # Route to server with fewest active connections
        
        # Docker Compose creates multiple backend containers
        # They all listen on port 3000 internally
        server backend:3000 max_fails=3 fail_timeout=30s weight=1;
        
        # Keep connections alive for better performance
        keepalive 32;
        keepalive_requests 100;
        keepalive_timeout 60s;
    }

    # ==================== BASIC SETTINGS ====================
    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    # Logging
    log_format main '$remote_addr - $remote_user [$time_local] "$request" '
                    '$status $body_bytes_sent "$http_referer" '
                    '"$http_user_agent" "$http_x_forwarded_for" '
                    'upstream: $upstream_addr upstream_status: $upstream_status '
                    'request_time: $request_time upstream_response_time: $upstream_response_time';

    access_log /var/log/nginx/access.log main;
    error_log /var/log/nginx/error.log warn;

    # Performance
    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    server_tokens off;

    # Timeouts
    client_body_timeout 12s;
    client_header_timeout 12s;
    send_timeout 10s;

    # Buffer sizes
    client_body_buffer_size 10K;
    client_header_buffer_size 1k;
    client_max_body_size 50m;
    large_client_header_buffers 4 16k;

    # Gzip compression
    gzip on;
    gzip_vary on;
    gzip_proxied any;
    gzip_comp_level 6;
    gzip_types text/plain text/css text/xml text/javascript 
               application/json application/javascript application/xml+rss 
               application/rss+xml font/truetype font/opentype 
               application/vnd.ms-fontobject image/svg+xml;

    # ==================== MAIN SERVER ====================
    server {
        listen 80;
        server_name _;  # Replace with your domain in production

        # ==================== HEALTH CHECK (No Auth, No Logging) ====================
        location = /api/v1/health {
            access_log off;
            proxy_pass http://backend;
            proxy_http_version 1.1;
            proxy_set_header Connection "";
        }

        location = /api/v1/ready {
            access_log off;
            proxy_pass http://backend;
            proxy_http_version 1.1;
            proxy_set_header Connection "";
        }

        # ==================== API ROUTES ====================
        location /api/ {
            # Proxy settings
            proxy_pass http://backend;
            proxy_http_version 1.1;
            
            # Headers
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "upgrade";
            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;
            proxy_set_header X-Forwarded-Host $host;
            proxy_set_header X-Forwarded-Port $server_port;
            
            # Timeouts
            proxy_connect_timeout 60s;
            proxy_send_timeout 60s;
            proxy_read_timeout 60s;
            
            # Buffering
            proxy_buffering on;
            proxy_buffer_size 4k;
            proxy_buffers 8 4k;
            proxy_busy_buffers_size 8k;
            
            # Error handling - try next upstream server on errors
            proxy_next_upstream error timeout http_502 http_503 http_504;
            proxy_next_upstream_tries 2;
            
            # Don't cache by default
            proxy_cache_bypass $http_upgrade;
            add_header X-Cache-Status $upstream_cache_status;
        }

        # ==================== ROOT ROUTE ====================
        location / {
            return 200 '{"message":"API Server Running","status":"ok"}';
            add_header Content-Type application/json;
        }

        # ==================== ERROR PAGES ====================
        error_page 502 503 504 /50x.html;
        location = /50x.html {
            return 503 '{"error":"Service temporarily unavailable","status":503}';
            add_header Content-Type application/json;
        }
    }
}

# ============================================================================
# NOTES:
# ============================================================================
# 1. Docker Compose networking automatically resolves "backend" to all 
#    backend service replicas. You don't need to list each IP/port manually.
#
# 2. The upstream block uses "least_conn" to route to the server with the
#    fewest active connections (better than round-robin for varying load).
#
# 3. Health checks at /api/v1/health don't create access logs (reduces noise).
#
# 4. Error handling with proxy_next_upstream ensures failed requests are
#    automatically retried on a different backend server.
#
# 5. For HTTPS in production, add SSL configuration:
#
#    server {
#        listen 443 ssl http2;
#        ssl_certificate /etc/nginx/ssl/cert.pem;
#        ssl_certificate_key /etc/nginx/ssl/key.pem;
#        ssl_protocols TLSv1.2 TLSv1.3;
#        ssl_ciphers HIGH:!aNULL:!MD5;
#        # ... rest of config ...
#    }
# ============================================================================