CDN Senior
Explain the concept of an 'Origin Shield' (or Tiered Caching). Why would a high-traffic site implement this between their edge and their origin?
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A DNS routing system that directs each user to their geographically nearest edge node.
A standby origin server that serves all content whenever the primary origin goes offline.
A dedicated firewall tier that filters malicious requests before they reach the origin.
An intermediate cache that consolidates edge misses so the origin gets far fewer requests.
What is the difference between the 'Control Plane' and the 'Data Plane' in a CDN?
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The control plane serves end users while the data plane synchronizes caches between regions.
The control plane handles TLS termination while the data plane manages DNS resolution rules.
The control plane manages configuration and routing while the data plane delivers content.
The control plane caches static assets while the data plane forwards dynamic requests on.
How does a CDN perform load balancing and failover across its edge servers, and what role do health checks play?
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Health checks count active connections so new sessions are billed to the correct customer.
Health checks validate cached content freshness so stale objects get purged automatically.
Health checks measure user latency so requests always travel to the fastest origin server.
Health checks detect failing servers so traffic is rerouted to healthy nodes for failover.
How does a CDN handle large file delivery (like 5GB video files) differently than small images?
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Large files bypass the cache entirely and stream directly from the origin server.
Large files use byte-range requests and chunked caching rather than whole-object.
Large files are compressed at the edge while small images are always served raw.
Large files are split across multiple POPs while small files stay on a single POP.
How does a CDN provide DDoS mitigation, and what is the difference between absorbing a volumetric attack at the edge vs. at the origin?
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The CDN forwards the flood to the origin, which adds bandwidth to absorb and survive the attack itself.
Absorbing at the origin scales better since one tuned server filters packets faster than spread edges.
Edge absorption handles only application-layer floods while the origin must soak up all volumetric ones.
Edge PoPs absorb and disperse attack traffic across the global network, shielding the limited origin.
What is the 'Thundering Herd' problem in a CDN context, and how does 'Request Collapsing' (or Cache Lock) prevent it?
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On a cache miss many requests hit origin at once; collapsing forwards one and shares its response.
On expiry the CDN preemptively refetches every object, so collapsing avoids serving any stale data.
On startup cold edges flood origin, so collapsing delays them until the cache is fully warmed up.
On a cache hit edges duplicate the response, so collapsing drops the extra copies to save memory.
What is 'Cache Poisoning,' and how can misconfigured CDN cache keys lead to it?
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Leaking private cached responses to users because TLS certificates expired silently.
Overloading the cache with junk objects so legitimate content gets evicted early.
Encrypting cached responses so that users cannot decrypt them without a key token.
Serving a malicious cached response to users when unkeyed inputs alter the response.
Explain 'Tag-based Invalidation' (Surrogate Keys). Why is it more efficient than URL-based purging?
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Tags compress many URLs into a single object so less storage is needed and purging therefore frees more space
Objects are tagged with keys, so one tag purge clears all related URLs in a single operation
Tags let the origin skip revalidation entirely because tagged objects are always treated as permanently fresh
Each tag stores a shorter cache key than a full URL, which lets the edge index and look up objects faster
How does a CDN actually route a user to the 'closest' edge server? Explain the difference between Anycast and GeoDNS-based routing.
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Anycast advertises one IP from many sites so routing picks the nearest, while GeoDNS returns different IPs by client location
Anycast measures latency at the TCP layer while GeoDNS measures it at the HTTP layer to find the closest server
Both Anycast and GeoDNS rely solely on browser geolocation APIs to choose which edge server should handle a request
Anycast returns a different IP per region via DNS, while GeoDNS shares one IP that network routing directs to nearest node
How does the transition from HTTP/2 to HTTP/3 (QUIC) at the CDN layer impact performance for users on high-latency mobile networks?
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QUIC drops encryption to save round-trips, which speeds up handshakes for users switching between mobile cells
QUIC runs over TCP with larger windows, so it lowers latency only on stable wired connections, not mobile
QUIC removes TCP head-of-line blocking and supports connection migration, improving lossy mobile performance
QUIC multiplexes streams over one TCP socket, so a single packet loss still stalls every concurrent stream
What is TLS session resumption, and how does the CDN reuse TLS sessions at the edge to reduce handshake overhead?
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It lets the CDN cache HTTP responses so repeated requests skip contacting the origin server fully.
It compresses the certificate chain so each new handshake transfers fewer bytes to the client.
It rotates session keys on every request to strengthen forward secrecy during the handshake.
It lets clients resume prior sessions with cached keys, avoiding a full handshake's extra round trips.
What are the tradeoffs of moving logic to 'Edge Functions' (Compute-at-the-Edge) vs. keeping it in the backend?
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Edge lowers latency and offloads origin but limits runtime, resources, and access to central data.
Edge removes all latency and resource limits while giving full access to the central database always.
Edge eliminates the backend entirely so no origin server or central data store is ever needed again.
Edge increases latency but simplifies deployment by keeping all the state in one centralized region.
Why would a company adopt a 'Multi-CDN' strategy? What are the architectural complexities of doing so?
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Lowers cost by always routing to the cheapest provider, but needs unified billing across all the vendors
Improves resilience and performance via provider redundancy, but needs complex cross-CDN traffic steering
Removes the origin server entirely from the stack, but needs apps rewritten to run fully serverless
Ensures zero downtime by caching all content forever, but needs manual cache purging on each provider