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Cache Purging Concepts

Cache purging is a critical mechanism in Content Delivery Network (CDN) management that allows you to control when cached content is refreshed or removed. Understanding the different approaches to cache purging helps you maintain optimal performance while ensuring content freshness.

Why Cache Purging Matters

CDN caching significantly improves website performance by serving content from edge locations closer to users. However, this creates a challenge: how do you ensure users receive updated content when changes are made to your website?

Cache purging solves this by providing controlled methods to refresh cached content, balancing the benefits of caching with the need for current information.

Types of Cache Purging

Soft Purge (Revalidation)

Soft purge is a cache refresh method that preserves the cached resource while validating its freshness:

How it works:

  • The cached resource remains in the CDN edge cache
  • Peakhour sends revalidation requests to your origin server
  • The origin responds with either "304 Not Modified" or provides updated content
  • If unchanged, the existing cache continues serving immediately
  • If changed, the cache is updated with new content

Benefits:

  • Faster response times for unchanged content
  • Reduced origin server load
  • Lower bandwidth usage between CDN and origin
  • Graceful handling of origin server unavailability

Best for:

  • Regularly scheduled cache refreshes
  • Content that changes infrequently
  • Large files where re-downloading is expensive
  • Maintaining performance during content updates

Hard Purge (Eviction)

Hard purge completely removes resources from the cache, forcing fresh retrieval:

How it works:

  • Targeted resources are immediately removed from all edge caches
  • Next request triggers fresh retrieval from origin server
  • No revalidation step - content is fetched unconditionally
  • Cache is rebuilt from scratch with new content

Benefits:

  • Guarantees fresh content delivery
  • Complete cache invalidation
  • Immediate effect across all edge locations
  • Removes any potentially corrupted cached versions

Considerations:

  • Higher origin server load during cache rebuilding
  • Slower response for first users after purge
  • Increased bandwidth usage
  • Temporary performance impact during rebuild

Purging Strategies

Full Site Purging

Complete cache invalidation affects all cached resources for a domain:

When to use:

  • Major website redesigns or restructuring
  • Platform migrations or updates
  • When unsure which specific resources need updating
  • Emergency situations requiring immediate global refresh

Impact considerations:

  • Significant temporary performance reduction
  • High origin server load
  • Increased bandwidth usage
  • Should be used sparingly

Selective Purging

Targeted cache invalidation for specific resources or resource groups:

Single Resource Purging:

  • Targets individual files by exact URL
  • Minimal performance impact
  • Precise control over what gets refreshed
  • Ideal for specific file updates

Cache Tag Purging:

  • Uses metadata tags to group related resources
  • Allows logical grouping beyond URL patterns
  • Efficient for complex content relationships
  • Requires planning during content creation

Purging Timing Considerations

Immediate Purging

Best for urgent updates like:

  • Security fixes
  • Critical content corrections
  • Breaking news updates
  • Legal compliance requirements

Scheduled Purging

Suitable for predictable updates:

  • Regular content publications
  • Daily data updates
  • Maintenance windows
  • Batch content updates

Event-Triggered Purging

Automated purging based on system events:

  • Content management system updates
  • Database changes
  • User-generated content submissions
  • Third-party data synchronization

Performance Impact

Origin Server Considerations

  • Purging increases origin server load temporarily
  • Plan capacity for cache rebuild traffic
  • Monitor origin response times during purges
  • Consider origin server scaling for major purges

User Experience Impact

  • First users after purge experience slower load times
  • Subsequent users benefit from fresh cached content
  • Regional users may experience purge effects at different times
  • Mobile users may be more sensitive to temporary slowdowns

Bandwidth Implications

  • Hard purges increase CDN-to-origin bandwidth usage
  • Soft purges use minimal additional bandwidth for revalidation
  • Consider bandwidth costs for frequently purged large files
  • Plan for traffic spikes during major purging events

Best Practices

Strategic Planning

  • Map your content update patterns to appropriate purging strategies
  • Use soft purges for regular maintenance, hard purges for critical updates
  • Implement cache tagging during content creation for flexible purging
  • Monitor purging effectiveness and adjust strategies accordingly

Automation Integration

  • Integrate purging with your content management workflows
  • Implement automated purging for predictable content updates
  • Use webhooks or APIs to trigger purges from external systems
  • Set up monitoring and alerting for purging operations

Understanding these concepts enables you to make informed decisions about when and how to purge your cache, maintaining optimal performance while ensuring content freshness.