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

Senior

Jul 2026

Summary

System design round at Crusoe for a software engineer role. One question, Slack, which sounds approachable until you actually start drawing it out.

Questions Asked (1)

Design a messaging platform like Slack, covering direct messaging, group chat, and file sharing.

System DesignData ModelingTechnical Trade-offs

Author's notes

I started with the DM flow and felt fine, but group chat at scale is where things got messy for me.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale, then propose a high-level architecture that separates real-time messaging, storage, and file handling. Dive into data modeling for conversations and messages, and discuss trade-offs for delivery guarantees, consistency, and scalability.

Pro tip: Emphasize idempotency and ordering in message delivery, as these are critical for a reliable chat system and often overlooked. Also, mention how you would handle presence and typing indicators without overloading the system.

1. Clarify Requirements and Scale

Ask about expected user count, message volume, latency requirements, and features like read receipts, presence, and search. This ensures you design for the right scale and avoid over-engineering.

2. High-Level Architecture

Outline components: WebSocket servers for real-time communication, a message queue for asynchronous processing, a database for persistence, and a blob store for files. Explain how they interact.

3. Data Modeling

Design schemas for users, conversations (direct and group), messages, and file metadata. Discuss how to efficiently query recent messages and handle group membership.

4. Deep Dive into Key Flows

Detail message sending/receiving, file upload/download, and group management. Address ordering, idempotency, and delivery guarantees (at-least-once vs exactly-once).

5. Trade-offs and Scalability

Discuss trade-offs: SQL vs NoSQL, push vs pull for message delivery, consistency vs availability. Explain how to scale horizontally and handle failures.

Key Points to Mention

Use WebSockets for real-time bidirectional communication and fallback to long polling if needed.
Data model: conversations table with participants, messages table with conversation_id, sender_id, content, timestamp, and file attachments.
Message ordering: use per-conversation sequence numbers or timestamps with tie-breakers; ensure idempotency with client-generated message IDs.
File sharing: store files in object storage (e.g., S3), metadata in database, and use pre-signed URLs for secure upload/download.
Scalability: shard conversations by conversation_id, use pub/sub for fan-out, and cache recent messages for fast retrieval.
Trade-offs: consistency vs latency for message delivery; consider eventual consistency for presence and read receipts.

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