Origin Shield - Learning Module

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Configuration Strategies

From Theory to Production

You understand what origin shield is, how it reduces load, where to place it, and the trade-offs involved. Now it's time to translate that knowledge into production configuration.

Origin shield effectiveness depends heavily on configuration details—TTL alignment between cache layers, coalescing window settings, cache key design, and monitoring. Poor configuration can negate the benefits of shield entirely, while excellent configuration can maximize the protection and efficiency gains.

This page provides practical guidance drawn from production deployments across e-commerce, media, SaaS, and API platforms. You'll learn the configuration patterns that work, the mistakes to avoid, and how to monitor and tune your shield setup over time.

What You Will Learn

This page covers practical configuration strategies: TTL coordination between layers, coalescing settings, cache key design, header configuration, monitoring setup, and rollout best practices. You'll leave with a playbook for production-ready origin shield deployment.

TTL Configuration Strategy

TTL (Time To Live) configuration in a two-tier cache system requires careful coordination. Misaligned TTLs between edge and shield can cause unexpected behavior—either serving stale content too long or hitting origin more than necessary.

The TTL Hierarchy:

With origin shield, you have three TTL considerations:

Edge TTL: How long edges cache content before revalidating
Shield TTL: How long shield caches content
Origin TTL directives: What Cache-Control headers origin sends

ttl-configuration.ts
TypeScript
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/**
 * TTL Configuration Matrix
 * 
 * Guidelines for coordinating TTLs across cache tiers.
 */
 
interface TTLConfiguration {
  contentType: string;
  description: string;
  originCacheControl: string;
  edgeTTL: string;
  shieldTTL: string;
  rationale: string;
}
 
const ttlConfigurations: TTLConfiguration[] = [
  {
    contentType: "Static Assets (JS, CSS, fonts)",
    description: "Immutable versioned files",
    originCacheControl: "public, max-age=31536000, immutable",
    edgeTTL: "1 year",
    shieldTTL: "1 year",
    rationale: "Versioned via filename hash; cache forever until version changes"
  },
  {
    contentType: "Images (product photos)",
    description: "Rarely changing visual assets",
    originCacheControl: "public, max-age=86400, stale-while-revalidate=3600",
    edgeTTL: "24 hours",
    shieldTTL: "48 hours",
    rationale: "Shield TTL longer to serve expired edges; SWR for freshness"
  },
  {
    contentType: "API Responses (public data)",
    description: "Shared data endpoints",
    originCacheControl: "public, max-age=60, s-maxage=300",
    edgeTTL: "5 minutes (s-maxage)",
    shieldTTL: "10 minutes",
    rationale: "Edges refresh more often; shield provides coalescing buffer"
  },
  {
    contentType: "HTML Pages (product pages)",
    description: "Dynamic but cacheable pages",
    originCacheControl: "public, max-age=300, stale-while-revalidate=600",
    edgeTTL: "5 minutes",
    shieldTTL: "15 minutes",
    rationale: "Shorter edge TTL for freshness; longer shield for protection"
  },
  {
    contentType: "Real-Time Data",
    description: "Frequently updating content",
    originCacheControl: "public, max-age=10",
    edgeTTL: "10 seconds",
    shieldTTL: "30 seconds",
    rationale: "Shield absorbs rapid edge expiration while staying reasonably fresh"
  },
];
 
// Golden Rule: shieldTTL >= edgeTTL
// This ensures shield can serve expired edges without hitting origin

The Golden TTL Rule:

Shield TTL should generally be ≥ Edge TTL

When edge TTL expires, the edge requests content from shield. If shield TTL is longer, shield can serve the edge without hitting origin. This is where coalescing and protection come from.

Configuration Pattern:

Origin sends:    Cache-Control: public, max-age=300, s-maxage=900
Edge interprets: Cache for 900 seconds (s-maxage overrides max-age)
Shield config:   Cache for 1800 seconds (2x edge, or via CDN override)

Using stale-while-revalidate:

The stale-while-revalidate directive is powerful with origin shield:

Cache-Control: public, max-age=300, stale-while-revalidate=600

Content is fresh for 5 minutes
For the next 10 minutes, serve stale immediately while refreshing in background
Combined with shield coalescing, only one background refresh hits origin

Use s-maxage for CDN Control

The s-maxage directive controls shared cache (CDN) TTL separately from browser cache (max-age). This lets you cache longer at CDN while sending shorter max-age to browsers, giving you more control over content freshness at each layer.

Request Coalescing Configuration

Request coalescing (also called request collapsing) is the mechanism that bundles multiple simultaneous requests into a single origin fetch. Configuring it correctly is essential for origin protection.

Coalescing Parameters:

Most CDNs expose these coalescing controls:

Enable/Disable: Whether coalescing is active at all
Coalescing Window: How long to wait for requests to bundle
Max Coalesced Requests: Upper limit on bundled requests
Content Type Filters: Which content types to coalesce

Coalescing Configuration Guidelines
Setting	Conservative	Aggressive	Recommendation
Enable coalescing	Yes	Yes	Always enable for cacheable content
Coalescing window	100ms	1000ms	200-500ms balances latency and efficiency
Max coalesced requests	100	Unlimited	1000 is reasonable; unlimited for high-traffic
Coalesce on cache miss only	Yes	No	Yes - don't coalesce cache hits

Understanding the Coalescing Window:

The coalescing window determines how long the shield waits for additional requests before fetching from origin:

Scenario: 100 edge servers request /image.png within 50ms window

With 100ms coalescing window:
1. First request arrives at t=0, initiates origin fetch
2. Requests at t=10, t=20, t=30... are queued (coalesced)
3. Origin response arrives at t=200
4. All 100 requests served from single origin fetch

With 10ms coalescing window:
1. First request at t=0 initiates fetch
2. Requests at t=10 may initiate SECOND fetch (window expired)
3. Multiple origin requests for same content

Longer window = better coalescing but higher latency for first request in batch

Shorter window = faster first response but less effective coalescing

coalescing-vcl.vcl

VCL (Fastly)

# Fastly VCL: Request Coalescing Configuration
# This is configured in Fastly's VCL (Varnish Configuration Language)
 
sub vcl_backend_fetch {
  # Enable request coalescing (enabled by default on Fastly)
  # Fastly calls this "request collapsing"
  
  # For specific paths, you might want to disable coalescing
  if (req.url ~ "^/api/user-specific/") {
    # Don't coalesce user-specific API calls
    set bereq.http.Fastly-No-Shield = "1";
  }
  
  # For API endpoints, coalesce but with shorter timeouts
  if (req.url ~ "^/api/") {
    # API calls: don't wait too long
    set bereq.first_byte_timeout = 10s;
  }
  
  # For large assets, allow longer coalescing
  if (req.url ~ "\.(iso|zip|tar|dmg)$") {
    # Large downloads benefit from aggressive coalescing
    set bereq.first_byte_timeout = 60s;
  }
}
 
sub vcl_recv {
  # Ensure consistent cache keys for coalescing to work
  # Strip query parameters that don't affect content
  set req.url = querystring.filter_except(req.url, 
    "v" + "version" + "id" + "page" + "size");
  
  # Normalize Accept-Encoding for better coalescing
  if (req.http.Accept-Encoding) {
    if (req.http.Accept-Encoding ~ "br") {
      set req.http.Accept-Encoding = "br";
    } elsif (req.http.Accept-Encoding ~ "gzip") {
      set req.http.Accept-Encoding = "gzip";
    } else {
      unset req.http.Accept-Encoding;
    }
  }
}

Don't Coalesce Non-Idempotent Requests

Coalescing is only safe for idempotent, cacheable requests (typically GET). Never coalesce POST, PUT, or DELETE requests, and ensure coalescing is disabled for user-specific or authenticated content that bypasses cache.

Cache Key Design

The cache key determines which requests are considered "the same" for caching purposes. With origin shield, cache key design is crucial—it affects both edge and shield cache efficiency.

Cache Key Components:

Typical cache key includes:

URL path
Relevant query parameters
Host header
Vary header values (Accept-Encoding, etc.)

The Consistency Requirement:

For origin shield to work effectively, cache keys must be consistent between edge and shield. If an edge computes a different cache key than the shield, coalescing fails and cache fragmentation occurs.

cache-key-design.ts
TypeScript
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/**
 * Cache Key Design Patterns
 * 
 * Best practices for cache key construction with origin shield.
 */
 
interface CacheKeyConfig {
  includeHost: boolean;
  includeQueryParams: 'all' | 'none' | 'allowlist' | 'denylist';
  queryParamList?: string[];
  normalizeCase: boolean;
  stripTrailingSlash: boolean;
  normalizeEncoding: boolean;
  varyHeaders: string[];
}
 
// GOOD: Consistent, minimal cache key
const productPageCacheKey: CacheKeyConfig = {
  includeHost: true,               // Different hosts = different sites
  includeQueryParams: 'allowlist', // Only include params that affect content
  queryParamList: ['id', 'color', 'size'], // Product variants
  normalizeCase: true,             // /Product/123 === /product/123
  stripTrailingSlash: true,        // /products/ === /products
  normalizeEncoding: true,         // %20 === space
  varyHeaders: ['Accept-Encoding'], // Gzip vs Brotli
};
 
// BAD: Over-broad cache key causing fragmentation
const badCacheKey: CacheKeyConfig = {
  includeHost: true,
  includeQueryParams: 'all',  // BAD: Includes tracking params (utm_source, etc.)
  normalizeCase: false,       // BAD: Case variations create duplicates
  stripTrailingSlash: false,  // BAD: /path and /path/ are different entries
  normalizeEncoding: false,   // BAD: Encoding variations create duplicates
  varyHeaders: ['Accept-Encoding', 'Accept-Language', 'User-Agent'], // BAD: Too many variations
};
 
// Cache key function for edge/shield consistency
function computeCacheKey(request: Request, config: CacheKeyConfig): string {
  let key = '';
  
  // Host
  if (config.includeHost) {
    key += request.headers.get('Host') || '';
  }
  
  // Path normalization
  let path = new URL(request.url).pathname;
  if (config.normalizeCase) {
    path = path.toLowerCase();
  }
  if (config.stripTrailingSlash && path.endsWith('/') && path !== '/') {
    path = path.slice(0, -1);
  }
  key += path;
  
  // Query parameters
  const url = new URL(request.url);
  if (config.includeQueryParams === 'allowlist' && config.queryParamList) {
    const filteredParams = new URLSearchParams();
    for (const param of config.queryParamList) {
      if (url.searchParams.has(param)) {
        filteredParams.set(param, url.searchParams.get(param)!);
      }
    }
    // Sort for consistency
    filteredParams.sort();
    if (filteredParams.toString()) {
      key += '?' + filteredParams.toString();
    }
  }
  
  // Vary headers
  for (const header of config.varyHeaders) {
    key += '|' + header + ':' + (request.headers.get(header) || '');
  }
  
  return key;
}
 
// The same function MUST be used at both edge and shield!

Common Cache Key Mistakes:

Cache Key Anti-Patterns

•Including all query params: Marketing params (utm_source, fbclid) create infinite cache variations for identical content
•Varying on User-Agent: Thousands of unique User-Agents = thousands of cache entries for same content
•Case sensitivity: /Product/123 and /product/123 should be the same but may cache separately
•Including session cookies: Session IDs in cache key = per-user cache = no sharing
•Different keys at edge vs shield: Breaks coalescing entirely

Audit Your Cache Keys

Most CDNs provide cache analytics showing key distribution. Look for suspiciously high numbers of unique cache keys—this indicates key over-specification. A site with 10,000 products shouldn't have 10 million cache entries.

Header Configuration

HTTP headers control caching behavior at every layer. Proper header configuration ensures edge, shield, and browsers all behave as intended.

Essential Caching Headers:

Critical Caching Headers
Header	Purpose	Shield Consideration
Cache-Control	Primary caching directive	Use s-maxage for CDN-specific TTL
Vary	Cache key variation	Minimize Vary headers; each adds fragmentation
ETag	Conditional revalidation	Shield uses for efficient origin revalidation
Last-Modified	Time-based revalidation	Fallback for ETag-based revalidation
Surrogate-Control	CDN-specific directives	Allows CDN config without affecting browsers
Surrogate-Key	Tag-based invalidation	Essential for efficient purging at scale

header-examples.http

HTTP

# Example: Static Asset (immutable, versioned)
HTTP/1.1 200 OK
Cache-Control: public, max-age=31536000, immutable
Surrogate-Control: max-age=31536000
Surrogate-Key: static-assets v2-assets
ETag: "abc123xyz"
Content-Type: application/javascript
 
# Example: Product Page (cacheable but needs freshness)
HTTP/1.1 200 OK
Cache-Control: public, max-age=60, s-maxage=300, stale-while-revalidate=600
Surrogate-Control: max-age=900
Surrogate-Key: product-pages product-12345 category-electronics
Vary: Accept-Encoding
ETag: "prod12345-v789"
Content-Type: text/html
 
# Example: API Response (shared data)
HTTP/1.1 200 OK
Cache-Control: public, max-age=10, s-maxage=60
Surrogate-Control: max-age=120
Surrogate-Key: api-catalog catalog-v3
Vary: Accept-Encoding
Content-Type: application/json
 
# Example: Personalized Content (not cacheable)
HTTP/1.1 200 OK
Cache-Control: private, no-store
Vary: Cookie
Content-Type: text/html
 
# Example: Edge computation allowed
HTTP/1.1 200 OK
Cache-Control: public, max-age=300, stale-if-error=86400
Surrogate-Control: max-age=600, stale-if-error=86400
CDN-Cache-Control: max-age=1800
Content-Type: text/html

The Surrogate-Key Pattern:

Surrogate keys (also called cache tags) enable efficient invalidation at scale:

# Product page includes multiple tags
Surrogate-Key: product-123 category-shoes brand-nike homepage-featured

# Invalidate by category (purges all shoes)
Purge: category-shoes

# Invalidate specific product (purges just this page)
Purge: product-123

# Invalidate homepage features (purges featured products)
Purge: homepage-featured

With origin shield, surrogate key invalidation must propagate to:

All edge servers
All shield servers
Correctly handle tag hierarchies

CDN-Specific Headers

Different CDNs use different headers for CDN-specific control. Fastly uses Surrogate-Control and Surrogate-Key. CloudFront uses x-cache headers. Cloudflare uses Cache-Tag. Consult your CDN's documentation for the correct header names.

Monitoring and Observability

You can't optimize what you can't measure. Comprehensive monitoring is essential for effective origin shield operation.

Essential Metrics Dashboard:

Origin Shield Monitoring Checklist

•Edge Cache Hit Rate: Percentage of requests served directly from edge (target: >90%)
•Shield Cache Hit Rate: Percentage of edge misses served from shield (target: >80%)
•Origin Request Rate: Requests per second reaching origin (should be minimal)
•Origin Bandwidth: GB transferred from origin (track for cost optimization)
•Coalescing Ratio: Requests bundled per origin fetch (higher during spikes)
•Edge→Shield Latency (P50, P95, P99): Time for edge to fetch from shield
•Shield→Origin Latency (P50, P95, P99): Time for shield to fetch from origin
•Error Rates by Layer: 4xx/5xx at edge, shield, and origin separately
•Top Cache Miss URLs: What content is hitting origin most (optimization targets)
•Invalidation Events: Track purge requests and their propagation

shield-dashboard-queries.sql
SQL (Analytics)
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-- Example queries for CDN log analytics
-- Adapt to your log format and analytics platform
 
-- 1. Cache Hit Rates by Layer
SELECT 
  DATE_TRUNC('hour', timestamp) as hour,
  ROUND(100.0 * SUM(CASE WHEN cache_status = 'HIT' THEN 1 ELSE 0 END) / COUNT(*), 2) as edge_hit_rate,
  ROUND(100.0 * SUM(CASE WHEN shield_status = 'HIT' THEN 1 ELSE 0 END) / 
        SUM(CASE WHEN cache_status = 'MISS' THEN 1 ELSE 0 END), 2) as shield_hit_rate
FROM cdn_logs
WHERE timestamp > NOW() - INTERVAL '24 hours'
GROUP BY 1
ORDER BY 1;
 
-- 2. Origin Load Over Time
SELECT 
  DATE_TRUNC('minute', timestamp) as minute,
  COUNT(*) as origin_requests,
  SUM(response_bytes) / 1e9 as origin_gb
FROM cdn_logs
WHERE cache_status = 'MISS' 
  AND shield_status = 'MISS'
  AND timestamp > NOW() - INTERVAL '1 hour'
GROUP BY 1
ORDER BY 1;
 
-- 3. Top Cache Miss URLs (Optimization Targets)
SELECT 
  url_path,
  COUNT(*) as miss_count,
  SUM(response_bytes) / 1e6 as total_mb,
  AVG(origin_response_time_ms) as avg_origin_ms
FROM cdn_logs
WHERE cache_status = 'MISS' 
  AND shield_status = 'MISS'
  AND timestamp > NOW() - INTERVAL '1 hour'
GROUP BY 1
ORDER BY miss_count DESC
LIMIT 50;
 
-- 4. Coalescing Effectiveness
SELECT 
  DATE_TRUNC('minute', timestamp) as minute,
  SUM(coalesced_requests) as total_coalesced,
  COUNT(*) as origin_fetches,
  ROUND(SUM(coalesced_requests)::numeric / NULLIF(COUNT(*), 0), 1) as coalesce_ratio
FROM shield_origin_logs
WHERE timestamp > NOW() - INTERVAL '1 hour'
GROUP BY 1
ORDER BY 1;
 
-- 5. Latency Percentiles
SELECT 
  PERCENTILE_CONT(0.50) WITHIN GROUP (ORDER BY edge_to_shield_ms) as p50_edge_shield,
  PERCENTILE_CONT(0.95) WITHIN GROUP (ORDER BY edge_to_shield_ms) as p95_edge_shield,
  PERCENTILE_CONT(0.99) WITHIN GROUP (ORDER BY edge_to_shield_ms) as p99_edge_shield,
  PERCENTILE_CONT(0.50) WITHIN GROUP (ORDER BY shield_to_origin_ms) as p50_shield_origin,
  PERCENTILE_CONT(0.95) WITHIN GROUP (ORDER BY shield_to_origin_ms) as p95_shield_origin
FROM cdn_logs
WHERE cache_status = 'MISS'
  AND timestamp > NOW() - INTERVAL '1 hour';

Alerting Strategy:

Set up alerts for abnormal shield behavior:

Condition	Threshold	Urgency	Action
Shield hit rate drops	<70% for 10 min	High	Investigate cache configuration
Origin requests spike	5x baseline	High	Check for invalidation storm or shield bypass
Shield→origin latency rises	>500ms P95	Medium	Check origin health, network issues
Error rate increases	>1% 5xx	High	Check shield and origin health
Coalescing ratio drops	<10 during peak	Medium	Check coalescing configuration

Add Cache Status Headers

Configure your CDN to add diagnostic headers like X-Cache (HIT/MISS), X-Cache-Layer (edge/shield), and X-Coalesced (count). These make debugging much easier and can be stripped before reaching end users.

Rollout Best Practices

Rolling out origin shield to production requires careful planning. A poor rollout can cause outages; a good rollout demonstrates value immediately.

Pre-Rollout Checklist:

Before Enabling Origin Shield

•Baseline current metrics: Record edge hit rate, origin load, latency distributions before shield
•Audit cacheability: Ensure content that should be cacheable has correct headers
•Verify cache key consistency: Confirm edge and shield compute same cache keys
•Test in staging: Deploy to a test environment first and validate behavior
•Configure monitoring: Set up dashboards and alerts before going live
•Prepare rollback plan: Know how to quickly disable shield if issues arise
•Notify stakeholders: Inform ops team, on-call rotation about the change

Phased Rollout Strategy:

Phase 1: Single Non-Critical Path (Day 1-3)
- Enable shield for static assets (/static/*, /images/*)
- Monitor hit rates and latency
- Validate coalescing works

Phase 2: Expand Coverage (Day 4-7)
- Add product pages, category pages
- Monitor origin load reduction
- Fine-tune TTLs based on observations

Phase 3: API Endpoints (Day 8-14)
- Enable shield for cacheable API endpoints
- Monitor API latency closely
- Adjust coalescing windows if needed

Phase 4: Full Traffic (Day 15+)
- Route all cacheable traffic through shield
- Optimize based on top cache miss analysis
- Document final configuration

Rollout Risk Mitigation
Risk	Mitigation	Rollback Trigger
Increased latency	Monitor P95 closely; start with non-critical paths	100ms P95 increase
Origin overload	Monitor origin requests; shield should REDUCE load	Origin requests increase
Cache consistency issues	Test invalidation works at all layers	Stale content served post-purge
Configuration errors	Test in staging; peer review configs	Any 5xx errors from shield
Cost overruns	Monitor CDN billing; set budget alerts	Costs exceed budget by 20%

Don't Launch on Friday

Enable origin shield early in the week when your team is fully staffed and alert. Issues may take time to surface, and you want engineers available to respond quickly.

Production Configuration Examples

Here are complete configuration examples for common CDN providers. These represent production-ready configurations adaptable to your needs.

cloudfront-origin-shield.tf

Terraform

# AWS CloudFront with Origin Shield - Terraform Configuration
 
resource "aws_cloudfront_distribution" "main" {
  enabled             = true
  is_ipv6_enabled     = true
  comment             = "Production CDN with Origin Shield"
  default_root_object = "index.html"
  price_class         = "PriceClass_All"
 
  origin {
    domain_name = aws_s3_bucket.content.bucket_regional_domain_name
    origin_id   = "S3Origin"
 
    s3_origin_config {
      origin_access_identity = aws_cloudfront_origin_access_identity.main.cloudfront_access_identity_path
    }
 
    # ORIGIN SHIELD CONFIGURATION
    origin_shield {
      enabled              = true
      origin_shield_region = "us-east-1"  # Same region as origin
    }
  }
 
  # Default cache behavior for static assets
  default_cache_behavior {
    allowed_methods  = ["GET", "HEAD", "OPTIONS"]
    cached_methods   = ["GET", "HEAD"]
    target_origin_id = "S3Origin"
 
    forwarded_values {
      query_string = false
      cookies {
        forward = "none"
      }
    }
 
    viewer_protocol_policy = "redirect-to-https"
    min_ttl                = 0
    default_ttl            = 86400    # 1 day
    max_ttl                = 31536000 # 1 year
    compress               = true
  }
 
  # Cache behavior for API endpoints
  ordered_cache_behavior {
    path_pattern     = "/api/*"
    allowed_methods  = ["GET", "HEAD", "OPTIONS"]
    cached_methods   = ["GET", "HEAD"]
    target_origin_id = "S3Origin"
 
    cache_policy_id = aws_cloudfront_cache_policy.api.id
 
    viewer_protocol_policy = "https-only"
    compress               = true
  }
}
 
resource "aws_cloudfront_cache_policy" "api" {
  name        = "api-cache-policy"
  comment     = "Cache policy for API endpoints"
  default_ttl = 60
  max_ttl     = 300
  min_ttl     = 0
 
  parameters_in_cache_key_and_forwarded_to_origin {
    cookies_config {
      cookie_behavior = "none"
    }
    headers_config {
      header_behavior = "whitelist"
      headers {
        items = ["Accept-Encoding"]
      }
    }
    query_strings_config {
      query_string_behavior = "whitelist"
      query_strings {
        items = ["id", "page", "limit"]
      }
    }
  }
}

Summary: Configuration Strategies

We've covered practical configuration strategies for production origin shield deployments. Let's consolidate the key takeaways:

Key Takeaways

•TTL coordination is essential: Shield TTL ≥ Edge TTL allows shield to serve expired edges without hitting origin
•Configure coalescing properly: Enable for cacheable content; tune window for your latency/efficiency balance
•Design cache keys consistently: Same key computation at edge and shield; minimize Vary headers
•Use appropriate headers: s-maxage, stale-while-revalidate, and Surrogate-Key enable fine-grained control
•Implement comprehensive monitoring: Track hit rates, origin load, latency by layer, and coalescing effectiveness
•Roll out in phases: Start with non-critical paths, validate metrics, then expand coverage

Module Complete:

You've now completed the comprehensive exploration of origin shield:

Page 1: Understood what origin shield is and why it exists
Page 2: Learned how it reduces origin load through multiple mechanisms
Page 3: Explored strategic placement decisions
Page 4: Analyzed the trade-offs inherent in shield implementations
Page 5: Mastered practical configuration strategies

You're now equipped to design, implement, and optimize origin shield configurations for production systems.

Module Complete: Origin Shield

Congratulations! You've mastered origin shield concepts and configuration. You understand how this intermediate caching layer protects origin infrastructure, the placement trade-offs, and practical configuration strategies. Apply these concepts to protect your origin servers, reduce bandwidth costs, and improve reliability during traffic spikes.