System design interview question
"Design Disqus — an embeddable commenting widget. Any website drops in a script tag and gets a full comment thread on their page. You serve millions of different sites, from blogs with 10 readers to news sites with 10 million. Design it."
multi-tenancy and third-party embedding. This is not Reddit with a different logo. The distinguishing constraints are: you run inside someone else's page (cross-origin, security, performance budget), you have extreme tenant skew (10 pageviews vs 10M), and you must not slow down the host site. Candidates who design a comment system and ignore the embedding model have missed the question.
On Practick another engineer interviews you live for 60 minutes on a question from this bank. They have the full interviewer guide beside the call: the answers to your clarifying questions, the numbers to hand out, a ladder of follow-ups and a scoring rubric. You get the model solution once the session settles. It is free: interview someone for an hour and you have earned your own.
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