Engineering notes

Practical answers from production full-stack work: preventing overselling, optimistic vs pessimistic locking, idempotent webhooks, exam timers and multi-vertical booking.

• How do you stop two people buying the last ticket?

  Prevent overselling in ticket checkout with a pessimistic row lock, a Redis reservation hold with a TTL, and an idempotent payment webhook.

  2026-10-10 · FindYourVibe

• Optimistic or pessimistic locking: which should a booking or checkout use?

  Choose between optimistic and pessimistic locking by how often two writers hit the same row. Four real cases: tickets, bookings, stock and exam submits.

  2026-10-10 · Trexind

• How do you make a payment or WhatsApp webhook idempotent?

  Key every webhook on a unique ID and publish work to a queue so retries cannot double-issue tickets, bookings or charges. Patterns from two production apps.

  2026-10-10 · FindYourVibe

• How do you build an exam timer students can't cheat?

  Make the server the source of truth for exam deadlines: Redis-held sessions, periodic flush to MongoDB, auto-submit, and a lock against double submits.

  2026-10-10 · PS Classes

• How do you serve clinics, restaurants and salons from one booking system?

  Model a generic booking entity that each vertical extends with a typed payload, so booking, WhatsApp and billing engines never fork per business type.

  2026-10-10 · Trexind

• How much does it cost to build a custom ERP, SaaS or booking system in India?

  Indicative price ranges and timelines for custom ERP, SaaS, booking, e-commerce and exam platforms, and the four things that move the quote.

  2026-10-10 · Pricing

• What breaks when you parse LLM output in production?

  Three real bugs from an AI backend: JSON wrapped in code fences, prompt legends copied into output, and validators that reject almost-right responses.

  2026-10-10 · Engineering notes