API Rate Limiter using Sliding Window Algorithm

What is Rate Limiting?

Rate limiting is a technique used to control the number of requests a user or client can send to a system within a specific time window.

It helps protect systems from:

• Server overload
• Excessive traffic
• API abuse
• DDoS attacks
• Resource exhaustion

Rate limiting is widely used in:

• Backend APIs
• Authentication systems
• Payment gateways
• Distributed systems
• Cloud services

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Why is Rate Limiting Important?

Without rate limiting, a large number of incoming requests can:

• degrade system performance
• increase server load
• exhaust system resources
• affect other users

Rate limiting helps:

• maintain system stability
• improve reliability
• ensure fair resource usage
• prevent malicious traffic spikes

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Sliding Window Algorithm

This project implements rate limiting using the Sliding Window algorithm.

The Sliding Window approach tracks requests continuously within a moving time window instead of fixed intervals.

Example:

• Maximum Requests Allowed = 5
• Time Window = 10 seconds

If a user sends more than 5 requests within the last 10 seconds, additional requests are denied.

Older expired requests are automatically removed from the tracking queue.

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Why Sliding Window is Commonly Used?

Sliding Window is widely used because it provides:

• smoother traffic control
• better accuracy
• fair request distribution
• reduced burst traffic problems

Compared to Fixed Window algorithms, Sliding Window prevents sudden request spikes near window boundaries.

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Why HashMap and Queue?

This project combines:

• HashMap
• Queue

to efficiently manage per-user request tracking.

HashMap

Used for:

• storing user-specific request data
• fast user lookup
• efficient request management

Queue

Used for:

• maintaining request timestamps in chronological order
• removing expired requests efficiently
• tracking requests within the active sliding window

Combining both data structures enables efficient rate limiting.

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Project Overview

This project implements an API Rate Limiter using the Sliding Window algorithm in Java.

The system:

• tracks requests for each user
• removes expired requests automatically
• validates incoming requests
• accepts or rejects requests based on configured limits

The implementation simulates real-world API traffic throttling mechanisms used in backend systems.

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Features

• Sliding Window based rate limiting
• Per-user request tracking
• Automatic expiration of old requests
• Request throttling mechanism
• HashMap + Queue implementation
• Real-time request validation
• Efficient timestamp management

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Tech Stack

• Java
• HashMap
• Queue
• ArrayDeque
• Object-Oriented Programming (OOP)

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Project Structure

RateLimiter.java
Main.java

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Sample Output

Request Accepted

Request Denied

Sliding Window Expiration

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How to Run

1. Clone the repository

git clone https://github.com/Monishohms/api-rate-limiter-sliding-window.git

2. Compile Java files

javac *.java

3. Run the project

java Main

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Future Improvements

• Thread-Safe Rate Limiter
• Distributed Rate Limiting
• Redis Integration
• REST API Integration
• Dynamic Configuration
• Monitoring and Analytics Dashboard
• Token Bucket and Leaky Bucket Algorithms