📝 System Design Interview Classics · Sequence
Movie Ticket Booking (BookMyShow-style)
How seat booking avoids double-selling: seats are locked for a few minutes during payment, and released if payment fails or times out.
Drawing diagram…
What this diagram shows
- Seats are locked in Redis with a 10-minute expiry
- The booking is confirmed only after payment succeeds
- Expired locks free the seats automatically
Prompt used
Seat booking sequence: the user picks a show and seats in the app. The booking service tries to lock the seats in Redis with a 10-minute expiry; if any seat is already locked it asks the user to choose again. It creates a pending booking in the database and sends the user to the payment gateway. On payment success the booking is confirmed, seats are marked sold and an e-ticket is sent. On failure or timeout the lock expires and seats become free.
Mermaid code
sequenceDiagram
actor U as User
participant APP as App
participant BS as Booking Service
participant R as Redis Seat Locks
participant DB as Booking DB
participant PG as Payment Gateway
participant N as Notification Service
U->>APP: Choose show and seats
APP->>BS: Book seats A5, A6
BS->>R: Lock A5, A6 for 10 minutes
alt Seats free
R-->>BS: Locked
BS->>DB: Create pending booking
BS-->>APP: Go to payment
APP->>PG: Pay
alt Payment success
PG-->>BS: Success webhook
BS->>DB: Confirm booking, mark seats sold
BS->>N: Send e-ticket
N-->>U: E-ticket by SMS and email
else Failure or timeout
PG-->>BS: Failed
BS->>DB: Cancel booking
Note over R: Lock expires, seats free again
end
else Already locked
R-->>BS: A5 taken
BS-->>APP: Pick other seats
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