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Twitter HLD: Fan-Out, the Celebrity Problem, and Merging a Timeline

A Twitter / X news feed system design (HLD): fan-out on write vs read, the celebrity problem, the push-pull hybrid, and assembling a timeline by merging K sorted streams at scale.

By fiveyearsdevJune 13, 202615 min read

"Design Twitter." The single most-asked system-design question, and for good reason: it looks trivial — post a tweet, show a timeline — and the moment you start drawing, it splits open into the deepest trade-off in feed design. You follow 800 accounts. You pull to refresh and a perfectly time-ordered feed appears in 100 milliseconds. Multiply that by 300 million users refreshing all day, and the naive answer — "query every tweet from everyone I follow, sort by time, take the top 50" — is a query that would run thousands of times a second against billions of rows. It cannot work. And then the i…

What’s inside

Let's start nowhere near a computer

Where the porch-paper trick runs

Step 1 — Functional requirements (sentences first)

Step 2 — Non-functional requirements

Step 3 — Nouns, and the two hidden ones

Step 4 — The two ways to build a timeline

Step 5 — The celebrity problem and the hybrid

Step 6 — Assembling the feed: merge K sorted streams

Step 7 — Trade-offs (each one keeping an NFR)

The complete implementation

Step 8 — Only now, the boxes

Step 9 — Scaling, one bottleneck at a time

Step 10 — When a piece fails: designing for failure

The interview corner

Where to go from here

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