# Design Reddit: system design interview question

> A discussion platform with changing vote totals, community post ranks, popular communities, and large comment trees.

Source: https://sysround.com/questions/design-reddit
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design a community discussion platform like Reddit. Users can create communities, publish posts, vote, and reply to comments. Keep community lists responsive under heavy read and write traffic."

## What to clarify first

- Users and content
- Scale
- Traffic and request mix
- Uneven demand and thread size
- Vote rules
- Post and comment order
- Latency and freshness
- Consistency and availability
- Durability and record sizes
- Scope limits

## Scope of a strong answer

Core requirements:

- Create public communities and publish text or link posts.
- Add comments and replies to posts.
- Set or remove one vote per user on each post or comment.
- Read ordered community post lists and comment threads.

Usually out of scope:

- Personalized home feeds
- Full-text search
- Private communities
- Media storage
- Full moderation tools

## What interviewers probe

### A user's vote and the displayed total

A vote replaces an earlier choice. Concurrent devices, retries, and delayed totals must not change the meaning of a successful vote read.

### Community post order under load

Votes change post order while readers request pages. Popular communities concentrate reads and score updates on a small set of data.

### Large comment threads

A thread can contain a million comments and deep reply chains. Each comment's changing score affects only its parent's direct-reply order.

## Common mistakes

- The design treats each vote request as an increment and loses the user's current choice.
- The design serves current vote reads from a stale cache or asynchronous replica.
- The read path scans and sorts all posts in a community.
- The API returns an entire comment tree.
- The design updates post ranks but never updates score-ordered replies.
- The design serves old ranks without degraded status or freshness failure counts.

## The bar by level

- **Senior:** Defines an authoritative vote read path. Connects durable vote changes to counts, post ranks, and each parent's reply order. Handles retries, duplicate events, page shifts, and shared caches.
- **Staff+:** Derives capacity limits for popular targets and parent indexes. Sets an end-to-end freshness budget, including outages that consume the error budget. Defines quorum failures, replay boundaries, and safe repair without stale vote reads.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=reddit


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