System requirements


Functional:

  1. Use should be able to see nearby Movie theaters.
  2. User should be able to see movies list in each theater.
  3. User should be able to select movie and seat for the ticket.
  4. The ticket should hold for the user for 5 ~ 10 minutes to complete payment.
  5. User should be able to complete the payment.


Non-Functional:

  1. Availability
  2. Scalability
  3. Strong consistency for movie ticket counts.
  4. mid level Latency. Checking ticket availability and completing payment could be allowed around 10 second delay.




Capacity estimation

Assume the system has 10m DAU.

Read QPS: 10m / 100k = 100

The peak read QPS will be twice the traffic: 2 * 100 = 200

Assume 1% user buy one ticket per day.

Write QPS: 10m * 1% / 100k = 1

The peak write QPS will be twice the traffic: 2 * 1 = 2


So this is a read heavy system.



API design

GET getMovieTheaters: The request will be user's locationInfo. The response will be a list of movie theater info.

GET getMoviesListForTheater: The request will be movie theater id. The response will be a list of movies. Each movie information will contain the available time for the movie.

GET getSeatsForMovie: The request will be theater id, movie id and date time info. The response will be a list of seatInfo. The seat info contains the seat position and whether the seat is occupied or not.

POST holdTicket: The request will be the theaterId, movieId, time of the movie and seatId. The response will be the ticket id and the status of the ticket.

GET getTicketStatus: The request will be ticketId. The response will be the status of the ticket.

POST completePaymentForTicket: The request will be ticketId. The response will be whether the payment succeed or not.


For each API request, there will be auth token attached to the request for authentication purpose.

The responses of each API may contain error. The error will have http status code, for example 4xx indicates request error from client side and 5xx indicates response error from server side. Also the error will contain specific error messages to display to the user.



Database design

I will use SQL database


Theater Table:

  • theaterId (primary key)
  • theaterInfo
  • movieIds (reference key to Movie table)

Movie Table:

  • movieId (primary key)
  • movieInfo

MovieTime Table:

  • movieTimeId (primary key)
  • theaterId (reference key to Theater table. This field should be indexed)
  • movieId (reference key to Movie table. This field should be indexed)
  • movieTime (timestamp)

Ticket Table:

  • ticketId (primary key)
  • movieId (reference key to Movie table. This field should be indexed)
  • movieTimeId (reference key to MovieTime table. This field should be indexed)
  • seatId (reference key to Seat table. This field should be indexed)
  • status (enum value: pending, complete, paymentFail, refund)



High-level design


Client: The client customer used to book a ticket. It could be a web app or mobile app.


Load Balance: Balance the traffic from client evenly to different servers. The load could be balanced with path based, round robin or consistent caching approaches.


Api Gateway: Api Gateway will be responsible with authentication, rate limiting and others.


CDN: Store static files, for example movie preview images, movie tailor and theater images. When user request static data, the request will be routed to the CDN which physically close to the client.


Info Service: Info service will be responsible to read request from clients. For example, get theater info, get movie info and get seat info.


Ticket Service: Ticket service will be responsible to update the database to hold a ticket and query a ticket status.


Task Scheduler: The task scheduler will be responsible to schedule a time after user hold a ticket. If the user didn't pay the ticket in a certain time (5 min or 10 min), the task scheduler will be responsible to update the status of the ticket.


SQL Database: The database will be responsible to store the information of the theater, movie, seat and ticket.


Database Cache: Will cache the SQL Database to read data faster.



Request flows

Explain how the request flows from end to end in your high level design. Also you could draw a sequence diagram using the diagramming tool to enhance your explanation...






Detailed component design

Dig deeper into 2-3 components and explain in detail how they work. For example, how well does each component scale? Any relevant algorithm or data structure you like to use for a component? Also you could draw a diagram using the diagramming tool to enhance your design...






Trade offs/Tech choices

Explain any trade offs you have made and why you made certain tech choices...






Failure scenarios/bottlenecks

Try to discuss as many failure scenarios/bottlenecks as possible.






Future improvements

What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?