How IRCTC Actually Works (and Why It Crashes) | System Breakdown Ep 6
At 10:00 a.m., millions of people tap "book" at the same second — and almost all of them have to lose.
Hotstar was millions of people READING the same thing: one cached chunk, copied everywhere, every request succeeds. IRCTC is the exact opposite. Millions of people WRITING, all converging on the same few thousand rows. You cannot cache a seat. Only one person gets berth 32 in coach S4. So the system's real job is to make most requests fail — correctly, instantly, without ever giving the same berth to two people.
This is the full teardown:
→ Who actually builds it (CRIS writes and runs the reservation system; IRCTC is the shop front)
→ CONCERT, the distributed database across 5 centres — and why there's no distributed lock
→ Two axes of partition: quota × journey segment, where one berth is sold twice as two legs
→ RAC: selling one side-lower berth to two passengers, by design
→ The payment two-step: lien → PNR → debit, or release
→ Charting, the batch that finally resolves everything 8 hours before departure
→ The bot war: 500 IPs from one machine, and why the CAPTCHA was finally deleted in July 2026
→ What the published data actually says about Tatkal selling out
Every number on screen is sourced. And where the engineering isn't public — like CONCERT's locking model — the video says so instead of guessing.
Chapters
0:00 Behind one over
0:27 Many writes, one row
1:22 The ladder — 2,000 to 37,410 tickets a minute
2:06 Who builds it
3:16 The whole pipeline
4:29 The machines
5:14 The front door
6:29 The read path
7:38 The inventory
9:05 Quota × segment
10:48 Click to claim
11:50 The two-step
13:03 The chart
14:22 The return path
15:13 The adversary
16:30 Two minutes, or ten hours
17:28 From human to unique
19:10 Belief vs the data
20:40 What breaks first
21:33 The close
Sources
· CAG of India, Report No. 11 of 2007 (Railways) — CONCERT architecture, the five PRS centres, the documented duplicate-berth failures
· PIB releases — the capacity ladder, chart preparation timings, RAC berths per coach, the 2016 tables on Tatkal's first 35 seconds
· indianrail.gov.in (CRIS) — the 19 quota codes and the road-side/remote-location redefinition rule
· IRCTC AutoPay documentation — the lien/debit/release protocol and the "No Room" error
· Lok Sabha and Rajya Sabha replies, 2025–26 — bot traffic share, deactivated user IDs, Aadhaar authentication
· LocalCircles survey (55,000+ respondents) and CRIS booking-pattern analysis — perception vs measured reality
Next episode: Aadhaar — a billion people, under a second.
⚠️ Educational system-design teardown. Figures are point-in-time and sourced as above.
#irctc #systemdesign