Design a one-to-one chat system
Company: Anthropic
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
Design a one-to-one chat system for a web application.
Assumptions:
- Each user is active on at most one browser device at a time.
- Group chat is out of scope.
- The system should support real-time messaging between two users.
Discuss the following:
- How clients connect and exchange messages in real time.
- How the system determines whether a user is online.
- How to handle message delivery when the recipient is offline.
- How to store messages, conversation metadata, unread counts, and delivery state.
- How to preserve message ordering and avoid duplicate sends.
- Whether Redis or Kafka should be used in different parts of the system, and why.
- The core ideas behind Kafka and when it is or is not a good fit for chat delivery.
Quick Answer: This System Design question evaluates a candidate's ability to architect a scalable, real-time one-to-one chat system, testing competencies in real-time protocols, presence detection, durable message storage, delivery state and ordering guarantees, deduplication, and the appropriate use of messaging technologies like Redis and Kafka.