Photo Sharing Feed

This is an illustrative target, not a claim about Instagram production: 30 million daily active users, 15 million posts/day, 1.3 billion feed loads/day and 250 million likes/day. Those inputs imply about 174 posts/s, 15,046 feed loads/s and 2,894 likes/s on average. Example peaks are 1,000 posts/s, 50,000 feed loads/s and 8,000 likes/s. At 2.4 MB per post, direct uploads peak near 2.4 GB/s; clients therefore upload to object storage rather than proxying bytes through application servers.

Correct write path

The media service reserves an owner-scoped key and issues a short-lived SigV4 presigned PUT constrained by key and checksum. Finalization performs HEAD-style metadata validation for owner, size, content type and checksum. A scan/thumbnail consumer decodes with hard resource limits and writes deterministic derivatives. A post can reference only MEDIA_READY objects.

PostgreSQL holds canonical relationships, post metadata and an outbox. A domain mutation and its outbox row commit in one transaction. The relay may publish more than once, so every event has a stable ID and every projector is idempotent. Cassandra is a query model, not a relational database queried arbitrarily: posts_by_id supports feed hydration; posts_by_user supports profiles, backfill and celebrity pull-on-read. Independent consumer groups can make a feed ID visible before its Cassandra projection; the read path suppresses/retries a missing projection until the projector catches up. Horizontal scaling still requires deliberate partition sizing, compaction, repair and capacity work.

Feed and likes