When running Ruby on Rails applications with Puma behind Nginx, you might encounter warnings like:

2023/11/15 14:22:35 [warn] 30587#0: *1123 an upstream response is buffered to a temporary file
    /var/lib/nginx/proxy/5/23/0000012376 while reading upstream

This occurs when Nginx's proxy buffers fill up and it needs to temporarily store response data on disk. While this prevents memory exhaustion, it can impact performance for dynamic applications.

Nginx provides several directives to control buffering behavior:

proxy_buffering on|off;
proxy_buffer_size 4k|8k;
proxy_buffers 8 4k|8k;
proxy_max_temp_file_size 1024m;
proxy_temp_path /var/lib/nginx/proxy;

For most Ruby on Rails applications, we recommend this balanced approach:

location @puma {
  proxy_pass http://puma;
  proxy_buffering on;
  proxy_buffer_size 4k;
  proxy_buffers 8 16k;
  proxy_max_temp_file_size 0;
  proxy_busy_buffers_size 24k;
}

The settings above provide:

• Small initial buffer (4k) to start sending data quickly
• Medium-sized secondary buffers (16k x 8 = 128k total)
• Disabled disk buffering to prevent I/O overhead
• Optimized busy buffers for concurrent requests

Testing with a sample Rails API endpoint returning 50KB JSON:

For endpoints returning >1MB responses:

location /large_resources {
  proxy_buffering on;
  proxy_buffer_size 16k;
  proxy_buffers 16 32k;
  proxy_max_temp_file_size 1m;
  proxy_temp_file_write_size 64k;
}

Monitor buffer usage with:

log_format buffers '$remote_addr - $upstream_cache_status '
                   '$upstream_bytes_received/$upstream_bytes_sent '
                   '$proxy_buffer_size/$proxy_busy_buffers_size';

access_log /var/log/nginx/buffer_debug.log buffers;

When running Rails applications with Puma behind Nginx, you might encounter warnings like:

2023/06/15 14:22:05 [warn] 4231#0: *1127 an upstream response is buffered to a temporary file
/var/lib/nginx/proxy/3/02/0000040231 while reading upstream

Nginx's proxy buffering mechanism serves three primary purposes:

• Protects upstream servers (like Puma) from slow clients
• Allows Nginx to reuse backend connections
• Enables response transformation (gzip, SSI, etc.)

The buffering occurs when:

1. Response exceeds proxy_buffer_size
2. Memory buffers (defined by proxy_buffers) are full
3. Nginx needs to continue receiving the response

Here's a recommended Nginx configuration for Rails/Puma:

location @puma {
  proxy_pass http://puma;
  proxy_http_version 1.1;
  proxy_set_header Connection "";
  
  # Buffer configuration
  proxy_buffering on;
  proxy_buffer_size 8k;
  proxy_buffers 8 32k;
  proxy_busy_buffers_size 64k;
  
  # Timeouts
  proxy_read_timeout 60s;
  
  # Disable temp files
  proxy_max_temp_file_size 0;
}

Buffering becomes problematic primarily when:

• Responses are very large (e.g., file downloads)
• You need real-time streaming
• Memory pressure is a concern

For typical Rails JSON/HTML responses, keeping buffering enabled with proper buffer sizes is often optimal.

In our load tests (1000 concurrent users):

For most Rails applications:

1. Keep proxy_buffering on
2. Set proxy_max_temp_file_size 0
3. Adjust buffer sizes based on average response sizes
4. Monitor /var/lib/nginx/proxy/ directory

Remember that these warnings don't necessarily indicate a problem - they're informational messages about Nginx's normal operation.