CDN caching rules in Azure - Step-by-Step Execution

Process Flow - CDN caching rules

Client Request

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Check CDN Cache

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Forward to Origin

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Origin Response

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Serve Cached Content

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Cache Content

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Apply Caching Rules

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Store/Update Cache

The CDN receives a client request, checks if content is cached, serves cached content if available, otherwise fetches from origin, applies caching rules, and updates the cache.

Execution Sample

Azure

1. Client requests /image.png
2. CDN checks cache for /image.png
3. Cache miss, CDN requests origin
4. Origin returns /image.png with Cache-Control: max-age=3600
5. CDN caches content for 3600 seconds
6. CDN serves /image.png to client

This example shows how a CDN handles a request, applies caching rules from origin headers, caches content, and serves it.

Process Table

Step	Action	Cache Status	Caching Rule Applied	Result
1	Client requests /image.png	N/A	N/A	Request received by CDN
2	CDN checks cache for /image.png	Miss	N/A	No cached content found
3	CDN forwards request to origin	Miss	N/A	Request sent to origin server
4	Origin responds with /image.png and Cache-Control: max-age=3600	Miss	max-age=3600	CDN reads caching rule
5	CDN caches /image.png for 3600 seconds	Cached	max-age=3600	Content stored in CDN cache
6	CDN serves /image.png to client	Cached	max-age=3600	Client receives cached content
7	Client requests /image.png again within 3600 seconds	Hit	max-age=3600	CDN serves cached content directly
8	Client requests /image.png after 3600 seconds	Expired	max-age=3600	Cache expired, CDN fetches from origin again

💡 Execution stops when content is served from cache or origin, depending on cache status and rules.

Status Tracker

Variable	Start	After Step 2	After Step 5	After Step 7	After Step 8
Cache Status	Empty	Miss	Cached	Hit	Expired
Cached Content TTL (seconds)	0	0	3600	3600 (valid)	0 (expired)

Key Moments - 3 Insights

Why does the CDN fetch content from the origin at step 3?

Because the cache status is 'Miss' at step 2, meaning the content is not yet stored in the CDN cache, so it must request it from the origin.

What does the 'max-age=3600' caching rule mean at step 4?

It tells the CDN to keep the content cached for 3600 seconds (1 hour) before considering it expired and needing to fetch fresh content.

Why does the cache status change to 'Expired' at step 8?

Because the cached content's TTL of 3600 seconds has passed, so the CDN must fetch the content from the origin again to update the cache.

Visual Quiz - 3 Questions

Look at the execution table, what is the cache status at step 5?

A Miss

B Cached

C Expired

D Hit

💡 Hint

Refer to the 'Cache Status' column in the execution table at step 5.

At which step does the CDN serve cached content directly to the client?

A Step 3

B Step 6

C Step 7

D Step 8

💡 Hint

Check the 'Result' column for when cached content is served without origin fetch.

If the origin sent Cache-Control: max-age=0, how would the cache status at step 5 change?

A It would be 'Cached' with TTL 0

B It would be 'Hit'

C It would be 'Miss' and never cache

D It would be 'Expired' immediately

💡 Hint

Look at how max-age affects TTL and cache expiration in the variable tracker.

Concept Snapshot

CDN caching rules control how long content is stored at the CDN edge.
Client requests check cache first; if miss, CDN fetches from origin.
Cache-Control headers like max-age set caching duration.
Cached content is served directly until TTL expires.
Expired content triggers a fresh fetch from origin.

Full Transcript

When a client requests content, the CDN first checks if it has the content cached. If not, it requests the content from the origin server. The origin server responds with the content and caching instructions, such as max-age, which tells the CDN how long to keep the content cached. The CDN stores the content and serves it to the client. Subsequent requests within the max-age period are served directly from the cache, improving speed. Once the max-age expires, the CDN fetches fresh content from the origin again. This process ensures efficient delivery and up-to-date content.