Case study

Design Instagram Photo Feed

Upload, store, and deliver photos in a social feed to 500M users with global CDN delivery.

CDNfeedobject storage

Requirements

Functional: upload photo + caption; follow users; home feed and explore
Like, comment; view full-resolution and thumbnail variants
Non-functional: feed load p99 < 1s; upload < 5s for 5 MB image
Scale: 500M DAU; 100M photos/day; avg 200 KB stored per photo (multi-res)

Back-of-envelope Estimation

Metric	Calculation	Result
Upload QPS	100M / 86,400	~1.2K/sec avg; ~5K peak
Feed reads/day	500M × 3	1.5B/day → ~17K/sec
Storage/day	100M × 200 KB	20 TB/day new blobs
CDN egress	17K feed × 20 thumbs × 50 KB	~17 GB/sec peak (CDN absorbs)

API Design

Endpoint	Description
POST /v1/media/upload	Init multipart upload → pre-signed URLs per part
POST /v1/posts	{ mediaId, caption } → postId
GET /v1/feed?cursor=	Posts with CDN URLs for thumb + full
POST /v1/posts/{id}/like	Idempotent like

Data Model

Entity	Storage
posts	post_id, user_id, media_id, caption, ts — SQL/Cassandra
media	media_id, user_id, s3_keys{thumb,medium,full}, status — SQL
follows	follower_id, followee_id — graph store
feed_timeline	user_id → post_ids — Redis (same pattern as news feed)

High-level Design

Upload & serve

Upload: Client ──► pre-signed S3 ──► S3 raw
              └──► Post Service ──► async resize workers ──► S3 variants ──► CDN purge

Feed: Client ──► Feed Service ──► timeline IDs ──► CDN URLs (no blob through API)

Deep Dive: Image Processing Pipeline

Client uploads to S3 via pre-signed URL (direct, no API bandwidth)
S3 event triggers Lambda/worker: generate 150px thumb, 640px medium, 1080p full
Store WebP + JPEG fallback; mark media status = ready
Fan-out post to followers (reuse news feed hybrid model)

Bottlenecks at Scale

20 TB/day storage cost — lifecycle to cold tier after 90 days for old media
Resize worker backlog during peak — autoscale on queue depth
Celebrity feed fan-out — hybrid merge on read
CDN cache miss on new viral post — origin shield layer

Failure Modes & Monitoring

SLO	Target
Feed p99	< 1 sec
Upload success	> 99.5%
Media processing lag	< 60 sec p99

Alert on resize queue depth, CDN 5xx, incomplete uploads orphaned > 24h. Track storage cost per DAU.

System Design