Coach Jack has provided the following feedback on your session:

Wei demonstrated good system design thinking 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.
  
• 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.
  
• Reasoned about scalability with fanout models.
  
  

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 Senior+ roles, aim for:
  → Requirements: 5 mins
  → APIs + Entities: 5 mins
  → HLD: 10–15 mins
  → Deep Dives: 15–20 mins or till the end of the interview
  
• On scale estimation, avoid over-investing time unless comparing tradeoffs or influencing architecture decisions.
  
• Proactively discuss durability, clarify which parts of the message lifecycle need persistence and for how long.
  
• 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.

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