This is the interview experience for round 2 for SDE2 at Zepto.
Round 1 : https://leetcode.com/discuss/interview-experience/5711152/zepto-sde-2-round-1/2601252

Design LLD of IRCTC train ticket booking system

Functional Requirements:

• Users should be able to search trains by train number and via source, destination and date.
• Users should be able to see the number of available seats, given the train, source, and destination and date
• Users should be able to book tickets (The system should be able to handle concurrent booking requests in a fair manner).
• Seat Reusability of booked seats IS allowed. (User A books a seat S1 from station1 to station2 then UserB should be able to book the same seat S1 from station3 onwards).
• Users should be able to cancel a booked ticket.

Assumptions:

• A train will run from source to destination once a day. In the case of the reverse journey, the ID of the train will change and it will be treated as a new train itself.
• To keep things simple, we will not consider the berth and coach preference in this article.
• There will be only one coach in a train.
• We will also only book tickets that are confirmed, waitlisting is not supported.

Discussed the solution step wise.

1. Flow
2. Actors and Actions
3. Entities and Relationship
4. DB Design

Interview was more interested on DB design. Explained the DB design using SQL. Started with these entities -

• User
• Train
• Seat
• Booking
  For seat reusablity, introduced a new entity segement. Interviewer was focussed on design more than actual implementation

Please feel free to post the solutions for above in the comments :)