Exam-Sync

Problem

Keep one exam countdown in sync for an examiner and all students, even when they are connected to different backend instances.

My role: Personal project. Built the NestJS backend and the Next.js examiner and student pages.

Constraints

• WebSocket connections are sticky to one instance, but a start or pause from the examiner must reach students on every instance.
• Concurrent control actions (start, pause, reset) must not corrupt the shared timer state.

Architecture

1. Next.js examiner / student pages

   socket.io-client on the /exam namespace

   ↓

2. NestJS ExamGateway

   joinExam, createAndStartExam, pauseExam, resetExam, restartExam

   ↓

3. Redis + Redlock

   Room state keyed by exam id, guarded by timer-lock:{examId}

   ↓

4. Kafka topics

   examStarted, examPaused, examReset, examTimerUpdate

   ↓

5. Every backend instance

   Consumes the topic and emits to its local socket room

What I Built

• NestJS ExamGateway (Socket.IO), ExamService, and RedisService using ioredis and Redlock.
• Kafka consumers with a per-instance group id, so every instance receives every event (fan-out) and relays it to its own sockets.
• Shared timer state in Redis, decremented once per second by the instance that started the exam.
• Next.js 15 examiner and student views.

Technical Challenges

Cross-instance broadcast

Kafka is used as a bus between instances: a unique consumer group per instance turns each topic into a broadcast.

Concurrent state changes

Control actions take a Redlock lock (2s TTL, 10 retries) before reading and writing the room state.

Trade-offs & Limitations

• Scope is timer synchronisation only: no auth, questions or persistence.
• The ticking interval lives on one instance; if it crashes, the countdown stops until restarted.
• Timer ticks write Redis without the lock, trading strictness for simplicity.