System Design Breakdown: How Instagram-Scale Photo Feeds Handle 500M Daily Users

A high-level system design walkthrough outlines the architecture needed to support a photo-sharing platform at Instagram's scale, handling 500 million daily active users and 100 million photo uploads per day. The design centers on direct client-to-S3 uploads via pre-signed URLs, ensuring origin servers never process raw image bytes, while a resize worker generates three image variants stored on S3 and served exclusively through a CDN. Feed delivery relies on pre-computed Redis caches using sorted sets, with a hybrid fan-out strategy that pushes updates to regular users' feeds on write but defers celebrity content to read-time merging. A tiered storage model transitions photos from S3 Standard to Infrequent Access and eventually Glacier based on age, managing a projected five-year storage footprint of roughly 730 petabytes. The social graph is handled by Cassandra for its write-heavy, large-scale relationship demands, while PostgreSQL manages user data and Snowflake IDs uniquely identify photos across the distributed system.