Purging CDN cache in Azure - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is CDN cache purging?

CDN cache purging is the process of removing stored content from the CDN servers so that fresh content is fetched from the origin server on the next request.

beginner

Why do we need to purge CDN cache?

We purge CDN cache to update or remove outdated or incorrect content so users get the latest version without waiting for cache expiration.

intermediate

Name two common methods to purge CDN cache in Azure.

1. Purge by content path (specific files or folders). 2. Purge everything (clear all cached content).

intermediate

What is the difference between 'purge' and 'invalidate' in CDN cache management?

'Purge' removes cached content immediately, forcing a fresh fetch. 'Invalidate' marks content as stale but may serve old content until refreshed.

advanced

How does purging CDN cache affect system performance?

Purging causes more requests to go to the origin server temporarily, which can increase load and latency until the cache is rebuilt.

What happens immediately after purging a CDN cache for a file?

A The CDN fetches the file fresh from the origin on next request

B The CDN serves the old cached file until expiration

C The file is deleted from the origin server

D The CDN disables caching for that file permanently

Purging removes the cached file so the CDN must get a fresh copy from the origin on the next request.

Which Azure service is used to manage CDN cache purging?

A Azure Virtual Machines

B Azure Blob Storage

C Azure CDN

D Azure Functions

Azure CDN service provides the tools to purge cached content on CDN endpoints.

Which purging method removes all cached content in Azure CDN?

A Purge by content path

B Invalidate by tag

C Refresh cache

D Purge everything

'Purge everything' clears all cached content on the CDN endpoint.

What is a potential downside of frequent CDN cache purging?

A Permanent loss of content

B Increased load on origin servers

C Slower CDN network speed

D Users see outdated content

Frequent purging causes more requests to origin servers, increasing their load temporarily.

Which HTTP header can help control CDN caching behavior?

A Cache-Control

B Content-Type

C Authorization

D User-Agent

The Cache-Control header instructs CDNs and browsers how to cache content.

Explain the process and reasons for purging CDN cache in Azure.

Think about how cached content can become outdated and how Azure lets you remove it.

Describe the difference between purging specific content paths and purging everything in CDN cache.

Consider when you want to update one file versus all files.

Practice

1. What is the main purpose of purging a CDN cache in Azure?

easy

A. To increase the storage capacity of the CDN

B. To remove outdated content so users get the latest version

C. To add new content to the CDN cache

D. To monitor traffic statistics on the CDN

Solution

1. Step 1: Understand CDN cache role

   CDN caches store copies of content to serve users faster.

2. Step 2: Purpose of purging cache

   Purging removes old cached content so fresh content is served.

3. Final Answer:

   To remove outdated content so users get the latest version -> Option B

4. Quick Check:

   Purging = Remove outdated content [OK]

Hint: Purging means clearing old cache to refresh content [OK]

Common Mistakes:

• Thinking purging adds content instead of removing
• Confusing purging with monitoring or scaling
• Believing purging increases storage capacity

2. Which Azure CLI command correctly purges the CDN cache for a specific content path?

easy

A. az cdn endpoint purge --resource-group MyGroup --profile-name MyProfile --name MyEndpoint --content-paths '/images/*'

B. az cdn endpoint create --resource-group MyGroup --profile-name MyProfile --name MyEndpoint --content-paths '/images/*'

C. az cdn endpoint start --resource-group MyGroup --profile-name MyProfile --name MyEndpoint --content-paths '/images/*'

D. az cdn profile purge --resource-group MyGroup --name MyProfile --content-paths '/images/*'

Solution

1. Step 1: Identify correct command for purging

   The command to purge cache is az cdn endpoint purge.

2. Step 2: Check parameters

   It requires resource group, profile name, endpoint name, and content paths.

3. Final Answer:

   az cdn endpoint purge --resource-group MyGroup --profile-name MyProfile --name MyEndpoint --content-paths '/images/*' -> Option A

4. Quick Check:

   Purging uses az cdn endpoint purge [OK]

Hint: Purge command uses 'endpoint purge' with content paths [OK]

Common Mistakes:

• Using 'create' or 'start' commands instead of 'purge'
• Confusing profile purge with endpoint purge
• Omitting required parameters like endpoint name

3. Given this Azure CLI command:
az cdn endpoint purge --resource-group RG1 --profile-name Profile1 --name Endpoint1 --content-paths '/css/*' '/js/app.js'
What will happen after running it?

medium

A. The command will fail due to incorrect syntax

B. Only the '/css/*' files will be purged from the CDN cache

C. Both '/css/*' and '/js/app.js' files will be purged from the CDN cache

D. Only '/js/app.js' will be purged, '/css/*' will be ignored

Solution

1. Step 1: Analyze content-paths parameter

   The command lists two paths: '/css/*' and '/js/app.js'. Both are valid and separated by space.

2. Step 2: Understand purge effect

   Both specified paths will be purged from the CDN cache after command runs.

3. Final Answer:

   Both '/css/*' and '/js/app.js' files will be purged from the CDN cache -> Option C

4. Quick Check:

   Multiple paths purge all listed [OK]

Hint: Multiple content paths purge all specified files [OK]

Common Mistakes:

• Assuming only first path is purged
• Thinking command syntax is invalid with multiple paths
• Believing wildcard and file path cannot be combined

4. You run this command to purge CDN cache:
az cdn endpoint purge --resource-group RG1 --profile-name Profile1 --name Endpoint1 --content-paths '/images/logo.png'
But the cache is not cleared. What is the most likely cause?

medium

A. The content path is incorrect or does not match cached files

B. The resource group name is misspelled

C. The Azure CLI command does not support purging images

D. The CDN endpoint is not created yet

Solution

1. Step 1: Check command correctness

   The command syntax is correct and supports purging images.

2. Step 2: Identify common purge failure reason

   If cache is not cleared, often the content path does not match cached files exactly.

3. Final Answer:

   The content path is incorrect or does not match cached files -> Option A

4. Quick Check:

   Incorrect path = purge fails [OK]

Hint: Verify content path matches cached files exactly [OK]

Common Mistakes:

• Assuming CLI cannot purge images
• Ignoring typos in resource group or endpoint names
• Thinking purge works instantly without matching paths

5. You manage a global website using Azure CDN. You want to purge cache for a frequently updated JavaScript file but minimize performance impact. Which strategy is best?

hard

A. Disable CDN caching for all files to avoid purging

B. Purge the entire CDN endpoint cache every hour

C. Purge cache only once a month to reduce load

D. Purge only the specific JavaScript file path instead of the entire CDN endpoint

Solution

1. Step 1: Understand impact of purging scope

   Purging entire endpoint cache causes high load and delays for all users.

2. Step 2: Choose targeted purging

   Purging only the specific JavaScript file updates that content with minimal performance impact.

3. Final Answer:

   Purge only the specific JavaScript file path instead of the entire CDN endpoint -> Option D

4. Quick Check:

   Targeted purge = minimal impact [OK]

Hint: Purge specific files, not whole endpoint, to save performance [OK]

Common Mistakes:

• Purging entire cache too often causing slowdowns
• Disabling caching loses CDN benefits
• Purging too rarely causing stale content