When examining your nginx configuration and test cases, we can see that while JavaScript files are being properly compressed, JSON responses remain uncompressed despite having application/json in your gzip_types directive. Let's break down the potential causes:

// Current configuration snippet
gzip on;
gzip_types text/plain application/javascript application/json;
gzip_min_length 256;

Here's what we need to verify in this situation:

• Response size meets minimum length requirement
• No conflicting directives in server/location blocks
• Proper MIME type detection
• Upstream isn't already compressing the response

Your current configuration looks correct at first glance, but let's enhance it with some additional parameters that often help with JSON compression:

gzip on;
gzip_vary on;
gzip_proxied any;
gzip_comp_level 6;
gzip_buffers 16 8k;
gzip_http_version 1.1;
gzip_min_length 20;  // Reduced for testing purposes
gzip_types application/json application/javascript text/plain text/css application/xml text/javascript;

For reliable testing, use this curl command that shows both headers and compressed content:

curl -H "Accept-Encoding: gzip" -v https://example.com/test.json --output - | gunzip

Compare this with the standard response:

curl -v https://example.com/test.json

Several factors could prevent JSON compression:

• Response size below gzip_min_length
• Upstream proxy already modifying headers
• Location-specific nginx directives overriding main config
• Missing Vary header configuration

Add these to your nginx config for better debugging:

add_header X-Gzip-Enabled $gzip_ratio;
add_header X-Original-Content-Length $content_length;

These custom headers will show:

• Whether gzip was attempted (X-Gzip-Enabled)
• Original content length before compression

If you're using reverse proxy setups, ensure proper proxy configuration:

proxy_set_header Accept-Encoding "";
proxy_http_version 1.1;

Here's a complete working configuration for JSON compression:

http {
    gzip on;
    gzip_disable "msie6";
    gzip_vary on;
    gzip_proxied any;
    gzip_comp_level 6;
    gzip_buffers 16 8k;
    gzip_http_version 1.1;
    gzip_min_length 20;
    gzip_types application/json application/ld+json 
               application/javascript text/plain 
               application/xml text/css text/javascript;

    server {
        listen 80;
        server_name example.com;

        location / {
            root /var/www/html;
            try_files $uri $uri/ =404;
            
            # Ensure proper content type detection
            types {
                application/json json;
                application/ld+json jsonld;
            }
        }
    }
}

---

While troubleshooting compression issues on my Nginx server, I discovered a peculiar behavior where JSON responses stubbornly refused gzip compression while other content types worked perfectly. Here's my deep dive into the problem and solution.

My nginx.conf contained what appeared to be proper gzip directives:

gzip on;
gzip_vary on;
gzip_proxied any;
gzip_comp_level 6;
gzip_buffers 16 8k;
gzip_http_version 1.1;
gzip_min_length 256;
gzip_types text/plain application/javascript text/javascript text/xml text/css 
          text/html application/json application/vnd.ms-fontobject 
          application/x-font-ttf font/opentype image/svg+xml image/x-icon;

Testing with curl revealed inconsistent behavior:

# Working JS compression
curl -H "Accept-Encoding: gzip" -I http://example.com/script.js
# Returns Content-Encoding: gzip

# Failed JSON compression
curl -H "Accept-Encoding: gzip" -I https://example.com/api/data.json
# Missing Content-Encoding header

Before diving deeper, verify these basic requirements:

1. The client must send Accept-Encoding: gzip header
2. Response size must exceed gzip_min_length (256 bytes in my config)
3. The Content-Type must exactly match one of the types in gzip_types

After extensive testing, I identified two critical issues:

# 1. Dynamic content needs proper caching headers
location ~* \.json$ {
  expires 1h;
  add_header Cache-Control "public";
  add_header Vary "Accept-Encoding";
}

# 2. Proxy configurations may interfere
location /api/ {
  proxy_pass http://backend;
  proxy_set_header Accept-Encoding "";
  gzip_proxied any;
}

Here's the configuration that finally resolved my JSON compression issues:

http {
  gzip on;
  gzip_vary on;
  gzip_proxied expired no-cache no-store private auth;
  gzip_comp_level 6;
  gzip_min_length 1024; # Increased from 256
  gzip_types
    application/atom+xml
    application/geo+json
    application/json
    application/ld+json
    application/manifest+json
    application/xml
    text/css
    text/javascript
    text/plain
    text/xml;

  server {
    location ~* \.(json|js|css|xml)$ {
      gzip_static on;
      expires 1h;
      add_header Cache-Control "public";
      add_header Vary "Accept-Encoding";
    }
  }
}

To confirm gzip is working for JSON responses:

# Check compressed size
curl -H "Accept-Encoding: gzip" -s -w "%{size_download}" -o /dev/null https://example.com/data.json

# Verify headers
curl -H "Accept-Encoding: gzip" -I https://example.com/data.json | grep -i "content-encoding"

Remember to test with different JSON payload sizes to ensure the gzip_min_length threshold is properly configured for your use case.