Automatic Interview Scheduling System

1. System Overview

This is a backend REST API developed in Java Spring Boot that allows interviewers to set availability and candidates to book, manage, and reschedule interview slots. The system is designed to handle high concurrency, ensuring that no two candidates can book the same slot simultaneously.

2. Architecture

The system follows Clean Architecture principles to separate concerns:

• Controller Layer: Handles HTTP requests and response formatting.
• Service Layer: Contains business logic (Booking rules, Rescheduling atomicity).
• Repository Layer: Abstracted database interactions using Spring Data JPA.
• Domain Layer: Core entities (InterviewSlot, Availability) representing the database schema.

3. Key Features & Implementation

A. Slot Generation

• Logic: Interviewers define weekly availability (e.g., "Mondays 9-12"). The system projects this 14 days into the future, creating individual 1-hour InterviewSlot records.
• Scalability: Slots are pre-generated, making the "Get Available Slots" API extremely fast (O(1) lookup vs. O(N) calculation on the fly).

B. Concurrency Handling (Race Conditions)

• Problem: Multiple candidates clicking "Book" on the same slot at the same millisecond.
• Solution: Optimistic Locking via JPA @Version.
• Trade-off Discussion: We chose Optimistic Locking over Pessimistic Locking because interview bookings are "low-contention" (rarely do 100+ people fight for one slot). Optimistic locking provides higher throughput and doesn't block database rows unnecessarily.

C. Atomic Rescheduling

• Implementation: The rescheduleSlot method uses the @Transactional annotation.
• Behavior: It performs two actions: (1) Releasing the old slot and (2) Booking the new slot. If either fails, the entire transaction rolls back, preventing data corruption.

4. API Endpoints

Method	Endpoint	Description
POST	/api/slots/generate/{id}	Generates slots based on availability rules.
GET	/api/slots/available	Lists all open slots for the next 14 days.
POST	/api/slots/{id}/book	Books a specific slot (Safe against race conditions).
POST	/api/slots/reschedule	Atomically swaps an existing booking for a new one.

5. Database Schema

• Users: Stores Interviewers and Candidates.
• Availability: Stores recurring weekly schedules.
• Interview_Slots: Stores actual actionable slots. Includes version column for locking.

6. Testing

• Integration Testing: Added ConcurrencyTest.java using ExecutorService to simulate multi-threaded user attacks.
• Result: Verified that when 2 threads attempt to book the same slot, exactly 1 succeeds and 1 fails.

7. Bonus Features Implementation

A. Cursor-Based Pagination

• Implementation: Instead of standard offset pagination (which gets slower as data grows), we implemented Cursor Pagination.
• Trade-off: * Pros: Extremely fast performance (O(1)) for infinite scroll UIs.
  • Cons: You cannot jump to a specific page number (e.g., "Go to Page 10"), but this is acceptable for a "Load More" style scheduling interface.

B. UI & Debouncing

• UI: A lightweight HTML/JS frontend located in src/main/resources/static/index.html.
• Debouncing: Implemented a custom debounce() function in JavaScript.

  • Purpose: If a user frantically clicks the "Book" button, the system only processes the last click after a 500ms delay. This reduces load on the API and prevents accidental double-submissions.

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