The Indian Railway Catering and Tourism Corporation (IRCTC) powers one of the world’s largest online train ticket booking platforms. Designing a scalable train ticket booking system architecture requires balancing real-time seat availability, dynamic scaling for peak hours, and stringent transactional integrity. Whether you’re building a similar platform or preparing for system design interviews, this crash course in scalable system design can help you master the fundamentals.

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Key Goals and Functional Requirements

Every train booking platform must handle user searches, bookings, cancellations, and additional services seamlessly. The IRCTC design ensures:

• Train and station listing across India with route and schedule details.
• Real-time seat availability with the option to view nearby dates (+3/–3 days).
• Class selection (1AC, 2AC, 3AC, Sleeper, General) with transparent fare breakdown.
• Tatkal booking support for next-day travel, scaling to millions of concurrent users at 10 AM and 11 AM.
• Fair berth allocation prioritizing senior citizens, single women, families, and berth preferences.
• Multiple payment methods (UPI, net banking, cards, wallets) to ensure a smooth transaction.
• Automated notifications for PNR status via SMS and email.
• Onboard services booking (meals, bedrolls, wheelchair assistance).
  
  

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Core Non-Functional Requirements

Beyond features, the platform must excel in:

• Scalability & Performance
  
  
  • Handle millions of requests per second during peak Tatkal windows.
  • Auto-scale infrastructure up at 10 AM and 11 AM and scale down afterward.
    
    
• Transactional Integrity (ACID Compliance)
  
  
  • No duplicate or failed bookings.
  • Payments processed only after seat confirmation.
    
    
• High Availability & Fault Tolerance
  
  
  • Active-passive database replication with automated failover.
  • Distributed caching (Redis cluster) to offload read traffic.
    
    
• Partial Booking Support
  • Confirm available seats; move the rest to RAC or waitlist.
    
    

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Critical Design Considerations

Microservices and API Gateway

• API Gateway routes requests to microservices:
  
  
  • /trains/search → Train Service
  • /tickets/book → Booking Service
  • /tickets/{pnr}/status → Status Service
    
    
• Messaging Queues (booking_queue, payment_queue, notification_queue) for asynchronous processing.
  
  

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Caching and Load Balancing

• Write-Through vs. Write-Behind cache strategies to maintain consistency and performance.
  
  
• Cache Invalidation on seat booking to ensure fresh data:
  
  
  • Invalidate the entire key (e.g., seat_availability:12001:1)
  • Or update specific fields (HSET seat_availability:12001:1 A1 false)