Design ticketing system with seat hold

Design an online ticketing system (similar to Ticketmaster) that allows users to browse events, select seats, and place a temporary 5-minute hold on seats while they complete the purchase.

The system should:

• Allow users to search for events, view seating charts, and select specific seats.
• Allow a user to place a 5-minute hold on selected seats so they are not sold to others during that time.
• Automatically release held seats if the user does not complete payment within 5 minutes.
• Ensure that no two users can successfully purchase the same seat.
• Scale to handle very high traffic during popular events (e.g., tens of thousands of concurrent users for a big concert).

Describe:

• The main use cases and external APIs.
• The high-level architecture and key components.
• The data model for events, seats, orders, and seat holds.
• How you will implement the 5-minute hold and its expiration.
• How you will handle high concurrency and prevent overselling seats.
• Consistency and availability trade-offs, and how you would scale the system.

Assume this is an internet-scale service deployed across multiple servers and data centers.