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.