Proxy cache key in Nginx - Step-by-Step Execution

Process Flow - Proxy cache key

Client Request Received

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Extract Cache Key Components

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Build Cache Key String

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Check Cache for Key

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Serve Cached

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Store Response

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Serve Response

The proxy cache key is built from parts of the client request to identify cached content. Nginx uses this key to find or store cached responses.

Execution Sample

Nginx

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

This sets the cache key to the full URL including scheme, host, and request URI.

Process Table

Step	Action	Value of $scheme	Value of $host	Value of $request_uri	Constructed cache key	Cache lookup result	Next action
1	Receive request	https	example.com	/index.html			Start processing
2	Extract variables	https	example.com	/index.html			Variables ready
3	Build cache key	https	example.com	/index.html	https://example.com/index.html		Cache key formed
4	Check cache				https://example.com/index.html	Miss	Fetch from backend
5	Fetch response						Get fresh content
6	Store response in cache				https://example.com/index.html		Cache updated
7	Serve response						Response sent to client
8	Next request same key	https	example.com	/index.html	https://example.com/index.html	Hit	Serve cached content

💡 Cache key lookup ends after serving cached or fresh response.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	After Step 8
$scheme		https	https	https	https
$host		example.com	example.com	example.com	example.com
$request_uri		/index.html	/index.html	/index.html	/index.html
cache_key			https://example.com/index.html	https://example.com/index.html	https://example.com/index.html
cache_status				Miss	Hit

Key Moments - 3 Insights

Why does the cache key include $scheme, $host, and $request_uri?

Including these variables ensures the cache key uniquely identifies the full URL requested, avoiding serving wrong cached content. See execution_table step 3 where the key is built.

What happens if the cache key is not found in cache?

Nginx fetches the content from the backend server, stores it in cache with the key, then serves it. This is shown in execution_table steps 4 to 7.

Can the cache key be customized?

Yes, you can include or exclude variables to control cache granularity. The example uses a simple full URL key, but you can add headers or cookies if needed.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What is the constructed cache key?

A https://example.com/

B http://example.com/index.html

C https://example.com/index.html

D /index.html

💡 Hint

Check the 'Constructed cache key' column at step 3 in the execution_table.

At which step does Nginx find a cache miss?

A Step 2

B Step 4

C Step 6

D Step 8

💡 Hint

Look at the 'Cache lookup result' column in the execution_table.

If the $host changes, what happens to the cache key?

A Cache key changes

B Cache key stays the same

C Cache key becomes empty

D Cache key is ignored

💡 Hint

Refer to variable_tracker for $host and cache_key values.

Concept Snapshot

proxy_cache_key directive builds a unique key from request parts.
Common variables: $scheme, $host, $request_uri.
Nginx uses this key to find or store cached responses.
Changing variables changes cache key and cache entries.
Proper key ensures correct cached content served.

Full Transcript

When Nginx receives a client request, it extracts parts like scheme, host, and request URI. These parts are combined to form a cache key string. Nginx checks if this key exists in the cache. If found (cache hit), it serves the cached content. If not found (cache miss), it fetches fresh content from the backend, stores it in cache with that key, and serves it. The cache key must uniquely identify the requested resource to avoid serving wrong content. You can customize the cache key by including different variables. This process ensures efficient and correct caching behavior.