Design a fair event registration queue API

Problem Statement

Design an in-memory Event Registration Service for a high-demand ticketing platform. The system must support thousands of concurrent requests while ensuring fair, first-come, first-served (FIFO) access.

Requirements

1. Register: Add a user to the waitlist for a specific event. The registration should succeed only if the event is not at full capacity. Return a unique registration ID.
2. Cancel: Remove a user from the waitlist by their registration ID. If the registration is removed, the spot should ideally be freed for the next person in line.
3. Get Status: Retrieve the current position of a user in the queue.

Constraints

• The system must be thread-safe.
• Operations Register, Cancel, and GetStatus should ideally run in better than O(N) time, where N is the number of registrations.
• Assume a maximum capacity per event (e.g., 10,000 spots).

API Interface

class RegistrationService:
    def __init__(self, event_id: str, capacity: int):
        # Initialize the system for a specific event
        pass

    def register(self, user_id: str) -> str:
        # Returns a unique registration ID if successful, else empty string
        pass

    def cancel(self, reg_id: str) -> bool:
        # Removes the user from the queue, returns True if successful
        pass

    def get_position(self, reg_id: str) -> int:
        # Returns the 1-based index in the queue, or -1 if not found
        pass

Example

• Event capacity is 2. Users A, B, and C register.
• A and B get spots 1 and 2. C is placed in the queue.
• If B cancels, C should move up to spot 2.
• get_position('C_reg_id') should return 2.