Design a comment service for documents
PremiumBackend systemMedium
- Comment anchoring
- Transactional outbox
- Async notifications
Quick answer
This backend system interview question asks you to design a comment service for documents. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Design a comment service for documents that adds threaded reviewer comments on selected text to an existing document app, like the review comments in online word processors. The hard part is keeping comments attached to text that keeps changing, and notifying people without slowing anyone down.
A thread is anchored to a text range and holds replies in time order, one level deep. Threads can be resolved and reopened. Comments can mention people with @name.
What must a comment service for documents do?
- Anchored threads: Select text and start a thread; the thread stays on the same words as others edit the document.
- Replies and edits: Reply to a thread, edit or delete your own comment; a deleted comment with replies shows as removed.
How well must it do it?
- Non-blocking: Posting a comment returns in under 200 ms at p99 and never waits for notifications or anchor updates.
- No lost notifications: Every saved comment produces its notifications at least once, and users never get the same one twice.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Comment anchoring
- Transactional outbox
- Async notifications
- Hot partition
- Cursor pagination
- Etag
How you’ll practise
- Requirements
- Estimate
- API and data
- High-level design
- Deep dives
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any backend system design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a backend system question. You design a comment service for documents and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team