Design Ticket Booking Auto Release
System Design: Auto-expiring Ticket Reservations
Problem
Design a ticket-booking system where a reserved ticket automatically returns to the inventory if payment is not completed within 30 minutes.
Discuss and propose a design for:
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Data model and state machine.
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Reservation and payment flow.
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Concurrency control to prevent overselling.
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Timeout processing (auto-expiry) at scale.
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Consistency guarantees and failure handling.
Assumptions
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Tickets can be either seat-based (assigned seats) or general admission (GA) with a capacity count.
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A reservation places a temporary hold for 30 minutes. If payment is completed within that window, the reservation becomes a purchase; otherwise, it expires and inventory is released.
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High read/write throughput; horizontally scalable services.
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Payment is processed by an external provider and can succeed/timeout/arrive late.
Constraints & Assumptions
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Preserve the scope, facts, inputs, and requested outputs from the prompt above.
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If the prompt leaves a detail unspecified, state a reasonable assumption before relying on it.
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Keep the answer interview-ready: concise enough to present, but concrete enough to implement or evaluate.
Clarifying Questions to Ask
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Clarify users, core use cases, read/write patterns, scale, latency, availability, and data retention.
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State explicit assumptions before making sizing or architecture decisions.
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Prioritize the functional path first, then address reliability, security, observability, and rollout.
What a Strong Answer Covers
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A scoped requirements summary with concrete non-goals and success metrics.
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API, data model, architecture, consistency, capacity, and operations.
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Reasoned trade-offs among simple and scalable designs, including bottlenecks and failure modes.
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A validation, monitoring, migration, and launch plan appropriate for the risk level.
Follow-up Questions
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What breaks first at 10x traffic or data volume?
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How would you degrade gracefully during dependency failures?
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What metrics and alerts would prove the design is healthy after launch?