Design scalable media storage and delivery
Overview
This question evaluates an engineer's ability to design globally distributed, highly scalable media storage and delivery systems, covering competencies in object storage and metadata modeling, CDN integration, replication and consistency strategies, security and access control, background processing, observability, quotas, and cost/performance trade-offs. It is commonly asked in system design interviews to assess reasoning about durability, availability, latency and cost trade-offs while requiring both high-level architectural thinking and practical implementation details; category: System Design; level of abstraction: combines conceptual architecture with practical application considerations.
System Design: Global UGC Photos and Short Videos Store/Delivery
Context
Design a globally distributed system to store and deliver user-generated photos and short videos. The system must support billions of users, high write/read throughput, multi-region presence, and strong reliability/security guarantees. Assume mobile and web clients; traffic is worldwide with regional peaks and diurnal patterns.
Functional Requirements
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Uploads
- Single-part and multipart uploads (for large videos, unstable networks).
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Reads/Downloads
- Range requests; CDN acceleration; thumbnails and transcoded variants.
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Delete
- Soft delete with retention; hard delete; privacy-compliant erase.
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List
- List by user/album with pagination and sort (time, size).
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Derived media
- Thumbnail generation for images; transcode videos to multiple bitrates/resolutions.
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Search
- Simple search by owner and metadata (filename, tags, mime, created_at).
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Signed access URLs
- Time-bounded, scoped URLs for upload and download.
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Quotas
- Per-user size/object-count quotas; enforcement on upload.
Non-Functional Requirements (address explicitly)
- Durability targets (e.g., 11 9s), availability/SLOs.
- Multi-region data layout and replication strategy.
- Consistency model (metadata vs. object data) and read-after-write behavior.
- CDN integration and cache invalidation.
- Security: encryption in transit/at rest, key management.
- Access control and authorization.
- Abuse prevention and rate limiting.
- Cost model and lifecycle policies (tiering, retention, cold storage, deletion).
- Background processing semantics (idempotency/exactly-once behavior).
- Metadata schema and indexing strategy.
- Observability (metrics, tracing, audit logs).
- Backfill/migrations approach.
- Disaster recovery and regional failover.
- Performance targets (p95/p99) and bottlenecks.
- Testing and rollout plan.
Deliverables
- API sketch (major endpoints for uploads, reads, listing, search, delete, signing).
- Component diagram (textual/ASCII description acceptable).
- Storage choices (object store vs. block/file; metadata store).
- Capacity planning with rough numbers and assumptions.
Solution
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