FastCGI cache in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is FastCGI cache in nginx?

FastCGI cache is a feature in nginx that stores dynamic content generated by FastCGI servers (like PHP-FPM) on disk. This helps serve repeated requests faster by avoiding running the backend script every time.

beginner

Which directive in nginx config enables FastCGI caching?

The directive fastcgi_cache enables FastCGI caching by specifying the cache zone name to use for storing cached responses.

intermediate

What does the fastcgi_cache_path directive do?

It defines the location on disk where cached files are stored, the cache size, and cache key zone name. It sets up the cache storage parameters.

intermediate

How does nginx decide if a cached FastCGI response is valid?

Nginx uses cache control headers like Cache-Control and Expires from the backend response, and directives like fastcgi_cache_valid to determine cache validity duration.

intermediate

What is the purpose of fastcgi_cache_key?

It defines the unique key used to store and retrieve cached content. Usually, it includes the request URI and other variables to differentiate cache entries.

Which directive sets the disk location for FastCGI cache in nginx?

A fastcgi_cache_path

B fastcgi_cache_key

C fastcgi_cache_valid

D fastcgi_cache_use_stale

The fastcgi_cache_path directive defines where cached files are stored on disk.

What does fastcgi_cache_valid 200 10m; mean?

A Cache all responses for 200 minutes

B Cache 200 OK responses for 10 minutes

C Cache only 10 responses with status 200

D Disable caching for 200 status

It tells nginx to cache responses with HTTP status 200 for 10 minutes.

How does nginx serve a cached FastCGI response?

A By reading the cached file from disk and sending it directly

B By running the backend script again

C By redirecting to the backend server

D By clearing the cache first

Nginx serves cached content directly from disk without contacting the backend.

Which directive defines the cache zone name used in fastcgi_cache?

A fastcgi_cache_valid

B fastcgi_cache_key

C fastcgi_cache_path

D fastcgi_cache_methods

The fastcgi_cache_path directive defines the cache zone name used later by fastcgi_cache.

What is the effect of fastcgi_cache_use_stale error timeout?

A Cache only error responses

B Always clear cache on error

C Disable cache on timeout

D Serve stale cache if backend returns error or times out

This directive allows nginx to serve stale cached content if the backend is down or slow.

Explain how to configure FastCGI cache in nginx from scratch.

Think about where to store cache, how to name it, and how to use it in your server block.

Describe how nginx decides when to serve cached FastCGI content versus fetching fresh content.

Consider cache expiration and backend response status.

Practice

1. What is the main purpose of enabling FastCGI cache in nginx?

easy

A. To store dynamic page responses and serve them faster next time

B. To block all dynamic content from being served

C. To increase the size of the nginx log files

D. To disable backend server processing completely

Solution

1. Step 1: Understand FastCGI cache function

   FastCGI cache stores dynamic content generated by backend servers temporarily.

2. Step 2: Identify the benefit

   Serving cached content reduces backend load and speeds up response time.

3. Final Answer:

   To store dynamic page responses and serve them faster next time -> Option A

4. Quick Check:

   FastCGI cache purpose = store and speed up [OK]

Hint: FastCGI cache speeds up dynamic pages by storing them [OK]

Common Mistakes:

• Thinking it blocks dynamic content
• Confusing cache with logging
• Assuming it disables backend servers

2. Which of the following is the correct syntax to define a FastCGI cache zone in nginx?

easy

A. fastcgi_cache /path/to/cache keys_zone=mycache inactive=60m;

B. fastcgi_cache_zone /path/to/cache keys_zone=mycache:10m inactive=60m;

C. fastcgi_cache_path keys_zone=mycache:10m /path/to/cache inactive=60m;

D. fastcgi_cache_path /path/to/cache levels=1:2 keys_zone=mycache:10m inactive=60m;

Solution

1. Step 1: Recall fastcgi_cache_path syntax

   The correct syntax includes path, levels, keys_zone with size, and inactive time.

2. Step 2: Match options with syntax

   fastcgi_cache_path /path/to/cache levels=1:2 keys_zone=mycache:10m inactive=60m; matches the correct order and parameters exactly.

3. Final Answer:

   fastcgi_cache_path /path/to/cache levels=1:2 keys_zone=mycache:10m inactive=60m; -> Option D

4. Quick Check:

   Correct fastcgi_cache_path syntax = fastcgi_cache_path /path/to/cache levels=1:2 keys_zone=mycache:10m inactive=60m; [OK]

Hint: Remember: fastcgi_cache_path needs path, levels, keys_zone [OK]

Common Mistakes:

• Mixing keys_zone and path order
• Omitting levels parameter
• Using fastcgi_cache instead of fastcgi_cache_path

3. Given this nginx config snippet:

fastcgi_cache_path /var/cache/nginx levels=1:2 keys_zone=cache_zone:10m inactive=30m;
server {
  location ~ \.php$ {
    fastcgi_cache cache_zone;
    fastcgi_cache_valid 200 10m;
    fastcgi_pass backend;
  }
}

What happens when a PHP page is requested twice within 10 minutes?

medium

A. The cache is cleared after the first request

B. Both requests are forwarded to the backend server

C. The second request is served from cache, speeding up response

D. The second request returns a 404 error

Solution

1. Step 1: Understand fastcgi_cache_valid 200 10m

   This means HTTP 200 responses are cached for 10 minutes.

2. Step 2: Analyze repeated request behavior

   The first request caches the response; the second request within 10 minutes uses cache.

3. Final Answer:

   The second request is served from cache, speeding up response -> Option C

4. Quick Check:

   Cache valid time means reuse cached response [OK]

Hint: fastcgi_cache_valid sets cache duration for status codes [OK]

Common Mistakes:

• Assuming backend always handles requests
• Thinking cache clears after first request
• Confusing cache duration with cache clearing

4. You configured FastCGI cache but notice no caching happens. Which nginx directive is most likely missing?

medium

A. fastcgi_cache_path

B. fastcgi_cache_key

C. fastcgi_cache_valid

D. fastcgi_pass

Solution

1. Step 1: Identify required directives for caching

   fastcgi_cache_path defines cache storage; fastcgi_cache_valid sets cache time; fastcgi_pass sends requests to backend.

2. Step 2: Understand fastcgi_cache_key role

   fastcgi_cache_key defines how requests are identified for caching; missing it disables caching.

3. Final Answer:

   fastcgi_cache_key -> Option B

4. Quick Check:

   Missing cache key disables caching [OK]

Hint: Without fastcgi_cache_key, nginx can't store cache properly [OK]

Common Mistakes:

• Forgetting to set fastcgi_cache_key
• Confusing fastcgi_cache_valid with cache enabling
• Assuming fastcgi_pass controls caching

5. You want to cache PHP responses but exclude logged-in users identified by a cookie user_session. Which configuration snippet correctly disables cache for logged-in users?

hard

A. fastcgi_no_cache $cookie_user_session;

B. fastcgi_cache_valid 200 10m if_not $cookie_user_session;

C. fastcgi_cache_key $cookie_user_session;

D. fastcgi_cache_bypass $cookie_user_session;

Solution

1. Step 1: Understand directives controlling cache bypass

   fastcgi_no_cache disables caching when its variable is true; fastcgi_cache_bypass skips cache lookup but may still cache response.

2. Step 2: Identify correct directive for excluding logged-in users

   fastcgi_no_cache $cookie_user_session; disables caching for requests with that cookie, preventing cached content for logged-in users.

3. Final Answer:

   fastcgi_no_cache $cookie_user_session; -> Option A

4. Quick Check:

   fastcgi_no_cache disables caching for matching requests [OK]

Hint: Use fastcgi_no_cache to exclude requests from caching [OK]

Common Mistakes:

• Using fastcgi_cache_bypass alone to prevent caching
• Trying to conditionally set fastcgi_cache_valid
• Misusing fastcgi_cache_key for cache control