Design a news feed / home timeline for a social app
Reported in Instacart USA engineering loops. Product + system design hybrid common at Meta, Twitter/X, LinkedIn, and Reddit US offices.
Interview scenario
Context for Instacart candidates:
Scale: 300M DAU, 500M posts/day, p99 feed load under 200ms in the US.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Instacart: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Read path: hybrid fan-out—push to cache for normal users (write fan-out on post), pull for celebrities with millions of followers. Store per-user timeline in Redis sorted sets keyed by post_id timestamp.
Write path: post service → async fan-out workers → timeline cache + search index. Rank with ML scoring service (engagement, recency, affinity).
Storage: posts in Cassandra/Dynamo; media in S3 + CDN; graph in separate service. Use idempotent consumers and dead-letter queues.
Trade-offs: eventual consistency acceptable for feed; strong consistency for posting confirmation. Mention cold start, pagination cursors, and abuse/ranking guardrails.
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