Purging cached content in Nginx - Step-by-Step Execution

Process Flow - Purging cached content

Client sends PURGE request

↓

Nginx receives PURGE request

↓

Check if PURGE allowed

↓

Remove cached [Return 403 Forbidden

↓

Return 200 OK

↓

Client receives confirmation

The client sends a PURGE request to nginx, which checks permission, deletes cached content if allowed, and confirms the purge.

Execution Sample

Nginx

location / {
    proxy_cache mycache;
}

location ~ ^/purge(/.*) {
    allow 127.0.0.1;
    deny all;
    proxy_cache_purge mycache $1;
}

This config allows purging cached content for requests from localhost only.

Process Table

Step	Action	Request Type	Cache State	Response Code	Notes
1	Client sends PURGE request to /purge/index.html	PURGE	Cache has /index.html	N/A	Start purge process
2	Nginx checks client IP	PURGE	Cache has /index.html	N/A	Client IP 127.0.0.1 allowed
3	Nginx checks if PURGE method allowed	PURGE	Cache has /index.html	N/A	Allowed for /purge/ location
4	Nginx removes /index.html from cache	PURGE	Cache missing /index.html	N/A	Cache entry deleted
5	Nginx returns 200 OK to client	PURGE	Cache missing /index.html	200	Purge successful
6	Client receives 200 OK	PURGE	Cache missing /index.html	200	Client confirms purge
7	Client sends PURGE request from disallowed IP	PURGE	Cache has /index.html	403	Access denied, cache intact
8	Client sends normal GET request for /index.html	GET	Cache missing /index.html	200	Cache miss, content fetched from origin

💡 Purge stops when cache entry is removed or access denied.

Status Tracker

Variable	Start	After Step 4	After Step 7	After Step 8
Cache State	Contains /index.html	Does not contain /index.html	Contains /index.html	Does not contain /index.html

Key Moments - 3 Insights

Why does the PURGE request from a disallowed IP get denied?

Because nginx checks client IP against allowed list (see execution_table step 7) and denies with 403 if not allowed.

What happens to the cache after a successful PURGE?

The cached content is removed (execution_table step 4), so subsequent GET requests will fetch fresh content.

Why does a GET request after PURGE result in a cache miss?

Because the cached content was deleted by PURGE (step 4), so GET must fetch from origin (step 8).

Visual Quiz - 3 Questions

Look at the execution_table, what is the response code returned after a successful PURGE?

A 403

B 200

C 404

D 500

💡 Hint

Check step 5 and 6 in the execution_table where PURGE succeeds.

At which step does nginx remove the cached content?

A Step 2

B Step 7

C Step 4

D Step 8

💡 Hint

Look for the step describing cache deletion in the execution_table.

If the client IP is not allowed, what happens to the cache?

A Cache remains intact

B Cache is deleted

C Cache is refreshed

D Cache is corrupted

💡 Hint

See step 7 where PURGE is denied and cache state is unchanged.

Concept Snapshot

PURGE cached content in nginx:
- Client sends PURGE request to nginx
- Nginx checks client IP and permissions
- If allowed, nginx deletes cached content
- Returns 200 OK on success, 403 if denied
- Subsequent GET requests fetch fresh content if cache purged

Full Transcript

Purging cached content in nginx involves the client sending a PURGE request to nginx. Nginx first checks if the client IP is allowed to perform PURGE. If allowed, nginx deletes the cached content for the requested URI and returns a 200 OK response. If not allowed, nginx returns a 403 Forbidden response and the cache remains unchanged. After a successful purge, subsequent GET requests will miss the cache and fetch fresh content from the origin server.

Practice

1. What is the main purpose of purging cached content in nginx?

easy

A. To remove specific cached files so fresh content is served

B. To increase the cache size automatically

C. To disable caching completely

D. To restart the nginx server

Solution

1. Step 1: Understand caching in nginx

   Caching stores copies of content to serve faster responses.

2. Step 2: Understand purging purpose

   Purging removes specific cached files to update content without restarting nginx.

3. Final Answer:

   To remove specific cached files so fresh content is served -> Option A

4. Quick Check:

   Purging = Remove cached files [OK]

Hint: Purging means deleting cached files to refresh content [OK]

Common Mistakes:

• Confusing purging with disabling cache
• Thinking purging restarts nginx
• Assuming purging increases cache size

2. Which of the following is the correct syntax to enable cache purging in an nginx location block?

easy

A. proxy_cache_purge true;

B. proxy_cache_purge off;

C. proxy_cache_purge enable;

D. proxy_cache_purge mycache;

Solution

1. Step 1: Recall nginx directive syntax

   Directives use the format: directive_name value; with zone name for cache purging.

2. Step 2: Identify correct value for enabling purge

   Enabling uses zone name 'mycache' not 'enable', 'true', or 'off'.

3. Final Answer:

   proxy_cache_purge mycache; -> Option D

4. Quick Check:

   Enable purge = proxy_cache_purge mycache; [OK]

Hint: Use the zone name to enable cache purging in nginx [OK]

Common Mistakes:

• Using 'enable' or 'true' instead of the zone name
• Forgetting the semicolon at the end
• Setting proxy_cache_purge to 'off' by mistake

3. Given this nginx config snippet:

location /purge/ {
    allow 192.168.1.0/24;
    deny all;
    proxy_cache_purge mycache;
}

What happens when a client from IP 192.168.1.10 sends a PURGE request to /purge/page1?

medium

A. The cached content for /purge/page1 is removed

B. The request is denied due to IP restriction

C. The cache is cleared for all pages

D. The server returns a 404 error

Solution

1. Step 1: Check IP access rules

   Allow 192.168.1.0/24 means IP 192.168.1.10 is allowed; others denied.

2. Step 2: Understand proxy_cache_purge mycache

   Enables purging cached content for matching requests.

3. Final Answer:

   The cached content for /purge/page1 is removed -> Option A

4. Quick Check:

   Allowed IP + PURGE request = cache removed [OK]

Hint: Allowed IP + PURGE request = cache cleared for that URL [OK]

Common Mistakes:

• Assuming all IPs can purge
• Thinking PURGE clears entire cache
• Confusing PURGE with GET request

4. You configured nginx to purge cache with:

location /purge/ {
    deny all;
    proxy_cache_purge mycache;
}

But PURGE requests still return 403 Forbidden. What is the likely cause?

medium

A. Cache is empty so purge fails

B. proxy_cache_purge directive is misspelled

C. Missing allow/deny rules to permit purge requests

D. PURGE method is not supported by nginx

Solution

1. Step 1: Understand default access control

   Without 'allow', nginx denies all due to 'deny all;' for security.

2. Step 2: Check purge access rules

   To allow PURGE requests, you must explicitly allow trusted IPs.

3. Final Answer:

   Missing allow/deny rules to permit purge requests -> Option C

4. Quick Check:

   deny all without allow = 403 Forbidden on PURGE [OK]

Hint: Always add allow/deny for purge locations [OK]

Common Mistakes:

• Assuming PURGE method is unsupported
• Ignoring access control rules
• Thinking empty cache causes 403

5. You want to purge cached content only for URLs starting with /images/ and restrict purge access to IP 10.0.0.5. Which nginx config snippet correctly achieves this?

hard

A. location /images/ {
proxy_cache_purge mycache;
allow 10.0.0.5;
deny all;
}

B. location /purge/images/ {
proxy_cache_purge mycache;
allow 10.0.0.5;
deny all;
}

C. location /images/ {
proxy_cache mycache;
allow 10.0.0.5;
deny all;
}

D. location /purge/ {
proxy_cache_purge mycache;
allow all;
}

Solution

1. Step 1: Identify purge location for /images/ URLs

   Purging usually uses a separate location like /purge/images/ to trigger cache removal.

2. Step 2: Check access control and purge directive

   Allow only IP 10.0.0.5 and deny others with proxy_cache_purge mycache enabled.

3. Final Answer:

   location /purge/images/ { proxy_cache_purge mycache; allow 10.0.0.5; deny all; } -> Option B

4. Quick Check:

   Correct purge path + access control = location /purge/images/ { proxy_cache_purge mycache; allow 10.0.0.5; deny all; } [OK]

Hint: Use a purge-specific location with allow/deny for security [OK]

Common Mistakes:

• Enabling purge in wrong location
• Using proxy_cache instead of proxy_cache_purge
• Allowing all IPs to purge