- Channel/Interviewer: Gaurav Sen
- Duration: 00:11:01
- Original Video:
https://www.youtube.com/watch?v=8zX0rue2Hic
This document summarizes the key content of a system design mock interview. I highly recommend watching the full video if you can.
Teach Me: 5 Years Old | Beginner | Intermediate | Advanced | (reset auto redirect)
Learn Differently: Analogy | Storytelling | Cheatsheet | Mindmap | Flashcards | Practical Projects | Code Examples | Common Mistakes
Check Understanding: Generate Quiz | Interview Me | Refactor Challenge | Assessment Rubric | Next Steps
- Problem Prompt (One-liner): How to serve static content (e.g., HTML pages) quickly and efficiently to users across countries by using caching and a content delivery network (CDN).
- Primary Scope: Static content delivery, geo-distributed caching, and reducing latency. Out of scope: dynamic data writes and database design beyond using origin as the single source of truth.
- Non-Functional Priorities: Low latency via proximity caches; availability by avoiding single points of failure; cost efficiency through shared infrastructure; simplicity of invalidation via provider tools.
- Key Constraints & Numbers: Regions mentioned: India, USA, Netherlands/Europe; device-aware variants of pages; rough combinatorics example (device × country) kept “manageable” for caching. (No concrete QPS/latency targets stated.)
- High-Level Architecture (Text):
- Clients (mobile/desktop) request pages.
- Requests for static assets first hit a global/edge cache layer (CDN).
- Cache serves hits; cache misses go to origin server.
- Origin server accesses the database (system-of-record) for dynamic data; CDN holds copies only (not the source of truth).
- Caches are geographically distributed and possibly sharded by location.
- Provider UI/controls manage TTLs and invalidations.
- Top Trade-offs:
- Latency vs. Consistency: Edge speed vs. ensuring updated content; solved via TTL/invalidation.
- Build vs. Buy: Roll your own distributed caches vs. using a CDN provider with global footprint and compliance.
- Coverage vs. Cost: More PoPs/regions reduce latency but raise spend/complexity. (Implied by multiple regional caches.)
- Biggest Risks/Failure Modes:
- Single cache node as a bottleneck or SPOF → fix with distribution/sharding.
- Incorrect cache invalidation leading to stale content.
- Users far from origin still suffering high latency if caches aren’t close.
- 5-Min Review Flashcards:
- Q: Why a CDN? A: Reduce latency by serving static content from nearby caches.
- Q: What stays in origin? A: The “single source of truth” (dynamic data).
- Q: How avoid SPOF in caching? A: Use distributed/sharded caches.
- Q: What if content changes? A: Invalidate/version and push new assets; use TTLs.
- Q: Do devices get different pages? A: Yes—device/location customized variants can be cached.
- Q: Who provides the infra/UI? A: CDN providers (example named: Akamai).
- Q: Is S3 called out? A: S3 is mentioned as used for hosting files. [Personal note: For CDN features like global edge caching and advanced invalidations, pair object storage (e.g., S3) with a CDN (e.g., CloudFront or equivalent); storage alone isn’t a CDN.]
- Domain/Industry:
ecommerce - Product Pattern:
cdn, caching, object-storage - System Concerns:
low-latency, geo-replication, high-availability, gdpr(compliance noted in spirit via “country regulations”) - Infra/Tech (mentioned):
cdn, edge, s3- [Personal note: Consider a CDN layer in front of S3 for true edge delivery and cache controls.]
- Original Prompt (paraphrase): Serve static HTML pages to users in India, USA, and Netherlands with minimal latency, handling device/location variants, while treating origin as the source of truth.
- Use Cases:
- Deliver desktop/mobile variants of pages.
- Regionalized content (e.g., per-country/state variants).
- Out of Scope: Database schema design; dynamic write paths; full application logic. (Implied—focus is on static delivery.)
- APIs: Not stated in video.
Given in Video
- Functional:
- Cache and serve static pages near users; route dynamic data to origin.
- Support device/location-specific variants.
- Non-Functional:
- Latency: Lower latency via geo-near caches.
- Availability: Avoid single cache bottlenecks/points of failure.
- Operability: TTL + invalidation via provider UI.
- Compliance: Provider handles regional regulations.
Assumptions (conservative)
- Static assets include HTML/CSS/JS/images; dynamic data remains behind app servers.
- DNS-based geo-routing to nearest PoP. (Assumption—typical CDN behavior; not stated verbatim.)
Ask AI: Requirements & Constraints
Not stated in video—skipping numerical estimation.
- Clients: Desktop and mobile.
- Edge/CDN Layer: Geo-distributed caches; requests for static content terminate here when possible.
- Origin/App Server: Serves cache misses and all dynamic content; authoritative source of truth.
- Database: Stores dynamic/user/session data.
- Control Plane: TTLs, invalidations via provider UI (e.g., Akamai-like controls).
- Sharding by Region/Location: Traffic segmented to appropriate regional caches.
Ask AI: High-Level Architecture
- Role & Responsibilities: Serve static content from nearest cache; reduce round-trips to origin.
- Data Model: Cached objects are replicas, not authoritative; origin remains source of truth.
- Scaling & Partitioning: Horizontally distribute caches; shard by geography/users.
- Caching Strategy: TTL-based expiry; invalidation/versioning when pages change.
- [Personal note: Prefer cache headers (Cache-Control/ETag) with versioned asset URLs to make invalidations safer and predictable.]
- Failure Handling: Avoid SPOF by multiple cache nodes/regions; fall back to origin on misses/errors.
- Cost Considerations: CDN providers offer shared infra and UI; cheaper than building global caches in-house (implied).
| Topic | Option A | Option B | Video’s Leaning | Rationale (from video) |
|---|---|---|---|---|
| Edge Delivery | Build your own distributed caches | Use a CDN provider | CDN provider | Global presence, TTL UI, compliance, easier ops. |
| Content Updates | Long TTLs, risk of staleness | Short TTLs + explicit invalidation | TTL + invalidation | Balance freshness and latency. |
| Origin Location | Single US origin | Regional origins/PoPs | Edge caches near users | Keep origin authoritative; use edge for speed. |
- Replication/Consistency: CDN stores replicas; origin is the single source of truth.
- Latency: Reduced via geo-near caches; users in India/US/Netherlands get served locally/regionally.
- Degradation Path: On invalidation/expiry or cache miss, fetch from origin.
Ask AI: Reliability & Performance
Not stated in video.
Not stated in video.
Not stated in video.
Not stated in video.
- Push static content to the edge; keep origin authoritative for dynamic data.
- Avoid single cache bottlenecks; distribute and shard geographically.
- Use provider UI for TTLs/invalidation to simplify ops.
- Device/location-specific variants can still be cached effectively.
- Storage like S3 is useful for hosting, but real CDN capabilities are delivered by edge networks. [Personal note: Pair storage with a CDN layer for global edge caching and controls.]
- CDN (Content Delivery Network): A distributed cache layer that serves static content from locations near users.
- TTL (Time To Live): How long an item stays cached before expiring.
- Origin: The authoritative server hosting the canonical content/data.
- PoP (Point of Presence): A cache location serving a geographic area. (Concept implied via regional caches.)
- Practice explaining origin vs. edge responsibilities and how invalidation works.
- Sketch diagrams for multi-region cache sharding and failover paths.
- Source Video:
https://www.youtube.com/watch?v=8zX0rue2Hic - Channel: Gaurav Sen
- Note: This document is a summary of the linked mock interview.
I'm Ali Sol, a PHP Developer. Learn more:
- Website: alisol.ir
- LinkedIn: linkedin.com/in/alisolphp