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Design a Chat App: System Design Interview Practice

Design a Chat App as a system design interview question: One-to-one and small group chat with delivery and read receipts, online presence and message history.

How do you answer "Design a Chat App" in a system design interview?

Keep a persistent WebSocket from every phone to a gateway, route each message through a chat service that numbers it per conversation, stores it and finds the recipient's gateway in a session registry. Deliver over the open socket, or keep it in an inbox and send a push notification when the recipient is offline. Receipts and presence travel as small messages on the same sockets.

  • Level: Intermediate
  • Scored out of: 100
  • Cost: Free to read; a free account gets the review

One-to-one and small group chat with delivery and read receipts, online presence and message history.

The scale

50 million daily users, each sending about 40 messages a day; groups up to 100 people.

What a strong answer covers

  1. Requirements (20 points). States the functional requirements in scope and the non-functional ones that drive the design (scale, latency, availability, consistency), and leaves out what is out of scope.
  2. Scale estimates (15 points). Turns the stated scale into requests per second (average and peak), storage and bandwidth, with the arithmetic shown, and uses the numbers to make a decision.
  3. API and data model (20 points). Defines the main API calls with their inputs and outputs, and a data model whose keys and store fit the access patterns.
  4. Architecture (25 points). Chooses components that fit the requirements and explains how a request moves through them, for the main read and write paths.
  5. Bottlenecks and trade-offs (20 points). Names the real bottlenecks and failure points at this scale and how the design handles them, weighing the alternatives (consistency vs availability, push vs pull, cost).

How the practice works

Write your design in sections — functional and non-functional requirements, scale estimates, the API, the data model, the request flow and a deep dive on the bottlenecks — and pick the components to draw the architecture. A review scores it on the five areas above in under a minute, then you can defend it in an AI mock interview.

The reference design

Design a Chat App (WhatsApp): the walkthrough, with animated figures.

Common questions

How do you answer "Design a Chat App" in a system design interview?

Keep a persistent WebSocket from every phone to a gateway, route each message through a chat service that numbers it per conversation, stores it and finds the recipient's gateway in a session registry. Deliver over the open socket, or keep it in an inbox and send a push notification when the recipient is offline. Receipts and presence travel as small messages on the same sockets.

What scale is "Design a Chat App" designed for here?

50 million daily users, each sending about 40 messages a day; groups up to 100 people. Your estimates turn that into requests per second, storage and bandwidth before you pick components.

How is a design scored?

Out of 100 on a fixed rubric: requirements (20), scale estimates (15), api and data model (20), architecture (25), bottlenecks and trade-offs (20). The review names what the design covered and what it missed, then points to the reference design.

Is "Design a Chat App" a good first system design problem?

It is rated intermediate. Start with the beginner problems — the URL shortener and the rate limiter — and the System Design notes on load balancing, caching and sharding first.

Do I need an account?

Not to read the problem or the reference design. Getting your design reviewed needs a free account.

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