System designMedium~16 min

Interview question

Design a backend comments feature with mentions and optional threaded replies.

Design comments, mentions, and threaded discussion

Designs comments with permissions, threading, mentions, notifications, editing history, and moderation concerns.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

I would model comments with id, parent resource, author, body, created time, edited time, deleted state, and optional parent comment for replies. The API would check that the user can view and comment on the target resource. Mentions would be parsed and validated server-side, then converted into notification jobs for mentioned users who have access to the resource. Lists would use cursor pagination so long discussions remain stable.

Full answer below

Practice focus

Comment modeling, permissions, threading, mentions, notifications, editing, deletion, moderation, and pagination.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Comments are deceptively rich: they touch authorization, notifications, activity feeds, editing history, moderation, and performance.
I would model comments with id, parent resource, author, body, created time, edited time, deleted state, and optional parent comment for replies. The API would check that the user can view and comment on the target resource. Mentions would be parsed and validated server-side, then converted into notification jobs for mentioned users who have access to the resource. Lists would use cursor pagination so long discussions remain stable.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would define editing and deletion behavior. Editing can update the current body while preserving edit history when auditability matters. Deletion might soft-delete the comment and keep the thread shape, especially when replies exist. Mentions need access checks so a user is not notified about content they cannot see. I would also guard against abuse: length limits, rate limits, sanitization, and moderation or reporting flows if the product needs them.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production I would make comments part of the collaboration system rather than a table of text. Notifications, activity feed events, search indexing, audit logs, and permission changes all need a coherent story. If a user loses access to the parent resource, comments and mention notifications stop being visible to them. I would monitor comment creation rate, notification failures, moderation actions, and slow thread loading. The design can start simple, but it needs boundaries for privacy and abuse from day one.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Trusting the client to decide who can be mentioned.
No access check on the parent resource.
Hard deleting comments in a way that breaks threads unexpectedly.
No pagination for long discussions.

Red flagsCandidateRed flagsSignals that an interviewer may treat as risky.Open details

Details

Cannot describe mention notification flow.
Ignores edit history or deletion policy.
No abuse controls.
Leaks private resource activity through notifications.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

How would threaded replies be stored?
What happens when a mentioned user lacks access?
How would you moderate or report comments?

Continue practice

No explicit flow link is set yet, so these are nearby drills from the same path or taxonomy.

Consistency Trade-offsArchitecture scenario / Hard / ~12 minContinue Report Export BackendSystem design / Hard / ~20 minContinue Dashboard Read ModelSystem design / Hard / ~20 minContinue

Opening drill

System designMedium~16 min

Interview question

Design a backend comments feature with mentions and optional threaded replies.

Design comments, mentions, and threaded discussion

Designs comments with permissions, threading, mentions, notifications, editing history, and moderation concerns.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

Full answer below

Practice focus

Comment modeling, permissions, threading, mentions, notifications, editing, deletion, moderation, and pagination.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Comments are deceptively rich: they touch authorization, notifications, activity feeds, editing history, moderation, and performance.
I would model comments with id, parent resource, author, body, created time, edited time, deleted state, and optional parent comment for replies. The API would check that the user can view and comment on the target resource. Mentions would be parsed and validated server-side, then converted into notification jobs for mentioned users who have access to the resource. Lists would use cursor pagination so long discussions remain stable.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would define editing and deletion behavior. Editing can update the current body while preserving edit history when auditability matters. Deletion might soft-delete the comment and keep the thread shape, especially when replies exist. Mentions need access checks so a user is not notified about content they cannot see. I would also guard against abuse: length limits, rate limits, sanitization, and moderation or reporting flows if the product needs them.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production I would make comments part of the collaboration system rather than a table of text. Notifications, activity feed events, search indexing, audit logs, and permission changes all need a coherent story. If a user loses access to the parent resource, comments and mention notifications stop being visible to them. I would monitor comment creation rate, notification failures, moderation actions, and slow thread loading. The design can start simple, but it needs boundaries for privacy and abuse from day one.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Trusting the client to decide who can be mentioned.
No access check on the parent resource.
Hard deleting comments in a way that breaks threads unexpectedly.
No pagination for long discussions.

Red flagsCandidateRed flagsSignals that an interviewer may treat as risky.Open details

Details

Cannot describe mention notification flow.
Ignores edit history or deletion policy.
No abuse controls.
Leaks private resource activity through notifications.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

How would threaded replies be stored?
What happens when a mentioned user lacks access?
How would you moderate or report comments?

Continue practice

No explicit flow link is set yet, so these are nearby drills from the same path or taxonomy.

Consistency Trade-offsArchitecture scenario / Hard / ~12 minContinue Report Export BackendSystem design / Hard / ~20 minContinue Dashboard Read ModelSystem design / Hard / ~20 minContinue

Design a backend comments feature with mentions and optional threaded replies.

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue practice

Design a backend comments feature with mentions and optional threaded replies.

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue practice