Every social media app makes a news feed look effortless. You open the app, refresh the page, and within a fraction of a second hundreds of new posts appear in an order that feels surprisingly relevant. Behind that seemingly simple experience is one of the most demanding distributed systems engineers build. A News Feed system design has to absorb massive write spikes, serve millions of low-latency read requests, rank content in real time, and remain responsive even when a single post suddenly reaches millions of users. Those competing requirements make it a classic System Design interview problem because every architectural decision introduces tradeoffs between latency, scalability, consistency, storage, and cost. In this guide, you’ll build a News Feed system from the ground up, explore the major architectural decisions, and understand why large-scale platforms rely on hybrid fanout strategies, multi-layer caching, and carefully defined consistency contracts to keep feeds fast and reliable. News Feed Design Problem Framing And Baseline Architecture A news feed breaks first at the tails, not the averages. A single celebrity post can create a write storm that saturates the systems that copy the post into follower feeds, and the visible symptom is usually read p99 blowing past a target like 200 ms because caches miss and storage queues back up. That failure mode matters because it pins down the shape of the problem. One action produces many downstream writes, while the product requirement is that reads stay fast and stable even when writes spike. Baseline two-path architecture A baseline design splits cleanly into a write path that ingests and distributes new posts, and a read path that serves a ranked slice of a user’s feed with aggressive caching. The split exists because the system can spend more work per post asynchronously than it can spend per feed read synchronously. Use the diagram to trace the write boundary versus the read boundary across the components. In...