Design Content Delivery Network - System Design | WinJob

Problem Statement and Global CDN Context

Design a multi-tenant CDN that terminates TLS at the edge, caches static and streaming objects, and shields customer origins from flash crowds.

Key points

Cloudflare/Akamai/Fastly interviews expect control-plane vs data-plane separation.
Success metrics: cache hit ratio, p95 TTFB on hits, purge propagation SLA, origin offload percentage.
Scale anchor: 50M RPS global, 300+ PoPs, 25 Tbps peak egress, 92% hit ratio baseline.

Deep dive

A CDN is not a load balancer with cache headers—it is a distributed storage and routing fabric. Clients reach the nearest healthy PoP via Anycast or GeoDNS; edge proxies consult a local index before fetching from shield or origin. Purge and certificate changes ride an async control bus; the hot path never blocks on Postgres.

Failure and degradation

If a PoP loses uplink, withdraw Anycast prefix after warming neighbors. If origin returns 503, serve stale-if-error within policy window. If purge bus lags, rely on versioned URLs for deploy safety while ops clears backlog.

Cost lens

Egress TB dominates COGS—improve hit ratio by 1% at 25 Tbps saves massive monthly spend. Shield RAM is cheaper than origin egress; spend memory to buy origin protection.

Security lens

Terminate TLS at edge with rotated keys, enforce tenant isolation on cache keys, rate-limit purge APIs, and sign URLs for hot objects. WAF optional module blocks obvious L7 abuse before cache lookup.

javaOne Dark Pro

1 public final class CacheKey {
2   private final String tenantId;
3   private final String assetPath;
4   private final String variant;
5   public String normalized() {
6     return tenantId + ":" + variant + ":" + assetPath.toLowerCase(Locale.ROOT);
7   }
8 }

pythonOne Dark Pro

1 from dataclasses import dataclass
2  
3 @dataclass(frozen=True)
4 class EdgeRequest:
5     cache_key: str
6     client_asn: int
7     colo: str

typescriptOne Dark Pro

1 interface SignedUrl {
2   assetId: string;
3   expiresAtEpochSec: number;
4   signature: string;
5 }
6  
7 export function isExpired(url: SignedUrl, nowSec: number): boolean {
8   return nowSec >= url.expiresAtEpochSec;
9 }

Why interviewers care

Content Delivery Network interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

Interview checkpoint

Name one failure story for Problem Statement and Global CDN Context that proves you understand real outages, not happy-path diagrams.

Key Highlights

•Cloudflare/Akamai/Fastly interviews expect control-plane vs data-plane separatio
•Success metrics: cache hit ratio, p95 TTFB on hits, purge propagation SLA, origi
•Scale anchor: 50M RPS global, 300+ PoPs, 25 Tbps peak egress, 92% hit ratio base

1	public final class CacheKey {
2	private final String tenantId;
3	private final String assetPath;
4	private final String variant;
5	public String normalized() {
6	return tenantId + ":" + variant + ":" + assetPath.toLowerCase(Locale.ROOT);
7	}
8	}

1	from dataclasses import dataclass
2
3	@dataclass(frozen=True)
4	class EdgeRequest:
5	cache_key: str
6	client_asn: int
7	colo: str

1	interface SignedUrl {
2	assetId: string;
3	expiresAtEpochSec: number;
4	signature: string;
5	}
6
7	export function isExpired(url: SignedUrl, nowSec: number): boolean {
8	return nowSec >= url.expiresAtEpochSec;
9	}