Global Accelerator vs CloudFront: When to Use Each

AWS Global Accelerator and Amazon CloudFront are both global services that leverage the AWS global network to improve end-user experience. However, they operate at fundamentally different layers of the OSI model and solve different problems.

Global Accelerator operates at Layer 3/4 (Network and Transport layers). It uses anycast IP addresses to direct user traffic to the optimal AWS edge location, then routes it over the AWS global network to your application endpoint (e.g., Application Load Balancer, Network Load Balancer, EC2 instance, or Elastic IP). It does not cache content; it improves performance by reducing internet latency, jitter, and packet loss by keeping traffic on AWS's private backbone as much as possible.

CloudFront operates at Layer 7 (Application layer). It is a content delivery network (CDN) that caches HTTP(S) objects at edge locations. When a user requests content, CloudFront serves it from the nearest edge cache if available (cache hit), or fetches it from the origin (cache miss) and caches it for subsequent requests. CloudFront also supports dynamic content acceleration via its origin shield and TCP optimizations, but its primary value is caching.

Global Accelerator Mechanism: - Global Accelerator provides two static anycast IP addresses (or you can bring your own IP via BYOIP). These IPs are advertised from all AWS edge locations (points of presence) globally via BGP. - When a user sends traffic to one of these IPs, the internet's routing system directs the packet to the nearest AWS edge location (by BGP best path). - At the edge, Global Accelerator terminates the user's TCP/UDP connection and creates a new connection over the AWS global network to the endpoint in the chosen AWS region. This is called "connection termination at the edge." - The service monitors endpoint health via health checks (every 10 seconds by default) and can automatically route traffic to healthy endpoints, even across regions, providing failover within seconds. - Global Accelerator supports both TCP and UDP traffic, making it suitable for non-HTTP protocols like gaming, VoIP, or IoT.

CloudFront Mechanism: - CloudFront uses a global network of edge locations (over 600 points of presence). When a user requests content via a CloudFront URL (e.g., d123.cloudfront.net), DNS resolves to the nearest edge location. - The edge location checks its cache for the requested object. If present and not expired (based on Cache-Control headers, TTL defaults to 24 hours), it serves the object directly. - If not cached, the edge forwards the request to the origin (e.g., S3 bucket, ALB, EC2, or custom origin). The origin responds, and the edge caches the object for future requests. CloudFront supports HTTP/HTTPS only (Layer 7). - CloudFront can also use origin shield to reduce load on the origin by centralizing requests at a regional edge cache before hitting the origin.

Global Accelerator: - Accelerator: The top-level resource. You configure listeners (ports and protocols) and endpoint groups (mapping to regions). - Listener: Processes inbound connections. Supports TCP and UDP. Default port ranges: 1-65535. You can specify port overrides. - Endpoint Group: Associates a listener with a specific AWS region. You can set traffic dials (0-99%) to control the percentage of traffic sent to a region (useful for gradual migration). - Endpoint: Can be an Application Load Balancer, Network Load Balancer, EC2 instance, or Elastic IP. Supports cross-region endpoints. - Health Check: By default, health checks run every 10 seconds with a 5-second timeout and 3 consecutive failures to mark unhealthy. You can customize interval, timeout, and thresholds. - Client Affinity: Optional. Ensures that requests from the same client are routed to the same endpoint (uses source IP and port). - Flow Stickiness: By default, Global Accelerator maintains flow stickiness for TCP connections (same source IP/port goes to same endpoint) for the duration of the flow.

CloudFront: - Distribution: The top-level resource. You configure origins, behaviors, and cache settings. - Origin: The source of content (S3 bucket, ALB, EC2, or custom HTTP server). You can set origin path, connection attempts, and timeout. - Cache Behavior: Controls which URLs are cached, TTL defaults (Minimum TTL: 0 seconds, Default TTL: 86400 seconds (24 hours), Maximum TTL: 31536000 seconds (1 year)). You can override with Cache-Control headers. - Edge Location: Over 600 locations. Content is cached here. - Regional Edge Cache: Larger cache in each region to reduce origin load. Used when origin shield is enabled. - Origin Shield: An additional layer that aggregates requests from all edge locations in a region, sending a single request to the origin. Reduces origin load by up to 80%. - Price Class: Controls which edge locations are used (Price Class All: all locations; Price Class 200: most locations except the most expensive; Price Class 100: only locations in North America and Europe). - Signed URLs/Cookies: For private content. You can restrict access using AWS WAF, geo-restriction, or IP whitelisting. - TLS/SSL: Supports custom SSL certificates via AWS Certificate Manager (ACM) or IAM. Default uses CloudFront's domain name.

Global Accelerator (AWS CLI):

# Create accelerator
aws globalaccelerator create-accelerator --name MyAccelerator --ip-address-type IPV4

# List accelerators
aws globalaccelerator list-accelerators

# Describe accelerator
aws globalaccelerator describe-accelerator --accelerator-arn arn:aws:globalaccelerator::123456789012:accelerator/abc123

# Create listener
aws globalaccelerator create-listener --accelerator-arn <arn> --port-ranges From=80,To=80 --protocol TCP

# Create endpoint group
aws globalaccelerator create-endpoint-group --listener-arn <arn> --endpoint-group-region us-east-1 --endpoint-configurations EndpointId=arn:aws:elasticloadbalancing:us-east-1:123456789012:loadbalancer/app/my-alb/123

CloudFront (AWS CLI):

# Create distribution (simple S3 origin)
aws cloudfront create-distribution --origin-domain-name my-bucket.s3.amazonaws.com --default-root-object index.html

# List distributions
aws cloudfront list-distributions

# Get distribution config
aws cloudfront get-distribution --id E123ABC

# Invalidate cache
aws cloudfront create-invalidation --distribution-id E123ABC --paths /images/*

Use Global Accelerator when:

Use CloudFront when:

The exam often presents a scenario where both services could theoretically work, but one is clearly better. For example: - "An application serves both static and dynamic content. Which service should be used?" The answer is CloudFront for static, and Global Accelerator for dynamic, but the question might ask for a single service. In such cases, if the focus is on dynamic content acceleration, choose Global Accelerator. If the focus is on caching static content, choose CloudFront. Sometimes the answer is "use both" — CloudFront for static and Global Accelerator for dynamic, but this is rare. More often, the question specifies the need (e.g., "requires UDP support") which eliminates CloudFront.

Another trap: "Need to reduce latency for an HTTP API with no caching possible." Many candidates choose CloudFront because it's HTTP, but Global Accelerator is better because it improves network path without caching. CloudFront's caching is useless here, and its dynamic acceleration is less effective than Global Accelerator's network optimization.

A multiplayer game company uses Global Accelerator to route UDP traffic from players worldwide to game servers in us-east-1 and eu-west-1. The game requires real-time communication with low jitter. Previously, players experienced high latency and packet loss on the public internet, especially from regions like South America and Asia. With Global Accelerator, each player's UDP packets enter the AWS edge at the nearest location (e.g., Sao Paulo for South American players) and travel over the AWS backbone to the game server. The accelerator's health checks detect if the primary region's servers are overloaded and shift traffic to the secondary region within 30 seconds. The company also uses Global Accelerator's static IP addresses to whitelist traffic in corporate firewalls. Misconfiguration: Initially, they set the traffic dial for eu-west-1 to 100% during a test, accidentally routing all traffic to Europe, causing increased latency for US players. They corrected by setting dials based on player geography.

A large e-commerce site uses CloudFront for all user-facing content. Product images, CSS, JS, and HTML are cached at edge locations, reducing load on the origin ALB by 70%. Dynamic content (user cart, checkout) is not cached; instead, CloudFront forwards these requests to the origin ALB. The site uses Lambda@Edge to rewrite URLs for A/B testing and to add security headers. They also use AWS WAF integrated with CloudFront to block SQL injection and XSS attacks. The origin is an S3 bucket for static assets and an ALB for dynamic content. Misconfiguration: They set a very long TTL (1 year) for product images, and when a price changed, the old image was served for days until they issued a cache invalidation. They learned to use versioned URLs or shorter TTLs for mutable content.

A financial services company exposes a REST API that processes real-time transactions. The API cannot be cached due to strict consistency requirements. They use Global Accelerator to reduce latency for clients in Asia and Europe connecting to the primary region (us-east-1). Global Accelerator provides static IPs for firewall whitelisting and ensures fast failover to a secondary region (eu-west-1) in case of a regional outage. The API runs on an Application Load Balancer behind Global Accelerator. Performance: Latency from Tokyo to us-east-1 dropped from 150ms to 110ms (27% improvement) because traffic stayed on AWS backbone from the Tokyo edge location. They also use Global Accelerator's TCP termination at the edge to offload TLS termination, reducing CPU load on the ALB.