# Design Trello: system design interview question

> Shared boards with concurrent card moves, real-time updates, and workspace access rules that apply to requests and live connections.

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


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design a shared task management service with boards, lists, and cards. Users need real-time updates as they collaborate. The service must handle concurrent card moves and enforce workspace access rules."

## What to clarify first

- Product behavior
- Scale
- Request volume
- Shared board activity
- Latency
- Concurrent moves and retries
- Workspace access
- Consistency and availability
- Durability and connection recovery
- Scope limits

## Scope of a strong answer

Core requirements:

- Create and read boards, lists, and cards within a workspace.
- Edit cards and move cards between lists on the same board.
- Show committed changes to connected users.
- Enforce workspace roles on requests and live connections.

Usually out of scope:

- Merge edits that users make while offline.
- Move cards between boards.
- Support comments, attachments, search, or external notifications.
- Support private boards within a workspace.
- Accept writes in multiple regions.

## What interviewers probe

### Concurrent card moves

Users move the same card or target the same position from different views. The server must preserve one valid card position.

### Connection recovery

A successful write can precede an update failure. Snapshot reads and live events can overlap or leave a gap. Hot boards can dominate update traffic.

### Workspace access changes

Workspace roles protect every board path. Revocation must reach existing connections and prevent later writes that use stale authority.

## Common mistakes

- Candidates use client array indexes as permanent card positions.
- Candidates treat real-time delivery as the source of truth.
- Candidates ignore the gap between a snapshot and its live subscription.
- Candidates size update delivery from average viewer counts and ignore simultaneous hot boards.
- Candidates check workspace access only at connection creation.
- Candidates assume a retry cannot repeat a successful move.

## The bar by level

- **Senior:** Defines move conflicts, stable positions, and safe retries. Closes snapshot gaps and detects missing final events. Handles revocation during writes and live connections.
- **Staff+:** Quantifies board contention, hot-board fanout, and recovery costs. Sizes delivery capacity beyond average viewer counts. Defines authority during failures and bounds stale access. Explains when board serialization needs replacement.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=collaborative-task-management


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SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.