FastCGI cache in Nginx - Step-by-Step Execution

Process Flow - FastCGI cache

Client Request

↓

Check FastCGI Cache

↓

Backend Response

↓

Store in Cache

↓

Serve Response

The flow shows how nginx checks the FastCGI cache for a request. If cached, it serves directly. If not, it forwards to backend, caches the response, then serves it.

Execution Sample

Nginx

fastcgi_cache_path /var/cache/nginx levels=1:2 keys_zone=MYCACHE:10m max_size=100m;
server {
  location ~ \.php$ {
    fastcgi_cache MYCACHE;
    fastcgi_pass backend;
  }
}

This config sets up FastCGI cache storage and enables caching for PHP requests passing to backend.

Process Table

Step	Action	Cache Status	Backend Request	Response Served
1	Client sends request for index.php	Check cache for index.php	No	No
2	Cache miss - no cached response found	Miss	Yes - forward to backend	No
3	Backend processes request and sends response	Miss	Yes	No
4	Cache stores backend response for index.php	Stored	No	No
5	Serve response to client from backend	Stored	No	Yes
6	Next client request for index.php	Check cache for index.php	No	No
7	Cache hit - cached response found	Hit	No	Yes

💡 Execution stops after serving cached response on cache hit.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 7
cache_status	empty	miss	stored	hit
backend_request	no	yes	no	no
response_served	no	no	no	yes

Key Moments - 2 Insights

Why does nginx forward the request to backend on a cache miss?

Because the cache does not have the response yet (see execution_table step 2), nginx must get the response from backend to serve and store it.

How does nginx know when to serve from cache?

It checks if the requested resource is in cache (execution_table step 7 shows a cache hit), then serves cached response without contacting backend.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the backend first contacted?

A Step 2

B Step 1

C Step 5

D Step 7

💡 Hint

Check the 'Backend Request' column for 'Yes' in the table rows.

According to variable_tracker, what is the cache_status after step 4?

A empty

B miss

C stored

D hit

💡 Hint

Look at the 'cache_status' row under 'After Step 4' column.

If the cache was disabled, how would the 'cache_status' variable change in the tracker?

A It would always be 'hit'

B It would always be 'miss'

C It would be 'stored' after first request

D It would alternate between 'hit' and 'miss'

💡 Hint

Without cache, nginx never stores responses, so cache_status never becomes 'stored' or 'hit'.

Concept Snapshot

FastCGI cache stores backend responses to serve future requests faster.
Configure cache path and zone with fastcgi_cache_path.
Enable cache in location with fastcgi_cache directive.
On request, nginx checks cache: if hit, serve cached response.
If miss, forward to backend, cache response, then serve.
Improves performance by reducing backend load.

Full Transcript

FastCGI cache in nginx works by checking if a requested resource is already stored in cache. When a client sends a request, nginx looks up the cache. If the response is cached (cache hit), nginx serves it immediately without contacting the backend server. If the response is not cached (cache miss), nginx forwards the request to the backend server, receives the response, stores it in the cache, and then serves it to the client. This process speeds up response times and reduces backend load. The configuration involves defining a cache path and zone with fastcgi_cache_path and enabling caching in the server location block with fastcgi_cache. The execution table shows the step-by-step flow from client request to cache check, backend request on miss, caching the response, and serving from cache on subsequent requests.

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