Entry and exit flow in LLD - System Design Exercise

Design: Entry and Exit Flow System

Design covers the flow of users entering and exiting a facility with tracking and control. Out of scope are physical hardware details like gates or sensors.

Functional Requirements

FR1: Allow users to enter through a controlled entry point

FR2: Allow users to exit through a controlled exit point

FR3: Track entry and exit times for each user

FR4: Prevent unauthorized entry or exit

FR5: Support up to 1000 concurrent users entering or exiting

FR6: Provide real-time status of current users inside

Non-Functional Requirements

NFR1: System must respond to entry or exit requests within 200ms

NFR2: Availability target of 99.9% uptime

NFR3: Support peak load of 1000 concurrent entry/exit requests

NFR4: Ensure data consistency for entry and exit records

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Authentication service for user verification

Entry and exit controllers managing gate access

Database to store entry and exit logs

Real-time dashboard for monitoring current users inside

Notification system for alerts on unauthorized access

Design Patterns

Event-driven architecture for entry and exit events

State machine pattern to track user status (inside/outside)

Circuit breaker for gate control to handle failures

Caching for quick access to current user status

Reference Architecture

User --> Entry Controller --> Auth Service --> Database
       |                 |               |
       v                 v               v
    Exit Controller --> Auth Service --> Database
       |
       v
    Monitoring Dashboard

Components

Entry Controller

Lightweight REST API service

Handles user entry requests, verifies identity, and controls gate access

Exit Controller

Lightweight REST API service

Handles user exit requests, verifies identity, and controls gate access

Authentication Service

OAuth 2.0 or custom auth service

Verifies user identity before allowing entry or exit

Database

Relational DB (e.g., PostgreSQL)

Stores entry and exit logs with timestamps and user status

Monitoring Dashboard

Web dashboard with WebSocket updates

Displays real-time count and list of users currently inside

Request Flow

1. User requests entry at Entry Controller

2. Entry Controller calls Authentication Service to verify user

3. If verified, Entry Controller records entry time in Database

4. Entry Controller signals gate to open for user entry

5. User enters facility

6. User requests exit at Exit Controller

7. Exit Controller calls Authentication Service to verify user

8. If verified and user is inside, Exit Controller records exit time in Database

9. Exit Controller signals gate to open for user exit

10. Monitoring Dashboard queries Database or listens to events to update current users inside

Database Schema

Entities: - User: user_id (PK), name, credentials - EntryExitLog: log_id (PK), user_id (FK), entry_time, exit_time Relationships: - One User can have many EntryExitLog records - EntryExitLog tracks paired entry and exit times per user

Scaling Discussion

Bottlenecks

Authentication Service becoming slow under high concurrent requests

Database write contention for entry and exit logs

Real-time dashboard lagging with many users inside

Gate control communication delays

Solutions

Use caching or token-based authentication to reduce auth service load

Partition database by user or time to reduce write contention

Use event streaming (e.g., Kafka) to push updates to dashboard efficiently

Implement retries and circuit breakers for gate control communication

Interview Tips

Time: Spend 10 minutes clarifying requirements and constraints, 20 minutes designing components and data flow, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing.

Clarify user verification and security needs

Explain separation of entry and exit controllers for clarity

Describe database schema supporting tracking of user presence

Discuss real-time monitoring importance

Address scaling challenges and solutions clearly