**Design a Rate Limiter**

### Introduction

In this document, we will explore the design of a system for "Design a Rate Limiter". The goal is to understand the requirements, challenges, and architectural decisions involved in building such a system.

### Requirements

#### Functional Requirements

* The rate limiter must be able to track the number of requests made by a user within a specified time window (e.g., 1 minute).
* The rate limiter must be able to block or throttle excessive requests from a user.
* The rate limiter must provide an API for developers to integrate with.

#### Non-Functional Requirements

* Performance: The system should be able to handle a high volume of requests without significant latency or downtime.
* Scalability: The system should be able to scale horizontally (add more nodes) to handle increased traffic.
* Reliability: The system should be highly available and resistant to failures.

### High-Level Architecture

The rate limiter system consists of the following components:

1. **Request Tracker**: responsible for tracking requests made by users within a specified time window.
2. **Rate Limiter**: determines whether a request should be blocked or throttled based on the user's request rate.
3. **API Gateway**: provides an API for developers to integrate with, handling incoming requests and forwarding them to the Request Tracker.
4. **Database**: stores information about users, their request rates, and other relevant data.

### Database Schema

The database schema consists of three tables:

1. **users**: stores user information (e.g., ID, username).
2. **requests**: tracks individual requests made by users (e.g., timestamp, user ID, request type).
3. **rate_limits**: stores the rate limit for each user (e.g., number of requests per minute).

### API Design

#### Key Endpoints

1. `POST /requests`: allows a user to make a new request.
2. `GET /requests/:user_id`: returns the request history for a specific user.

Example Request:
```bash
curl -X POST \
  http://example.com/requests \
  -H 'Content-Type: application/json' \
  -d '{"request_type": "some_request"}'
```

Example Response:
```json
{
  "status": "ok",
  "request_id": "1234567890"
}
```

### System Flow

1. A user makes a request to the API Gateway.
2. The API Gateway forwards the request to the Request Tracker.
3. The Request Tracker checks if the user has exceeded their rate limit.
4. If the user is within their rate limit, the Request Tracker allows the request and increments the user's request count.
5. If the user is over their rate limit, the Rate Limiter blocks or throttles the request.

### Challenges and Solutions

1. **Handling high traffic**: Solution: use a distributed database and horizontal scaling to handle increased traffic.
2. **Preventing abuse**: Solution: implement IP blocking and analytics for identifying suspicious behavior.

### Scalability and Performance

To ensure scalability and performance, we will:

1. Use a distributed database to store request data.
2. Implement horizontal scaling by adding more nodes as needed.
3. Optimize database queries for high-performance.

### Security Considerations

We will implement the following security measures:

1. **Authentication**: require users to authenticate before making requests.
2. **Authorization**: control access to the API based on user roles and permissions.
3. **Encryption**: use SSL/TLS encryption for all data transmitted between the client and server.

### ASCII Diagrams

[Insert ASCII diagram illustrating the system architecture]

### Sample SQL Schema
```sql
CREATE TABLE users (
  id INT PRIMARY KEY,
  username VARCHAR(255) NOT NULL
);

CREATE TABLE requests (
  id INT PRIMARY KEY,
  user_id INT NOT NULL,
  timestamp TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  request_type VARCHAR(255) NOT NULL,
  FOREIGN KEY (user_id) REFERENCES users(id)
);

CREATE TABLE rate_limits (
  user_id INT PRIMARY KEY,
  rate_limit INT NOT NULL
);
```

### Example JSON API Response

```json
{
  "status": "ok",
  "request_id": "1234567890"
}
```

### Summary

The design of the rate limiter system involves tracking requests made by users, implementing a rate limit to prevent abuse, and providing an API for integration. The system must be scalable, performant, and secure to handle high traffic and protect user data.

Open questions and areas for further exploration:

* How will we handle cases where multiple users are making requests simultaneously?
* What is the optimal rate limit for each user, and how will we determine it?