PS Classes

Problem — Concurrent test-takers on a shared test series need a countdown nobody can extend by editing their system clock or letting the tab sit in the background — and with 500+ students starting a series at the same slot time, that timer state can't be a slow round trip to the primary database on every answer.

Approach — Made the server the source of truth for the timer, backed by Redis: each answer is written to a Redis-held session on selection (sub-second, no DB round trip) and periodically flushed to MongoDB, with the server auto-submitting the attempt once the deadline passes regardless of what the client believes the time is.

Problem — A flaky connection can make a student hit submit twice from two tabs — scoring the same attempt twice would double-count it in the rank aggregate.

Approach — A short-lived Redis lock (SETNX-style) on the attempt ID makes the first submit win; the second is rejected as already-processed instead of racing the first into the scoring queue — pessimistic by design, since a submit only happens once and losing a few milliseconds to the lock is cheaper than a corrupted rank.

Problem — All-India Rank and chapter-wise breakdowns need to reflect every attempt on a series without re-scanning every submission each time a student opens their result.

Approach — Submitted attempts go on a RabbitMQ queue; a scoring worker consumes them, updates the score/percentile rollup incrementally, and writes the result to Redis so rank and chapter-wise analytics are read from a fast cache instead of computed live per page view.

Problem — Free-trial students, paying students, and admin-authored content all sit behind the same auth — access needs checking on every test and course request without copy-pasting that check into each route.

Approach — Centralized the entitlement check (trial vs. purchased vs. admin) in one middleware layer every test/course endpoint routes through, instead of re-implementing access logic per feature.