Tyler demonstrated strong system design thinking and Staff-level communication throughout the mock interview. He proactively drove the conversation and articulated many advanced tradeoffs relevant to Slack-style messaging systems.

Areas of strength included:

• ✅ Clearly enumerated functional and non-functional requirements, including quantitative metrics where appropriate.
  
  
• ✅ Applied CAP theorem understanding to the message ordering problem—e.g., consistency guarantees within group chats to ensure context alignment.
  
  
• ✅ Thoughtfully modeled entity relationships, including how threads can nest within message structures.
  
  
• ✅ Demonstrated protocol fluency by comparing WebSocket vs Long-Polling, correctly opting for WebSocket for real-time bi-directional messaging.
  
  
• ✅ Designed a session manager module for maintaining persistent connections and user sessions.
  
  
• ✅ Discussed media message handling using pre-signed S3 URLs and blob storage.
  
  
• ✅ Proposed a watermark table + client tracking approach to handle multiple-device sync across clients.
  
  
• ✅ Included heartbeat mechanisms to track connection liveness and regenerate WebSocket sessions as needed.
  
  
• ✅ Reasoned about scalability with pub/sub models and local cache layers for mapping channel-to-server connections.
  
  
• ✅ Avoided blindly suggesting Kafka—highlighted its tradeoffs and explained when it would (and wouldn’t) help during burst scenarios.
  
  

Areas for improvement:

• Condense and combine requirement statements for clarity—see ShowOffer’s system design writing standards for examples.
  
  
• Time allocation: Requirements took ~10 mins; high-level design ~25 mins. For Staff+ roles, aim for:
  → Requirements: 5 mins
  → APIs + Entities: 5 mins
  → HLD: 10–15 mins
  → Deep Dives: 15–20 mins
  
  
• On scale estimation, avoid over-investing time unless comparing tradeoffs or influencing architecture decisions.
  
  
• When discussing durability, clarify which parts of the message lifecycle need persistence and for how long.
  
  
• State your assumptions early—e.g., "assume channel already exists and client B is online"—to reduce ambiguity.
  
  
• Consider structuring feature rollout progressively:
  
  
  
  • 1:1 text chat (online)
    
    
  • Add media support (upload workflow)
    
    
  • 1:1 with offline client
    
    
  • Deletion support (e.g., is_message_deleted boolean column)
    
    
  • Expand to group chats
    
    
• Improve clarity on media upload flow:
  
  
  
  • When is media_id generated?
    
    
  • When is the S3 pre-signed URL returned?
    
    

For deletions, rather than physical removal, a soft delete using a boolean flag is often sufficient and more efficient.