Proxy cache key in Nginx - Mini Project: Build & Apply

Configuring Proxy Cache Key in Nginx

📖 Scenario: You are managing a web server using Nginx. To improve performance, you want to cache responses from a backend server. The cache key determines how Nginx stores and retrieves cached content.In this project, you will configure the proxy_cache_key directive to customize the cache key based on the request URI and a custom header.

🎯 Goal: Build an Nginx configuration snippet that sets a proxy cache key using the request URI and the X-User-Type header. This will help cache different versions of content for different user types.

📋 What You'll Learn

Create a variable called $cache_key that combines $request_uri and the X-User-Type header

Set the proxy_cache_key directive to use the $cache_key variable

Use the proxy_cache_key directive inside the location / block

💡 Why This Matters

🌍 Real World

Web servers often cache content to speed up responses. Using a custom cache key lets you serve different cached content to different users, improving performance and user experience.

💼 Career

Understanding how to configure proxy cache keys is important for DevOps engineers managing web infrastructure to optimize server performance and resource usage.

1

Create the initial Nginx server block

Write an Nginx server block with a location / block that proxies requests to http://backend_server. Use the exact server block syntax shown below.

Nginx

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://backend_server;
        # Your code here
    }
}

Hint

Start by writing a basic server block that listens on port 80 and proxies all requests to http://backend_server.

2

Create a variable combining URI and header

Inside the location / block, create a variable called $cache_key that combines $request_uri and the X-User-Type header using the syntax: set $cache_key "$request_uri:$http_x_user_type";

Nginx

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://backend_server;
        # Create $cache_key variable combining $request_uri and X-User-Type header
        # Your code here
    }
}

Hint

Use the set directive to create a variable combining the URI and the header value.

3

Configure proxy_cache_key directive

Inside the location / block, add the directive proxy_cache_key $cache_key; to use the variable $cache_key as the cache key.

Nginx

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://backend_server;
        set $cache_key "$request_uri:$http_x_user_type";
        # Add proxy_cache_key directive here
        # Your code here
    }
}

Hint

Use the proxy_cache_key directive to specify the cache key variable.

4

Print the final Nginx configuration

Print the entire Nginx server block configuration to verify your setup. Use print() with triple quotes to display the configuration exactly as written.

Nginx

# Print the full Nginx configuration below
# Your code here

Hint

Use a print statement with triple quotes to show the full configuration exactly.

Practice

1. What is the main purpose of the proxy_cache_key directive in nginx?

easy

A. To define how nginx identifies and stores cached content

B. To set the maximum size of the cache

C. To specify the cache expiration time

D. To enable SSL for proxy connections

Solution

1. Step 1: Understand the role of proxy_cache_key

   The proxy_cache_key tells nginx how to uniquely identify cached content by combining variables like scheme, host, and URI.

2. Step 2: Differentiate from other cache settings

   Other directives control cache size or expiration, but proxy_cache_key specifically defines the cache lookup key.

3. Final Answer:

   To define how nginx identifies and stores cached content -> Option A

4. Quick Check:

   proxy_cache_key = cache identifier [OK]

Hint: proxy_cache_key sets cache identity, not size or time [OK]

Common Mistakes:

• Confusing cache key with cache size or expiration
• Thinking it enables SSL
• Assuming it controls cache storage location

2. Which of the following is the correct syntax to set a proxy cache key using scheme, host, and request URI in nginx?

easy

A. proxy_cache_key $scheme$host$request_uri;

B. proxy_cache_key $scheme://$host$request_uri;

C. proxy_cache_key $scheme://host$request_uri;

D. proxy_cache_key "$scheme://$host$request_uri";

Solution

1. Step 1: Check syntax for proxy_cache_key

   The value must be a string, so it should be enclosed in double quotes if it contains special characters like ://.

2. Step 2: Verify variable usage

   Variables like $scheme, $host, and $request_uri must be concatenated properly inside quotes.

3. Final Answer:

   proxy_cache_key "$scheme://$host$request_uri"; -> Option D

4. Quick Check:

   Quotes needed for complex keys = proxy_cache_key "$scheme://$host$request_uri"; [OK]

Hint: Use quotes when combining variables with symbols like :// [OK]

Common Mistakes:

• Omitting quotes around the key string
• Missing $ before variables
• Incorrect concatenation without quotes

3. Given this nginx config snippet:

proxy_cache_key "$scheme://$host$request_uri";

What will be the cache key for a request to https://example.com/path/page?query=1?

medium

A. example.com/path/page?query=1

B. https://example.com/path/page?query=1

C. https://example.com/path/page

D. http://example.com/path/page?query=1

Solution

1. Step 1: Substitute variables with request values

   $scheme is 'https', $host is 'example.com', and $request_uri includes path and query: '/path/page?query=1'.

2. Step 2: Combine variables as per the key format

   Joining them gives 'https://example.com/path/page?query=1'.

3. Final Answer:

   https://example.com/path/page?query=1 -> Option B

4. Quick Check:

   Variables combined exactly = https://example.com/path/page?query=1 [OK]

Hint: Remember $request_uri includes query string [OK]

Common Mistakes:

• Ignoring the query string in $request_uri
• Using http instead of https for scheme
• Leaving out the scheme or host

4. You set proxy_cache_key "$scheme://$host$uri"; but nginx caches different query strings as the same content. What is the likely cause?

medium

A. You did not enable proxy_cache_path

B. You used $request_uri which already includes query string

C. You forgot to include $args in the cache key

D. You used double quotes instead of single quotes

Solution

1. Step 1: Understand $uri vs $request_uri

   $uri contains only the path (normalized), excluding the query string. $request_uri includes the query.

2. Step 2: Analyze the symptom

   Different query strings yield the same $uri, hence same cache key, causing shared cache content.

3. Final Answer:

   You forgot to include $args in the cache key -> Option C

4. Quick Check:

   $uri excludes query; add $args [OK]

Hint: $uri lacks query string; add $args [OK]

Common Mistakes:

• Confusing cache key with cache size or expiration
• Thinking it enables SSL
• Assuming it controls cache storage location

5. You want to cache content separately for mobile and desktop users. Which proxy cache key configuration achieves this if user agent contains 'Mobile' for mobile devices?

hard

A. proxy_cache_key "$scheme://$host$request_uri:$is_mobile"; # where $is_mobile is set by map

B. proxy_cache_key "$scheme://$host$request_uri:$http_user_agent ~* Mobile";

C. proxy_cache_key "$scheme://$host$request_uri:$http_user_agent";

D. proxy_cache_key "$scheme://$host$request_uri";

Solution

1. Step 1: Understand the need for conditional cache key

   We want to separate cache for mobile and desktop, so the key must include a variable indicating device type.

2. Step 2: Use a map to set $is_mobile variable

   nginx can use map to set $is_mobile to 'mobile' or 'desktop' based on $http_user_agent matching 'Mobile'. Including $is_mobile in cache key separates caches.

3. Step 3: Evaluate options

   proxy_cache_key "$scheme://$host$request_uri:$http_user_agent"; uses full user agent, which is too specific and causes cache fragmentation. proxy_cache_key "$scheme://$host$request_uri:$http_user_agent ~* Mobile"; is invalid syntax. proxy_cache_key "$scheme://$host$request_uri"; ignores device type. proxy_cache_key "$scheme://$host$request_uri:$is_mobile"; # where $is_mobile is set by map correctly uses $is_mobile set by map.

4. Final Answer:

   proxy_cache_key "$scheme://$host$request_uri:$is_mobile"; # where $is_mobile is set by map -> Option A

5. Quick Check:

   Use map variable for device type in cache key = proxy_cache_key "$scheme://$host$request_uri:$is_mobile"; # where $is_mobile is set by map [OK]

Hint: Use map to create $is_mobile variable for cache key [OK]

Common Mistakes:

• Using full user agent causing too many cache entries
• Trying regex inside proxy_cache_key (invalid)
• Ignoring device type in cache key