Movie ticket booking

The problem

A theatre chain runs screens that show movies at set times. Customers browse shows, pick seats, pay and receive a ticket. Many customers pick seats for the same popular show at once. A seat must never be sold twice.

Entities

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Figure SDA-LLD-05-1 · Movie booking classes · © forcsguy.com

The key idea: ShowSeat is a seat in one show. A physical seat exists for all shows; it is available or taken per show. Status values: AVAILABLE, HELD, BOOKED.

The seat lifecycle

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Figure SDA-LLD-05-2 · Show seat status · © forcsguy.com

class ShowSeat {
  status: "AVAILABLE" | "HELD" | "BOOKED" = "AVAILABLE";
  heldBy?: string; heldUntil = 0;

  // must be atomic: a lock per seat in memory, a conditional UPDATE in a database
  tryHold(user: string, ms = 10 * 60_000): boolean {
    if (this.status === "HELD" && Date.now() > this.heldUntil) this.status = "AVAILABLE";
    if (this.status !== "AVAILABLE") return false;
    this.status = "HELD"; this.heldBy = user; this.heldUntil = Date.now() + ms;
    return true;
  }
  confirm(user: string): boolean {
    if (this.status !== "HELD" || this.heldBy !== user) return false;
    this.status = "BOOKED"; return true;
  }
}

In a database this is one statement: UPDATE show_seat SET status='HELD', held_by=?, held_until=? WHERE id=? AND (status='AVAILABLE' OR held_until < now()) and the affected-row count tells you whether you won.

Booking several seats

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Figure SDA-LLD-05-3 · Booking flow · © forcsguy.com

If any seat in the group cannot be held, release the ones you did hold and tell the customer. This avoids a half-booked group.

Design notes

• Pricing by seat category, time and day is a strategy or a price table on ShowSeat.
• Expiry can be a background job or a check at read time (as above). A job keeps the displayed seat map accurate.
• Hot shows: at huge scale put the seat state in a fast store with atomic operations, and queue requests; this overlaps with the flash-sale design in the real-world systems course.
• Idempotent payment: a retried payment callback must not issue a second ticket.
• Extensions: seat recommendations, food add-ons, cancellations with refund policy, and notifications on confirm.

Real-world reading