Design a content delivery network (CDN)
PremiumDistributed infrastructureMedium
- CDN
- Cache keys
- Cache invalidation
Quick answer
This distributed infrastructure interview question asks you to design a content delivery network (CDN). Aim for a design that keeps working across many machines and failures, then explain partitioning, replication, coordination and trade-offs. Interviewers want to see how you reason about failures, consistency and scale.
The problem
Design a content delivery network (CDN) that serves websites, images and video for 20,000 sites from 150 points of presence (PoPs). Users reach a nearby PoP, which serves cached copies and fetches misses from the origin. The hard parts are keeping origins nearly idle, purging worldwide within seconds, and staying up when servers or PoPs fail.
Customers control caching with standard HTTP headers and can purge by URL or by tag at any time.
What must a content delivery network (CDN) do?
- Edge caching: Serve cacheable responses from the nearest healthy PoP, honouring Cache-Control, s-maxage and Vary.
- Purge: Customers purge by exact URL, by tag (surrogate key) or by prefix; purged content is never served after the purge completes.
How well must it do it?
- Purge speed: A purge reaches all 150 PoPs within 5 s at p99.
- Origin load: Origins see at most 2% of requests; target 1%.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- CDN
- Cache keys
- Cache invalidation
- Anycast
- Tiered caching
- Consistent hashing
How you’ll practise
- Requirements
- Estimate
- Data model and API
- Partitioning and replication
- Consistency and failures
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any distributed infrastructure design round.
- States the consistency and availability guarantees up front
- Explains partitioning and replication choices
- Describes how nodes agree and recover after failures
- Handles network partitions and slow nodes
- Plans for adding and removing capacity safely
- Names the trade-offs instead of hiding them
Questions people ask about this interview question
- Which design type is this question?
- It is a distributed infrastructure question. You design a content delivery network (CDN) and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with the guarantees the system must give and the failures it must survive. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team