Design a Kanban board like Trello
PremiumLow-level designMedium
- Fractional indexing
- Composite pattern
- Command pattern
Quick answer
This low-level design interview question asks you to design a Kanban board like Trello. Aim for a design that is modelled as clear classes, interfaces and relationships that can handle new requirements without a rewrite. Interviewers want to see how you name responsibilities, keep classes small and reason about change.
The problem
Kanban board like Trello, low level design: build the object model for boards with ordered lists and cards that users drag, label, assign and comment on. The hard parts are reordering a card without renumbering the list, two users moving one card at once, an undoable activity log and board roles.
A workspace has boards; a board has ordered lists; a list has ordered cards. Users drag cards between lists and reorder them, add labels, members, due dates, checklists, attachments and comments, archive cards and lists, and see an activity log. Boards are private, shared with a team or public, with roles for members.
What must a Kanban board like Trello do?
- Structure: Workspace has boards; a board has up to 50 lists; a list has up to 500 cards. Lists and cards keep a user-defined order.
- Move and reorder: Move a card within its list or to another list at any position; reorder lists. A move must not renumber every other card.
How well must it do it?
- Cheap reorder: Moving a card must update one card's position, not every card in the list, in the common case.
- Consistent: Two users moving the same card at the same time must leave it in exactly one list at one position.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Fractional indexing
- Composite pattern
- Command pattern
- RBAC
- Observer pattern
- Activity log
How you’ll practise
- Requirements
- Core objects
- Class relationships
- Key flows
- Extensibility
- 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 low-level design round.
- Lists the use cases before writing classes
- Gives each class one clear responsibility
- Uses interfaces where behaviour is likely to change
- Applies a design pattern only when it solves a real problem
- Handles edge cases and invalid states explicitly
- Walks through one use case end to end with the model
Questions people ask about this interview question
- Which design type is this question?
- It is a low-level design question. You design a Kanban board like Trello 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 use cases and the core entities, then the responsibilities of each one. 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