Question bank / Software Engineering, System Design, Advanced
How to answer: Design a real-time feed ranking and delivery system.
Whether you understand the central trade-off in feeds: do the work when posting or when reading.
What the interviewer is testing.
Fan-out, ranking, freshness, abuse. Whether you understand the central trade-off in feeds: do the work when posting or when reading.
An outline for this question.
- Clarify the scale and freshness: how quickly should a new post appear for friends?
- For most users, fan out on write: push new posts into each follower's prepared feed.
- For accounts with huge followings, fetch on read and merge, since writing to millions is too slow.
- Rank in two steps, a cheap candidate pass and a heavier model on the shortlist, and add checks for spam and abuse.
The mistake to avoid.
Choosing one approach for everyone. The point is that the right choice depends on follower count.
The general method: requirements, design, trade-offs.
The same shape works for most system design questions, whatever the topic.
- Pin down what the system must do, how many users, and what matters most (speed, cost, correctness).
- Sketch the main parts and how data moves between them, starting simple.
- Pick one or two parts to go deep on, and say what you gave up for each choice.
- Finish with how it fails and how you would know: monitoring, retries, limits.
Practice it, out loud.
Reading an outline is not the same as saying it. Answer this question in a mock interview, in your own words, then compare it with the outline above.
More to practice.
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This is a Replysis practice question. It is not a leaked or official question from any company, and the outline is a guide to structure, not a script. Use your own experience and facts.