- auth (refresh token + access token)
- pagination (cursor based + offset)
- email service (email react/html templating + smtp transfer + node mail(resend) + fallback mechanisms + bouncer/spam mail protection for a good email score)
- file-upload-pipeline (multer + cloudinary + dual pipeline + sharp processing + ram resizing/normalisation + magic number verification + direct client upload (cloudinary signed urls)
- logging-metrics (mock ticket booking system under load -- k6 benchmarking+grafana dashboards+prom+tests using vitests/jest)
- rate-limiter (three rate limiting algorithms from scratch -- token bucket + sliding window + fixed window)
- redis-and-caching (cache-aside pattern + TTL based expiry + leaderboards)
- splittab-payment-backend (mock implementation of splitwise payment backend -- dlq + realtime backend contracts + HMAC/sha-256 webhook verification + OCC + redis pub/sub)
- webhook-delivery system (fastify backends + horizontal and vertical scaling + worker-queue batching + concurrency + backpressure handling + dlqs)
- webhooks/ (queue decoupling + ci/cd patterns + service-event segregation)
the code scope and goal was to implement atleast 2-3 new concepts in every project, and avoid repitition of concepts