Origin Shield - Learning Module

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Shield Placement

Where You Place the Shield Matters

Origin shield provides powerful benefits—but these benefits aren't uniform across all configurations. Where you place your origin shield fundamentally affects latency, cache hit rates, operational complexity, and cost. The wrong placement can actually increase latency for certain users while providing suboptimal origin protection.

Consider the tradeoffs: A shield placed close to your origin minimizes origin-to-shield latency but maximizes edge-to-shield latency for distant edges. A shield placed centrally to edge servers optimizes for edge communication but may be far from your origin. Multiple shields add redundancy and reduce latency but increase complexity and cost.

This page explores the strategic decision of shield placement—the factors to consider, the tradeoffs to evaluate, and the patterns that work for different architectures.

What You Will Learn

By the end of this page, you will understand the factors that influence shield placement decisions, how to evaluate geographic tradeoffs, when to use single vs multi-shield architectures, and how major CDN providers approach placement. You'll be equipped to design or evaluate shield placement for real-world systems.

The Placement Decision Framework

Shield placement involves balancing multiple competing objectives. Before diving into specific strategies, let's establish the framework for making these decisions.

The Three Latency Components:

Total request latency for a cache miss involves three hops:

User → Edge: Fixed by edge network topology (not affected by shield placement)
Edge → Shield: Affected by shield location relative to edges
Shield → Origin: Affected by shield location relative to origin

For edge cache hits, shield and origin latency are irrelevant. But for cache misses, the total latency is:

Total Miss Latency = Edge→Shield RTT + Shield→Origin RTT + Origin Processing Time

Placement Strategy Trade-offs
Strategy	Edge→Shield	Shield→Origin	Best For
Near Origin	Higher (edges traversing distance)	Minimal	Origin protection priority, simple setup
Central to Edges	Balanced (equidistant)	Variable	Global edge distribution, latency balancing
Near Traffic Center	Lower for majority traffic	Depends on origin location	Traffic-weighted optimization
Multi-Shield	Lowest (regional shields)	Variable per shield	Large scale, strict latency SLAs

Key Decision Factors:

Origin Location(s): Where are your origin servers? Single region or multi-region?
Traffic Distribution: Where does your traffic come from? Concentrated or globally distributed?
Latency Requirements: What's your cache miss latency budget? How strict are SLAs?
High Availability Needs: Do you need shield redundancy for failover?
Cost Constraints: Shield in premium locations costs more; multi-shield multiplies costs
Cache Efficiency: Smaller shield count = better cache efficiency; more shields = cache fragmentation

No Universal Answer

There's no single 'best' shield placement. The optimal configuration depends on your specific traffic patterns, origin architecture, and business requirements. The framework above helps you make informed tradeoffs for your situation.

Single Shield Placement Strategies

Most organizations start with a single origin shield. This maximizes cache efficiency (all requests go through one cache) and simplifies operations. The question is: where to place it?

Strategy 1: Co-Located with Origin

Place the shield in the same region as your origin servers, either in the same data center or a nearby CDN POP.

Advantages:

Minimal shield-to-origin latency (often <5ms)
Simplest to reason about and debug
Shield can use private network to origin (lower cost, more secure)
Maximum origin protection (fastest response to origin)

Disadvantages:

Distant edge servers experience higher edge-to-shield latency
All edges route to single location regardless of user proximity
If origin region has issues, shield is also affected

Converting Mermaid diagram...

Strategy 2: Geographic Center of Edge Traffic

Place the shield at a location that minimizes average edge-to-shield latency, weighted by traffic volume.

Calculation Approach:

Optimal Location = argmin Σ (traffic_weight[edge] × latency[edge, shield_location])

Advantages:

Minimizes average cache miss latency across all users
Balances latency impact across edge servers
Often a good middle-ground solution

Disadvantages:

May increase shield-to-origin latency
'Average' optimization may not help latency-sensitive use cases
Requires traffic analysis to determine optimal location

Strategy 3: Traffic-Weighted Placement

Place the shield close to where most of your traffic originates, accepting higher latency for lower-traffic regions.

Example: If 80% of your traffic comes from North America, place the shield in North America even if your origin is in Europe. The 80% majority gets lower latency; the 20% minority experiences higher latency but represents fewer users.

Advantages:

Optimizes experience for the majority of users
Clear rationale for placement decision
Easy to adjust as traffic patterns shift

Disadvantages:

Minority regions experience degraded performance
Can create geo-based user experience disparities
May not align with business priorities (e.g., expanding Asia market)

Start Simple, Measure, Iterate

For most organizations, starting with a single shield co-located with origin is the right first step. This provides immediate origin protection with minimal complexity. Measure latency from different regions, then consider optimization only if you identify specific problems.

Multi-Shield Architectures

For global organizations with strict latency requirements or high availability needs, a single shield may not suffice. Multi-shield architectures deploy origin shields in multiple geographic regions.

Why Multiple Shields?

Latency Reduction: Regional shields serve nearby edges with lower latency
High Availability: Shield failure doesn't take down content delivery
Regulatory Compliance: Keep certain traffic within geographic boundaries
Origin Redundancy: Multiple shields can route to different origin regions

Converting Mermaid diagram...

Multi-Shield Considerations:

Cache Fragmentation: With multiple shields, each shield maintains its own cache. Content cached in the US shield isn't available to the EU shield. This reduces overall cache efficiency:

Single Shield: 1 cache miss per content item per TTL period
3 Shields: Up to 3 cache misses per content item per TTL period

For globally popular content, this triples origin load compared to single shield. For regionally popular content, it may have no impact (content only requested in one region anyway).

Consistency Challenges: With multiple shields:

Cache invalidation must propagate to all shields
Different shields may serve different content versions temporarily
Debugging becomes more complex (which shield served this request?)

Multi-Shield Benefits

•Lower latency for all regions
•Shield-level redundancy
•Regional isolation for compliance
•Multi-origin support
•Better peak load distribution

Multi-Shield Costs

•Higher infrastructure costs
•Cache fragmentation
•More complex invalidation
•Harder to debug and monitor
•Potential consistency issues

The Fragmentation Trade-off

Every additional shield you deploy trades origin request reduction for latency improvement. If your origin can handle the load, fewer shields is often better. Only add shields when latency SLAs demand it or when you need high availability at the shield tier.

Hierarchical Shield Topologies

For the most demanding deployments, some architectures implement hierarchical shield topologies—multiple tiers of caching between edges and origin.

Three-Tier Architecture:

User → Edge → Regional Shield → Global Shield → Origin

This creates a cache hierarchy:

Edge: Per-POP cache, smallest capacity, lowest latency
Regional Shield: Per-region cache, aggregates regional edges
Global Shield: Single global cache, aggregates all regional shields
Origin: Source of truth

Benefits of Hierarchical Caching:

Regional popularity captured: Content popular in Asia stays cached in Asia shield even if not globally popular
Global popularity optimized: Content popular everywhere is cached in global shield, reducing origin to single request
Latency tiers: Regional cache misses only traverse to regional shield (low latency); only regional shield misses go to global shield

Request Flow in Hierarchical Topology
Cache Hit At	Hops	Typical Latency	Origin Impact
Edge	User→Edge	1-10ms	None
Regional Shield	User→Edge→Regional	10-50ms	None
Global Shield	User→Edge→Regional→Global	50-150ms	None
Origin (cache miss)	Full chain	150-500ms	Single request

When to Use Hierarchical Topologies:

Hierarchical topologies are appropriate for:

Very large scale: Thousands of edge servers, millions of content items
Mixed popularity content: Some content is regionally popular, some globally
Strict latency SLAs: Need regional caching for latency-sensitive majority
Complex origin protection: Need both regional aggregation and global coalescing

Implementation Complexity:

Hierarchical topologies significantly increase complexity:

More layers to monitor and debug
Cache invalidation must propagate through all tiers
Capacity planning for multiple cache tiers
More configuration to manage and optimize

Most organizations don't need this level of sophistication. Major CDN providers implement hierarchical caching internally, abstracting complexity from customers.

CDN-Managed Hierarchy

When you enable 'origin shield' on major CDNs, you're often getting a simplified view of their internal hierarchical topology. Akamai, Cloudflare, and Fastly all implement multi-tier caching internally. Understanding the concept helps you reason about their behavior even if you don't manage the hierarchy directly.

Latency Analysis and Optimization

Optimal shield placement requires understanding and measuring latency across your architecture. Here's how to analyze and optimize.

Measuring Current State:

Before optimizing, establish baseline measurements:

Edge-to-Shield Latency: Measure RTT from each edge region to current shield location
Shield-to-Origin Latency: Measure RTT from shield to origin
Cache Miss Latency: End-to-end time for requests that miss all caches
Cache Hit Latency: Time for edge-served requests (your target)

latency-analysis.ts
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/**
 * Shield Placement Latency Optimizer
 * 
 * Models latency for different shield placement strategies.
 */
 
interface EdgeLocation {
  region: string;
  trafficPercent: number;  // % of total traffic from this edge
}
 
interface ShieldOption {
  location: string;
  latencyFromEdges: Map<string, number>;  // Edge region -> RTT in ms
  latencyToOrigin: number;                 // RTT to origin in ms
}
 
function calculateWeightedLatency(
  edges: EdgeLocation[],
  shield: ShieldOption,
  edgeCacheHitRate: number
): number {
  // Only cache misses traverse to shield, so we weight by miss rate
  const cacheMissRate = 1 - edgeCacheHitRate;
  
  let totalLatency = 0;
  for (const edge of edges) {
    const edgeToShield = shield.latencyFromEdges.get(edge.region) || 100;
    const contribution = edge.trafficPercent * cacheMissRate * (edgeToShield + shield.latencyToOrigin);
    totalLatency += contribution;
  }
  
  return totalLatency;
}
 
// Example: Compare shield placements
const edges: EdgeLocation[] = [
  { region: 'us-east', trafficPercent: 0.40 },
  { region: 'us-west', trafficPercent: 0.25 },
  { region: 'europe', trafficPercent: 0.20 },
  { region: 'asia', trafficPercent: 0.15 },
];
 
const shieldOptions: ShieldOption[] = [
  {
    location: 'US-East (near origin)',
    latencyFromEdges: new Map([
      ['us-east', 5], ['us-west', 70], ['europe', 80], ['asia', 200]
    ]),
    latencyToOrigin: 2,
  },
  {
    location: 'Central US',
    latencyFromEdges: new Map([
      ['us-east', 35], ['us-west', 35], ['europe', 100], ['asia', 180]
    ]),
    latencyToOrigin: 30,
  },
  {
    location: 'Europe',
    latencyFromEdges: new Map([
      ['us-east', 80], ['us-west', 140], ['europe', 10], ['asia', 150]
    ]),
    latencyToOrigin: 80,
  },
];
 
for (const shield of shieldOptions) {
  const weightedLatency = calculateWeightedLatency(edges, shield, 0.92);
  console.log(`${shield.location}: ${weightedLatency.toFixed(1)}ms weighted miss latency`);
}
 
// Output comparison helps select optimal placement

Optimization Techniques:

Traffic-Weighted Analysis: Weight latency by traffic volume from each region. 5% of traffic from Asia at 200ms matters less than 40% from US-East at 50ms.
P95/P99 Focus: Average latency can hide problems. Analyze percentiles to understand worst-case experience.
Dynamic Routing: Some CDNs support dynamic shield selection based on real-time conditions. Edge chooses nearest healthy shield.
Anycast Shields: Shield behind anycast IP routes edges to nearest instance automatically, adjusting as conditions change.
Time-of-Day Patterns: Traffic patterns shift by time zone. Consider whether static placement works for your traffic's daily variation.

The 92% Rule

With a 92% edge cache hit rate, only 8% of requests traverse to shield. This means shield latency optimizations affect only 8% of your traffic. Don't over-engineer shield placement at the expense of edge performance, which affects 100% of requests.

CDN Provider Placement Options

Major CDN providers offer different approaches to origin shield placement. Understanding these options helps you choose the right configuration for your needs.

AWS CloudFront:

CloudFront offers regional edge caches and optional Origin Shield:

Origin Shield available in 12 regions globally
You select a single region for Origin Shield
Regional edge caches (different from Origin Shield) provide first-tier aggregation
Pricing: Additional per-request charge for Origin Shield

Placement recommendation: Choose the Origin Shield region closest to your origin for maximum protection, or closest to your primary traffic for latency optimization.

CloudFront Origin Shield Features

•Available Regions: 12 global regions (US, EU, Asia, Australia, South America)
•Placement: Single shield region selected per distribution
•Pricing: $0.0075-0.016 per 10,000 requests (varies by region)
•Integration: Native integration with AWS origins, including S3 and ALB
•Best Practice: Place shield in same region as origin, or use regional caches if origin is highly distributed

Vendor-Specific Optimization

Each CDN provider has unique approaches to shield topology. Work with your provider's solutions engineers to optimize placement for your specific use case. They have visibility into network topology and traffic patterns that external analysis can't replicate.

Failover and High Availability for Shields

Origin shield is a critical piece of infrastructure—if it fails, your origin may be exposed to unprotected CDN traffic. Planning for shield failover is essential for production systems.

Single Shield Risks:

With a single origin shield:

Shield POP outage = all traffic goes directly to origin
Network issues to shield = elevated latency or timeouts
Shield capacity saturation = degraded performance

Failover Strategies:

Shield High Availability Patterns

•Primary/Secondary Shield: Configure secondary shield that activates on primary failure. Edges route to secondary automatically.
•Active-Active Multi-Shield: Multiple shields share traffic. Failure of one redistributes load to others.
•Anycast Shield Routing: Shield behind anycast IP routes to nearest healthy instance. Automatic geographic failover.
•Fallback to Direct Origin: If all shields fail, edges route directly to origin. Requires origin to handle potential load spike.
•Shield Health Checks: Continuous health monitoring with automatic routing updates on failure detection.

Implementing Shield Failover:

Shield Failover Configuration:

1. Health Check:
   - Frequency: Every 5 seconds
   - Endpoint: /health (on shield)
   - Failure threshold: 3 consecutive failures
   - Success threshold: 2 consecutive successes

2. Failover Priority:
   - Primary:   us-east-shield-1.cdn.example.com
   - Secondary: us-west-shield-2.cdn.example.com
   - Fallback:  origin.example.com (direct)

3. Failback:
   - After primary healthy for 60 seconds
   - Gradual traffic shift (10% → 50% → 100%)

Testing Failover:

Regularly test your failover configurations:

Simulate shield failure (disable health check endpoint)
Verify edge routing shifts to secondary
Monitor origin load during failover
Measure failover and failback timing

Origin Must Handle Direct Traffic

Even with shield failover, your origin must be sized to handle direct edge traffic in worst-case scenarios. If all shields fail or are bypassed, origin receives unaggregated CDN traffic. Design origin capacity with this failure mode in mind.

Summary: Shield Placement Strategy

We've covered the strategic considerations for origin shield placement. Let's consolidate the key insights:

Key Takeaways

•Placement is a trade-off: Balance edge-to-shield latency against shield-to-origin latency based on your priorities
•Start with single shield near origin: Maximum origin protection, simplest configuration
•Multi-shield trades efficiency for latency: More shields reduce regional latency but fragment cache and increase complexity
•Hierarchical topologies serve large scale: Regional shields feeding global shield optimizes both regional and global popularity patterns
•Weight by traffic, focus on cache misses: Shield placement only affects cache miss latency (typically 5-10% of requests)
•Plan for failover: Shield is critical infrastructure; implement health checks and secondary routing

What's Next:

With placement strategy established, we'll next explore the trade-offs inherent in origin shield configurations. Every benefit has a cost—latency additions, cache fragmentation, operational complexity—and understanding these trade-offs is essential for making informed decisions.

Page Complete

You now understand the strategic considerations for shield placement. From single-shield strategies to multi-tier hierarchies, you can evaluate placement options and design configurations appropriate for your scale and requirements. Next, we'll examine the trade-offs involved in origin shield configurations.

Shield Placement

Where You Place the Shield Matters

This page explores the strategic decision of shield placement—the factors to consider, the tradeoffs to evaluate, and the patterns that work for different architectures.

What You Will Learn

The Placement Decision Framework

Shield placement involves balancing multiple competing objectives. Before diving into specific strategies, let's establish the framework for making these decisions.

The Three Latency Components:

Total request latency for a cache miss involves three hops:

User → Edge: Fixed by edge network topology (not affected by shield placement)
Edge → Shield: Affected by shield location relative to edges
Shield → Origin: Affected by shield location relative to origin

For edge cache hits, shield and origin latency are irrelevant. But for cache misses, the total latency is:

Total Miss Latency = Edge→Shield RTT + Shield→Origin RTT + Origin Processing Time

Placement Strategy Trade-offs
Strategy	Edge→Shield	Shield→Origin	Best For
Near Origin	Higher (edges traversing distance)	Minimal	Origin protection priority, simple setup
Central to Edges	Balanced (equidistant)	Variable	Global edge distribution, latency balancing
Near Traffic Center	Lower for majority traffic	Depends on origin location	Traffic-weighted optimization
Multi-Shield	Lowest (regional shields)	Variable per shield	Large scale, strict latency SLAs

Key Decision Factors:

Origin Location(s): Where are your origin servers? Single region or multi-region?
Traffic Distribution: Where does your traffic come from? Concentrated or globally distributed?
Latency Requirements: What's your cache miss latency budget? How strict are SLAs?
High Availability Needs: Do you need shield redundancy for failover?
Cost Constraints: Shield in premium locations costs more; multi-shield multiplies costs
Cache Efficiency: Smaller shield count = better cache efficiency; more shields = cache fragmentation

No Universal Answer

Single Shield Placement Strategies

Most organizations start with a single origin shield. This maximizes cache efficiency (all requests go through one cache) and simplifies operations. The question is: where to place it?

Strategy 1: Co-Located with Origin

Place the shield in the same region as your origin servers, either in the same data center or a nearby CDN POP.

Advantages:

Minimal shield-to-origin latency (often <5ms)
Simplest to reason about and debug
Shield can use private network to origin (lower cost, more secure)
Maximum origin protection (fastest response to origin)

Disadvantages:

Distant edge servers experience higher edge-to-shield latency
All edges route to single location regardless of user proximity
If origin region has issues, shield is also affected

Converting Mermaid diagram...

Strategy 2: Geographic Center of Edge Traffic

Place the shield at a location that minimizes average edge-to-shield latency, weighted by traffic volume.

Calculation Approach:

Optimal Location = argmin Σ (traffic_weight[edge] × latency[edge, shield_location])

Advantages:

Minimizes average cache miss latency across all users
Balances latency impact across edge servers
Often a good middle-ground solution

Disadvantages:

May increase shield-to-origin latency
'Average' optimization may not help latency-sensitive use cases
Requires traffic analysis to determine optimal location

Strategy 3: Traffic-Weighted Placement

Place the shield close to where most of your traffic originates, accepting higher latency for lower-traffic regions.

Advantages:

Optimizes experience for the majority of users
Clear rationale for placement decision
Easy to adjust as traffic patterns shift

Disadvantages:

Minority regions experience degraded performance
Can create geo-based user experience disparities
May not align with business priorities (e.g., expanding Asia market)

Start Simple, Measure, Iterate

Multi-Shield Architectures

For global organizations with strict latency requirements or high availability needs, a single shield may not suffice. Multi-shield architectures deploy origin shields in multiple geographic regions.

Why Multiple Shields?

Latency Reduction: Regional shields serve nearby edges with lower latency
High Availability: Shield failure doesn't take down content delivery
Regulatory Compliance: Keep certain traffic within geographic boundaries
Origin Redundancy: Multiple shields can route to different origin regions

Converting Mermaid diagram...

Multi-Shield Considerations:

Cache Fragmentation: With multiple shields, each shield maintains its own cache. Content cached in the US shield isn't available to the EU shield. This reduces overall cache efficiency:

Single Shield: 1 cache miss per content item per TTL period
3 Shields: Up to 3 cache misses per content item per TTL period

For globally popular content, this triples origin load compared to single shield. For regionally popular content, it may have no impact (content only requested in one region anyway).

Consistency Challenges: With multiple shields:

Cache invalidation must propagate to all shields
Different shields may serve different content versions temporarily
Debugging becomes more complex (which shield served this request?)

Multi-Shield Benefits

•Lower latency for all regions
•Shield-level redundancy
•Regional isolation for compliance
•Multi-origin support
•Better peak load distribution

Multi-Shield Costs

•Higher infrastructure costs
•Cache fragmentation
•More complex invalidation
•Harder to debug and monitor
•Potential consistency issues

The Fragmentation Trade-off

Hierarchical Shield Topologies

For the most demanding deployments, some architectures implement hierarchical shield topologies—multiple tiers of caching between edges and origin.

Three-Tier Architecture:

User → Edge → Regional Shield → Global Shield → Origin

This creates a cache hierarchy:

Edge: Per-POP cache, smallest capacity, lowest latency
Regional Shield: Per-region cache, aggregates regional edges
Global Shield: Single global cache, aggregates all regional shields
Origin: Source of truth

Benefits of Hierarchical Caching:

Regional popularity captured: Content popular in Asia stays cached in Asia shield even if not globally popular
Global popularity optimized: Content popular everywhere is cached in global shield, reducing origin to single request
Latency tiers: Regional cache misses only traverse to regional shield (low latency); only regional shield misses go to global shield

Request Flow in Hierarchical Topology
Cache Hit At	Hops	Typical Latency	Origin Impact
Edge	User→Edge	1-10ms	None
Regional Shield	User→Edge→Regional	10-50ms	None
Global Shield	User→Edge→Regional→Global	50-150ms	None
Origin (cache miss)	Full chain	150-500ms	Single request

When to Use Hierarchical Topologies:

Hierarchical topologies are appropriate for:

Very large scale: Thousands of edge servers, millions of content items
Mixed popularity content: Some content is regionally popular, some globally
Strict latency SLAs: Need regional caching for latency-sensitive majority
Complex origin protection: Need both regional aggregation and global coalescing

Implementation Complexity:

Hierarchical topologies significantly increase complexity:

More layers to monitor and debug
Cache invalidation must propagate through all tiers
Capacity planning for multiple cache tiers
More configuration to manage and optimize

Most organizations don't need this level of sophistication. Major CDN providers implement hierarchical caching internally, abstracting complexity from customers.

CDN-Managed Hierarchy

Latency Analysis and Optimization

Optimal shield placement requires understanding and measuring latency across your architecture. Here's how to analyze and optimize.

Measuring Current State:

Before optimizing, establish baseline measurements:

Edge-to-Shield Latency: Measure RTT from each edge region to current shield location
Shield-to-Origin Latency: Measure RTT from shield to origin
Cache Miss Latency: End-to-end time for requests that miss all caches
Cache Hit Latency: Time for edge-served requests (your target)

latency-analysis.ts
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/**
 * Shield Placement Latency Optimizer
 * 
 * Models latency for different shield placement strategies.
 */
 
interface EdgeLocation {
  region: string;
  trafficPercent: number;  // % of total traffic from this edge
}
 
interface ShieldOption {
  location: string;
  latencyFromEdges: Map<string, number>;  // Edge region -> RTT in ms
  latencyToOrigin: number;                 // RTT to origin in ms
}
 
function calculateWeightedLatency(
  edges: EdgeLocation[],
  shield: ShieldOption,
  edgeCacheHitRate: number
): number {
  // Only cache misses traverse to shield, so we weight by miss rate
  const cacheMissRate = 1 - edgeCacheHitRate;
  
  let totalLatency = 0;
  for (const edge of edges) {
    const edgeToShield = shield.latencyFromEdges.get(edge.region) || 100;
    const contribution = edge.trafficPercent * cacheMissRate * (edgeToShield + shield.latencyToOrigin);
    totalLatency += contribution;
  }
  
  return totalLatency;
}
 
// Example: Compare shield placements
const edges: EdgeLocation[] = [
  { region: 'us-east', trafficPercent: 0.40 },
  { region: 'us-west', trafficPercent: 0.25 },
  { region: 'europe', trafficPercent: 0.20 },
  { region: 'asia', trafficPercent: 0.15 },
];
 
const shieldOptions: ShieldOption[] = [
  {
    location: 'US-East (near origin)',
    latencyFromEdges: new Map([
      ['us-east', 5], ['us-west', 70], ['europe', 80], ['asia', 200]
    ]),
    latencyToOrigin: 2,
  },
  {
    location: 'Central US',
    latencyFromEdges: new Map([
      ['us-east', 35], ['us-west', 35], ['europe', 100], ['asia', 180]
    ]),
    latencyToOrigin: 30,
  },
  {
    location: 'Europe',
    latencyFromEdges: new Map([
      ['us-east', 80], ['us-west', 140], ['europe', 10], ['asia', 150]
    ]),
    latencyToOrigin: 80,
  },
];
 
for (const shield of shieldOptions) {
  const weightedLatency = calculateWeightedLatency(edges, shield, 0.92);
  console.log(`${shield.location}: ${weightedLatency.toFixed(1)}ms weighted miss latency`);
}
 
// Output comparison helps select optimal placement

Optimization Techniques:

Traffic-Weighted Analysis: Weight latency by traffic volume from each region. 5% of traffic from Asia at 200ms matters less than 40% from US-East at 50ms.
P95/P99 Focus: Average latency can hide problems. Analyze percentiles to understand worst-case experience.
Dynamic Routing: Some CDNs support dynamic shield selection based on real-time conditions. Edge chooses nearest healthy shield.
Anycast Shields: Shield behind anycast IP routes edges to nearest instance automatically, adjusting as conditions change.
Time-of-Day Patterns: Traffic patterns shift by time zone. Consider whether static placement works for your traffic's daily variation.

The 92% Rule

CDN Provider Placement Options

Major CDN providers offer different approaches to origin shield placement. Understanding these options helps you choose the right configuration for your needs.

AWS CloudFront:

CloudFront offers regional edge caches and optional Origin Shield:

Origin Shield available in 12 regions globally
You select a single region for Origin Shield
Regional edge caches (different from Origin Shield) provide first-tier aggregation
Pricing: Additional per-request charge for Origin Shield

Placement recommendation: Choose the Origin Shield region closest to your origin for maximum protection, or closest to your primary traffic for latency optimization.

CloudFront Origin Shield Features

•Available Regions: 12 global regions (US, EU, Asia, Australia, South America)
•Placement: Single shield region selected per distribution
•Pricing: $0.0075-0.016 per 10,000 requests (varies by region)
•Integration: Native integration with AWS origins, including S3 and ALB
•Best Practice: Place shield in same region as origin, or use regional caches if origin is highly distributed

Vendor-Specific Optimization

Failover and High Availability for Shields

Origin shield is a critical piece of infrastructure—if it fails, your origin may be exposed to unprotected CDN traffic. Planning for shield failover is essential for production systems.

Single Shield Risks:

With a single origin shield:

Shield POP outage = all traffic goes directly to origin
Network issues to shield = elevated latency or timeouts
Shield capacity saturation = degraded performance

Failover Strategies:

Shield High Availability Patterns

•Primary/Secondary Shield: Configure secondary shield that activates on primary failure. Edges route to secondary automatically.
•Active-Active Multi-Shield: Multiple shields share traffic. Failure of one redistributes load to others.
•Anycast Shield Routing: Shield behind anycast IP routes to nearest healthy instance. Automatic geographic failover.
•Fallback to Direct Origin: If all shields fail, edges route directly to origin. Requires origin to handle potential load spike.
•Shield Health Checks: Continuous health monitoring with automatic routing updates on failure detection.

Implementing Shield Failover:

Shield Failover Configuration:

1. Health Check:
   - Frequency: Every 5 seconds
   - Endpoint: /health (on shield)
   - Failure threshold: 3 consecutive failures
   - Success threshold: 2 consecutive successes

2. Failover Priority:
   - Primary:   us-east-shield-1.cdn.example.com
   - Secondary: us-west-shield-2.cdn.example.com
   - Fallback:  origin.example.com (direct)

3. Failback:
   - After primary healthy for 60 seconds
   - Gradual traffic shift (10% → 50% → 100%)

Testing Failover:

Regularly test your failover configurations:

Simulate shield failure (disable health check endpoint)
Verify edge routing shifts to secondary
Monitor origin load during failover
Measure failover and failback timing

Origin Must Handle Direct Traffic

Summary: Shield Placement Strategy

We've covered the strategic considerations for origin shield placement. Let's consolidate the key insights:

Key Takeaways

•Placement is a trade-off: Balance edge-to-shield latency against shield-to-origin latency based on your priorities
•Start with single shield near origin: Maximum origin protection, simplest configuration
•Multi-shield trades efficiency for latency: More shields reduce regional latency but fragment cache and increase complexity
•Hierarchical topologies serve large scale: Regional shields feeding global shield optimizes both regional and global popularity patterns
•Weight by traffic, focus on cache misses: Shield placement only affects cache miss latency (typically 5-10% of requests)
•Plan for failover: Shield is critical infrastructure; implement health checks and secondary routing

What's Next:

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