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Microsoft·Software Engineer·Onsite - System Design / Architecture·Senior

Senior
Apr 2026

Summary

Microsoft system design round for a software engineer role, full hour on designing a Slack-like messaging platform. Pretty thorough coverage of everything from WebSocket architecture to storage tiering and cross-region replication trade-offs.

Questions Asked (1)

Q1

Design a real-time team messaging platform similar to Slack, covering core features like channels, threads, reactions, file uploads, presence, and notifications at scale.

System DesignTechnical Trade-offsData Modeling
Author's notes

This is a beast of a question because it looks like one question but it's really about ten.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale (e.g., number of users, messages per day, latency expectations), then design a high-level architecture that separates real-time messaging (WebSockets) from persistent storage and asynchronous processing. Focus on data modeling for channels, messages, and threads, and discuss trade-offs in consistency, availability, and scalability, especially for presence and notifications.

Pro tip: Emphasize idempotency and ordering guarantees for message delivery, and discuss how to handle offline users and push notifications without overwhelming the system. Showing awareness of cost and operational complexity (e.g., using managed services vs. self-hosted) can set you apart.

1. Clarify Requirements and Scale

Ask questions to understand functional and non-functional requirements: expected number of users, concurrent connections, messages per second, latency targets, and consistency needs. This ensures the design meets actual needs.

2. High-Level Architecture

Outline the main components: WebSocket gateways for real-time communication, a message service for processing and storage, a presence service, a notification service, and a file service. Discuss how they interact and scale independently.

3. Data Modeling and Storage

Design schemas for channels, messages, threads, and reactions. Choose appropriate databases: e.g., wide-column store for messages (high write throughput), relational for metadata, and object storage for files. Consider partitioning and indexing strategies.

4. Real-Time Messaging and Presence

Detail how WebSockets maintain connections, how messages are routed to channel members, and how presence is tracked and propagated. Address fan-out, ordering, and delivery guarantees.

5. Scalability, Reliability, and Trade-offs

Discuss scaling WebSocket servers, sharding, replication, and handling failures. Cover trade-offs like consistency vs. availability, and how to ensure idempotency and exactly-once semantics where needed.

Key Points to Mention

  • Use WebSockets for real-time bidirectional communication, with a pub/sub system (e.g., Redis, Kafka) for message fan-out.
  • Data model: messages stored in a distributed database like Cassandra with time-based UUIDs for ordering; channels and threads as metadata.
  • Presence: use a heartbeat mechanism with TTL in a fast store like Redis; propagate changes via pub/sub.
  • Notifications: separate service that consumes events and sends push notifications via APNs/FCM, with batching and rate limiting.
  • File uploads: use pre-signed URLs to object storage (e.g., S3), with metadata stored in the database.
  • Scalability: shard by channel or user, use consistent hashing for WebSocket servers, and consider multi-region deployment for low latency.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.