System Design Mock Interview: Designing IRCTC

Introduction

• Interview conducted by Kirti with an expert known for system design.
• Focus is on a complex system design, specifically for IRCTC (Indian Railway Catering and Tourism Corporation).
• IRCTC is a train booking website for booking seats on trains.

Key Features to Design

1. Schedule of Trains

   • Need a rough schedule for trains traveling from one location to another.
   • Trains have routes with multiple stops.
   • Users need to see available seats and book them.
   • Support for booking multiple seats.
   • Routes can change based on dates.
   • Maximum booking allowed is 3 months in advance.

2. Search Feature

   • Users can search for trains between a source and destination for specific dates.
   • Show a list of available trains, timings, and other details.

3. Concurrent Bookings

   • Handle multiple users booking the same seat simultaneously.
   • Atomicity guarantees to ensure transactions are complete or rolled back.

Additional Requirements

• Consideration of cancellation policies.
• No booking if cancellations occur before money is debited.

Design Considerations

Train Service

• Components of a Train
  • Train ID, schedule, seating arrangement (simplified assumption).
  • Use of SQL database due to manageable scale (around 1 million records).
  • Important to handle search queries efficiently.

Searching Trains

• Implement indexes on estimated departure and arrival times for efficient querying.
• Considerations for handling multiple stop searches and transitive relations between source and destination.
• Discussed potential use of graph databases for complex route mappings.

Booking Seats

• Address concurrent booking issues with locking mechanisms (pessimistic and optimistic locks).
• Booking involves decrementing seat availability between multiple stations.
• Discussed seat assignment and cancellation impacts.
• Handling of scenarios where users need to change seats or partial bookings occur.

Feedback and Observations

• Importance of defining scope and requirements explicitly (e.g., focus on train booking only).
• Discussion on handling high load scenarios like 'Tatkal' booking.
• The need for communication with the interviewer during design discussions.
• Recognize system complexity and potential features beyond basic booking.

Conclusion

• The interview addressed key design aspects but noted missing considerations for certain use cases.
• Emphasis placed on the need for more interactive problem-solving with the interviewer.
• General feedback on improving questioning and involving the interviewer more in discussions.
• Overall, a valuable learning experience for system design interviews.

Additional Notes

• Potential scope for future design discussions including other IRCTC functionalities like hotel bookings, catering, etc.
• Importance of early estimation and assumptions in the system design process.