Cache Invalidation at Scale - Learning Module

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Soft Purge vs Hard Purge: Choosing the Right Invalidation Strategy

The Art of Graceful Invalidation

Not all cache invalidations are created equal. When content becomes stale, you face a fundamental choice: immediately remove the cached content (hard purge) or mark it as stale while keeping it available as a fallback (soft purge). This distinction might seem subtle, but it profoundly affects system resilience, user experience, and origin server load.

Imagine a scenario where your origin server is experiencing elevated latency. With hard purge, every request for invalidated content hammers the struggling origin directly. With soft purge, edge nodes can still serve stale content while attempting background revalidation—protecting both users and infrastructure during stress.

Understanding when to use each approach is what separates CDN operators who merely configure caching from those who architect resilient content delivery at scale.

What You Will Master

By the end of this page, you will deeply understand both hard and soft purge mechanisms, their internal operations at the edge node level, performance implications, and the decision framework for selecting the appropriate strategy for any content type or operational scenario.

Hard Purge: Immediate and Complete Removal

A hard purge (also called instant purge or immediate invalidation) completely removes content from the CDN edge cache. Once executed, the cached content ceases to exist at that edge location—subsequent requests must fetch fresh content from origin.

The Hard Purge Process:

When a hard purge instruction reaches an edge node:

The edge server locates the specified cache entry by its cache key
The entry is deleted from the edge cache immediately
Any in-flight requests for that content complete with the old version
The next request triggers a full origin fetch
The fresh response is cached with new TTL timers

This is the most intuitive form of cache invalidation—content is gone, and the next request gets fresh data.

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Timeline: Hard Purge Behavior at Edge Node
 
T0: Cached content exists (Age: 45min, TTL: 60min)
    |
T1: Hard purge instruction arrives
    |
T2: Cache entry DELETED immediately
    |
T3: Request arrives for content
    │
    ├─► Edge cache: MISS (no entry exists)
    │
    └─► Origin fetch initiated (SYNCHRONOUS)
        │
        ├─► If origin healthy: Fresh response (200ms latency)
        │   └─► Cache populated with new content
        │
        └─► If origin DOWN: ERROR returned to client (no fallback!)
            └─► User sees error page
 
Key Insight: Hard purge trades resilience for immediacy.
             There is no safety net if origin fails.

When Hard Purge Is Essential

•Security-sensitive content removal — Leaked credentials, exposed PII, or content that must never be served again under any circumstance.
•Legal compliance requirements — DMCA takedowns, GDPR right-to-be-forgotten, court-ordered content removal where stale serving is legally prohibited.
•Malicious content elimination — Malware-infected files, phishing pages, or any content actively harming users.
•Completely replaced resources — When old content is not just outdated but fundamentally incorrect or harmful if served.
•Cache key conflicts — When cache key structure changes and old entries would cause routing or serving errors.

The Hidden Cost of Hard Purge

Hard purge creates a 'thundering herd' scenario. When popular content is hard-purged across all edge nodes simultaneously, the next wave of requests all hit origin. For viral content, this can mean thousands of concurrent origin requests where previously there were zero. Plan origin capacity accordingly.

Soft Purge: Graceful Stale-While-Revalidate

A soft purge (also called stale purge or mark-stale) doesn't delete cached content—it marks it as stale while retaining the cached copy. The edge server will attempt to revalidate with origin, but can fall back to serving stale content if origin is unavailable or slow.

This approach implements the stale-while-revalidate pattern at the CDN level:

The Soft Purge Process:

The edge server locates the specified cache entry
The entry is marked stale (expired) but not deleted
The next request triggers background revalidation
User receives stale content immediately (sub-millisecond latency)
Background request fetches fresh content from origin
Cache is updated when fresh content arrives
Subsequent requests receive fresh content

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Timeline: Soft Purge Behavior at Edge Node
 
T0: Cached content exists (Age: 45min, TTL: 60min)
    |
T1: Soft purge instruction arrives
    |
T2: Cache entry marked STALE (but retained in cache)
    |
T3: Request arrives for content
    │
    ├─► Edge cache: STALE (entry exists but expired)
    │
    ├─► IMMEDIATELY return stale content to user (<1ms)
    │
    └─► BACKGROUND origin fetch initiated (ASYNC)
        │
        ├─► If origin healthy: Cache updated with fresh content
        │   └─► Next request gets fresh content
        │
        └─► If origin DOWN: Stale content continues serving
            └─► System degrades gracefully, users unaffected
 
Key Insight: Soft purge prioritizes availability and user experience.
             Freshness is eventually consistent.

The Grace Period Concept:

Soft purge introduces the concept of a stale grace period—the time window during which stale content can be served while revalidation occurs. This is typically configured as a CDN setting:

stale-while-revalidate: Serve stale during background revalidation (typically 60-300 seconds)
stale-if-error: Serve stale if origin returns 5xx errors (typically 3600 seconds or more)

These settings determine how long the CDN will use stale content as a fallback, providing a configurable tradeoff between freshness and availability.

When Soft Purge Excels

•Routine content updates — Blog posts, product descriptions, or any content where briefly stale data is acceptable.
•High-traffic content invalidation — Popular pages where origin protection is critical during cache warming.
•Origin instability periods — When origin health is degraded and you need cache resilience.
•Scheduled content refreshes — Planned updates where immediate freshness is ideal but not mandatory.
•Progressive content rollouts — When you want fresh content to propagate gradually rather than simultaneously.

Soft Purge Is Your Default

For most content types, soft purge should be your default strategy. Reserve hard purge for scenarios where serving stale content would cause actual harm. This mindset dramatically improves system resilience while rarely impacting user experience.

Technical Comparison: Side-by-Side Analysis

Understanding the precise behavioral differences between hard and soft purge enables informed decision-making for any invalidation scenario.

Hard Purge vs Soft Purge: Detailed Comparison
Characteristic	Hard Purge	Soft Purge
Cache Entry After Purge	Deleted completely	Marked stale, retained
First Request Response Time	Origin fetch latency (100-500ms+)	Cached stale content (<1ms)
Origin Load	Immediate synchronous requests	Background async requests
Origin Failure Behavior	Error returned to user	Stale content served
Content Freshness Guarantee	Immediate upon origin response	Eventually consistent
Thundering Herd Risk	High for popular content	Low (request coalescing)
Storage Impact	Immediate space reclamation	Delayed until stale expires
Propagation Latency	Similar (3-10 seconds)	Similar (3-10 seconds)
Typical Use Case	Security/legal compliance	Routine content updates

Request Flow Comparison:

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// Simplified edge server request handling logic
 
interface CacheEntry {
  content: Buffer;
  headers: Headers;
  cachedAt: Date;
  ttl: number;
  stale: boolean;  // Soft purge sets this to true
}
 
async function handleRequest(request: Request, cache: EdgeCache): Promise<Response> {
  const cacheKey = computeCacheKey(request);
  const entry = cache.get(cacheKey);
  
  // HARD PURGE: Entry doesn't exist
  if (!entry) {
    // Synchronous origin fetch - user waits
    const response = await fetchFromOrigin(request);
    cache.set(cacheKey, createCacheEntry(response));
    return response;
  }
  
  // SOFT PURGE: Entry exists but marked stale
  if (entry.stale || isExpired(entry)) {
    // Background revalidation - user doesn't wait
    triggerBackgroundRevalidation(request, cacheKey);
    
    // Check stale-while-revalidate window
    if (withinStaleWindow(entry)) {
      // Serve stale immediately
      return createResponse(entry, { 'X-Cache': 'STALE' });
    }
    
    // Stale window exceeded, must revalidate synchronously
    const response = await fetchFromOrigin(request);
    cache.set(cacheKey, createCacheEntry(response));
    return response;
  }
  
  // Fresh cache hit
  return createResponse(entry, { 'X-Cache': 'HIT' });
}
 
async function triggerBackgroundRevalidation(
  request: Request, 
  cacheKey: string
): Promise<void> {
  // Non-blocking origin fetch
  setImmediate(async () => {
    try {
      const response = await fetchFromOrigin(request);
      if (response.ok) {
        cache.set(cacheKey, createCacheEntry(response));
      }
    } catch (error) {
      // Origin failed, stale content continues serving
      metrics.increment('background_revalidation.failed');
    }
  });
}

Request Coalescing with Soft Purge

When soft purge triggers background revalidation, most CDNs coalesce multiple concurrent requests into a single origin fetch. 1,000 concurrent requests for stale content result in 1 origin request, not 1,000. This is a massive origin protection benefit that hard purge cannot provide.

Performance Implications

The choice between hard and soft purge has measurable performance implications that compound across high-traffic systems.

User-Perceived Latency:

Consider a product page with 10,000 requests per minute during a flash sale:

Hard Purge Scenario:

Purge issued at T0
Propagation completes at T0+5s
First 500 requests (T0+5s to T0+5.5s) all hit origin simultaneously
Origin response time degrades from 200ms to 800ms due to load
Users experience 800ms TTFB (Time To First Byte)
Origin may queue or reject requests, causing errors

Soft Purge Scenario:

Purge issued at T0
Propagation completes at T0+5s
First request triggers background revalidation
499 concurrent requests receive stale content instantly (<5ms TTFB)
Single coalesced request hits origin (200ms)
After ~200ms, all subsequent requests receive fresh content
Users experience <5ms TTFB throughout

Performance Metrics Comparison (High-Traffic Scenario)
Metric	Hard Purge	Soft Purge	Impact
p50 User Latency	350ms	4ms	87.5x improvement
p99 User Latency	1,200ms	8ms	150x improvement
Origin Requests/sec (peak)	500 RPS	1 RPS	500x reduction
Error Rate During Purge	0.5-5%	~0%	Eliminated
Time to Full Freshness	~5 seconds	~5.2 seconds	Nearly equivalent

Origin Protection:

Soft purge's request coalescing provides crucial origin protection during invalidation events:

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Hard Purge: Origin Request Pattern (high-traffic content)
═══════════════════════════════════════════════════════════
Time   | Edge Requests | Origin Requests | Origin Load
───────┼───────────────┼─────────────────┼─────────────────
T+0s   | 0             | 0               | Baseline
T+5s   | 500           | 500             | ████████████████████ SPIKE!
T+6s   | 450           | 50              | █████ Elevated
T+7s   | 420           | 10              | █ Normalizing
T+8s   | 400           | 5               | Normal
───────┴───────────────┴─────────────────┴─────────────────
 
Soft Purge: Origin Request Pattern (same traffic)
═══════════════════════════════════════════════════════════
Time   | Edge Requests | Origin Requests | Origin Load
───────┼───────────────┼─────────────────┼─────────────────
T+0s   | 0             | 0               | Baseline
T+5s   | 500           | 1               | Normal (coalesced)
T+6s   | 450           | 0               | Normal (cache warm)
T+7s   | 420           | 0               | Normal
T+8s   | 400           | 0               | Normal
───────┴───────────────┴─────────────────┴─────────────────
 
500 origin requests vs 1 origin request for identical traffic

Hard Purge Can Cause Cascading Failures

When hard purging popular content during peak traffic, the resulting origin load spike can trigger cascading failures: origin overload → increased latency → timeout failures → retry storms → complete outage. Soft purge prevents this cascade by design.

Implementation Patterns

Implementing intelligent purge type selection requires understanding your content's characteristics and building adaptive systems that choose the right approach.

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interface ContentMetadata {
  contentType: string;
  sensitivity: 'public' | 'sensitive' | 'critical';
  popularity: number;  // Requests per minute
  lastModified: Date;
}
 
interface PurgeDecision {
  type: 'hard' | 'soft';
  reason: string;
  riskLevel: 'low' | 'medium' | 'high';
}
 
class PurgeTypeSelector {
  /**
   * Determines optimal purge type based on content characteristics
   * and current system state.
   */
  selectPurgeType(
    content: ContentMetadata,
    context: PurgeContext
  ): PurgeDecision {
    
    // HARD PURGE: Security/Legal requirements override all
    if (content.sensitivity === 'critical') {
      return {
        type: 'hard',
        reason: 'Critical sensitivity requires immediate removal',
        riskLevel: this.assessOriginRisk(content.popularity)
      };
    }
    
    // HARD PURGE: Explicit security incident flag
    if (context.isSecurityIncident) {
      return {
        type: 'hard',
        reason: 'Security incident mandates hard purge',
        riskLevel: 'high'  // Accept origin load risk
      };
    }
    
    // SOFT PURGE: High popularity content (origin protection)
    if (content.popularity > 1000) {
      return {
        type: 'soft',
        reason: `High traffic content (${content.popularity} RPM) - origin protection`,
        riskLevel: 'low'
      };
    }
    
    // SOFT PURGE: Origin health is degraded
    if (context.originHealthScore < 0.8) {
      return {
        type: 'soft',
        reason: `Origin degraded (health: ${context.originHealthScore}) - preserve availability`,
        riskLevel: 'low'
      };
    }
    
    // SOFT PURGE: Recent content (likely being actively edited)
    const contentAgeMinutes = (Date.now() - content.lastModified.getTime()) / 60000;
    if (contentAgeMinutes < 5) {
      return {
        type: 'soft',
        reason: 'Recently modified content - may be rapid editing',
        riskLevel: 'low'
      };
    }
    
    // Default: SOFT PURGE for routine updates
    return {
      type: 'soft',
      reason: 'Standard content update - soft purge preferred',
      riskLevel: 'low'
    };
  }
  
  private assessOriginRisk(popularity: number): PurgeDecision['riskLevel'] {
    if (popularity > 5000) return 'high';
    if (popularity > 500) return 'medium';
    return 'low';
  }
}
 
// Usage in purge workflow
class PurgeWorkflow {
  constructor(
    private selector: PurgeTypeSelector,
    private cdnClient: CDNClient,
    private metrics: MetricsClient
  ) {}
  
  async executePurge(
    urls: string[],
    metadata: ContentMetadata[],
    context: PurgeContext
  ): Promise<PurgeResult> {
    // Group URLs by optimal purge type
    const hardPurgeUrls: string[] = [];
    const softPurgeUrls: string[] = [];
    
    for (let i = 0; i < urls.length; i++) {
      const decision = this.selector.selectPurgeType(metadata[i], context);
      
      if (decision.type === 'hard') {
        hardPurgeUrls.push(urls[i]);
        if (decision.riskLevel === 'high') {
          this.metrics.recordHighRiskHardPurge(urls[i]);
        }
      } else {
        softPurgeUrls.push(urls[i]);
      }
    }
    
    // Execute purges by type
    const results: PurgeResult = { hard: [], soft: [] };
    
    if (hardPurgeUrls.length > 0) {
      results.hard = await this.cdnClient.purge({
        urls: hardPurgeUrls,
        type: 'hard'
      });
    }
    
    if (softPurgeUrls.length > 0) {
      results.soft = await this.cdnClient.purge({
        urls: softPurgeUrls,
        type: 'soft'
      });
    }
    
    return results;
  }
}

Context-Aware Purging

The best purge systems don't use a single strategy—they adapt based on content type, current traffic, origin health, and operational context. Build intelligence into your purge workflows rather than hardcoding purge types.

CDN Provider Implementation Differences

Different CDN providers implement soft and hard purge with varying terminology, features, and behaviors. Understanding these differences is crucial when working across multiple providers or evaluating CDN options.

CDN Provider Purge Implementation Comparison
Provider	Hard Purge Term	Soft Purge Term	Key Features
Fastly	Purge (immediate)	Soft Purge	Instant purge (<150ms), native soft purge support
Cloudflare	Purge	N/A (via Cache-Control)	No native soft purge; use stale-while-revalidate headers
AWS CloudFront	Invalidation	N/A	No soft purge; invalidation only. Use versioned URLs instead.
Akamai	Fast Purge (delete)	Fast Purge (invalidate)	Invalidate marks stale; delete removes completely
Varnish	purge	softpurge (via VMOD)	Requires softpurge VMOD for soft purge behavior

Fastly: The Gold Standard for Purge Granularity

Fastly provides the most sophisticated purge options, including native soft purge support with sub-second propagation:

# Hard purge
PURGE /path/to/content HTTP/1.1
Host: example.com
Fastly-Key: your-api-key

# Soft purge (mark stale, serve while revalidating)
PURGE /path/to/content HTTP/1.1
Host: example.com
Fastly-Key: your-api-key
Fastly-Soft-Purge: 1

AWS CloudFront: Invalidation-Only Architecture

CloudFront doesn't support soft purge natively. Invalidations are always hard—content is removed from edge caches. To achieve soft-purge-like behavior, you must:

Use versioned URLs (/assets/main.v123.css) for cache busting
Configure short TTLs with stale-while-revalidate origin headers
Implement application-level cache fallback logic

Akamai: Invalidate vs Delete

Akamai's Fast Purge offers two distinct actions:

Invalidate: Marks content stale (equivalent to soft purge)—edge servers will attempt revalidation but serve stale if origin unavailable
Delete: Removes content completely (hard purge)—no fallback available

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interface UnifiedPurgeRequest {
  urls: string[];
  type: 'hard' | 'soft';
  priority?: 'normal' | 'urgent';
}
 
class MultiCDNPurgeClient {
  private providers: Map<string, CDNProviderClient>;
  
  constructor(providers: CDNProviderClient[]) {
    this.providers = new Map(providers.map(p => [p.name, p]));
  }
  
  async purge(request: UnifiedPurgeRequest): Promise<void> {
    const promises = Array.from(this.providers.values()).map(provider => 
      this.purgeProvider(provider, request)
    );
    
    await Promise.all(promises);
  }
  
  private async purgeProvider(
    provider: CDNProviderClient,
    request: UnifiedPurgeRequest
  ): Promise<void> {
    switch (provider.name) {
      case 'fastly':
        // Native soft purge support
        await provider.purge(request.urls, {
          soft: request.type === 'soft'
        });
        break;
        
      case 'cloudflare':
        // No soft purge - always hard, rely on origin headers
        await provider.purge(request.urls);
        if (request.type === 'soft') {
          console.warn('Cloudflare: Soft purge simulated via origin headers');
        }
        break;
        
      case 'cloudfront':
        // Invalidation only (hard purge)
        await provider.createInvalidation(request.urls);
        if (request.type === 'soft') {
          console.warn('CloudFront: No soft purge support. Consider versioned URLs.');
        }
        break;
        
      case 'akamai':
        // Explicit invalidate vs delete
        if (request.type === 'hard') {
          await provider.fastPurge(request.urls, { action: 'delete' });
        } else {
          await provider.fastPurge(request.urls, { action: 'invalidate' });
        }
        break;
        
      default:
        throw new Error(`Unknown CDN provider: ${provider.name}`);
    }
  }
}

Provider Capability Evaluation

When evaluating CDN providers, purge capabilities should be a key criterion. Native soft purge support (Fastly, Akamai) provides better origin protection than providers requiring workarounds (CloudFront, Cloudflare). Factor this into architecture decisions.

Hybrid Strategies: Combining Approaches

Sophisticated invalidation architectures often combine hard and soft purge in layered strategies that optimize for both freshness and resilience.

Advanced Hybrid Patterns

•Soft-Then-Hard Escalation — Start with soft purge for immediate response. If origin revalidation fails for N seconds, escalate to hard purge to force fresh fetch on next request.
•Regional Soft, Critical Hard — Use soft purge for most regions, but hard purge in regions with known origin proximity (low latency) or where content accuracy is legally mandated.
•Content-Type Differentiation — Soft purge for static assets (CSS, JS, images), hard purge for API responses or personalized content where staleness could cause logical errors.
•Time-Based Transition — Soft purge initially for gradual rollout, then hard purge after a grace period to ensure full freshness.
•Fallback Chain Strategy — Hard purge primary content, but soft purge fallback/default content that would serve if primary is unavailable.

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class EscalatingPurgeStrategy {
  constructor(
    private cdnClient: CDNClient,
    private verifier: PurgeVerifier,
    private softPurgeTimeout: number = 30000  // 30 seconds
  ) {}
  
  async purgeWithEscalation(urls: string[]): Promise<void> {
    // Phase 1: Soft purge for immediate stale but available content
    const softPurgeId = await this.cdnClient.purge({
      urls,
      type: 'soft'
    });
    
    console.log(`Initiated soft purge: ${softPurgeId}`);
    
    // Wait for soft purge propagation
    await this.cdnClient.waitForCompletion(softPurgeId);
    
    // Phase 2: Allow time for background revalidation
    await this.waitForRevalidation(urls);
    
    // Phase 3: Verify freshness achieved
    const verified = await this.verifier.verifyFreshness(urls);
    
    if (verified.allFresh) {
      console.log('Soft purge revalidation successful');
      return;
    }
    
    // Phase 4: Escalate to hard purge for any stale URLs
    const staleUrls = verified.staleUrls;
    
    if (staleUrls.length > 0) {
      console.warn(`Escalating to hard purge for ${staleUrls.length} URLs`);
      
      await this.cdnClient.purge({
        urls: staleUrls,
        type: 'hard'
      });
      
      metrics.increment('purge.escalation.hard', { count: staleUrls.length });
    }
  }
  
  private async waitForRevalidation(urls: string[]): Promise<void> {
    // Poll until revalidation complete or timeout
    const startTime = Date.now();
    
    while (Date.now() - startTime < this.softPurgeTimeout) {
      const status = await this.checkRevalidationStatus(urls);
      
      if (status.allRevalidated) {
        return;
      }
      
      await sleep(2000);  // Check every 2 seconds
    }
  }
}

Escalation Prevents Infinite Staleness

Soft purge with escalation provides the best of both worlds: immediate availability with eventual freshness guarantee. If background revalidation fails (e.g., origin transient error), escalation ensures content eventually refreshes.

Decision Framework: When to Use Each

Use this decision framework to systematically choose the appropriate purge type for any content invalidation scenario:

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PURGE TYPE DECISION TREE
═══════════════════════════════════════════════════════════════════
 
START: Content needs invalidation
   │
   ├─► Is this a security incident? (malware, data leak, etc.)
   │      │
   │      ├─► YES ──► HARD PURGE (immediate removal required)
   │      │
   │      └─► NO ──► Continue evaluation
   │
   ├─► Is there legal/compliance requirement for immediate removal?
   │      │
   │      ├─► YES ──► HARD PURGE (legal mandate)
   │      │
   │      └─► NO ──► Continue evaluation
   │
   ├─► Would serving stale content cause user harm or logical errors?
   │      │
   │      ├─► YES ──► HARD PURGE (user safety)
   │      │
   │      └─► NO ──► Continue evaluation
   │
   ├─► Is origin currently healthy? (response time, error rate)
   │      │
   │      ├─► NO ──► SOFT PURGE (preserve availability)
   │      │
   │      └─► YES ──► Continue evaluation
   │
   ├─► Is this high-traffic content? (>1000 RPM)
   │      │
   │      ├─► YES ──► SOFT PURGE (origin protection)
   │      │
   │      └─► NO ──► Continue evaluation
   │
   └─► DEFAULT ──► SOFT PURGE (standard content update)
 
QUICK REFERENCE:
• Hard Purge: Security, legal, actively harmful content
• Soft Purge: Everything else
 
When in doubt, soft purge is safer.

Key Takeaways

•Soft purge should be your default — It provides origin protection, user experience benefits, and graceful degradation with minimal freshness tradeoff.
•Hard purge is for exceptional cases — Security incidents, legal compliance, and actively harmful content justify the availability trade-off.
•Provider capabilities vary — Not all CDNs support native soft purge; plan architecture accordingly.
•Hybrid strategies combine benefits — Soft-then-hard escalation, regional differentiation, and content-type policies enable sophisticated control.
•Monitor purge outcomes — Track cache hit rates, origin load, and user latency after purges to validate strategy effectiveness.

What's Next:

With purge types mastered, the next page explores Tag-Based Invalidation—a powerful abstraction that allows logical grouping of cached content for efficient, targeted invalidation at scale.

Page Complete

You now understand the critical distinction between hard and soft purge, their performance implications, implementation patterns, and how to make data-driven decisions about which approach to use for any invalidation scenario.