HLD Phase 3: Specialized Infrastructure

Microservices, Message Queues (Kafka/RabbitMQ), CDNs, and Rate Limiting algorithms.

1. Message Queues (Kafka)

In monolithic applications, modules call each other directly. If one fails, the whole process fails. Microservices use **Message Queues** to operate asynchronously. For example, an OrderService writes an event to Kafka, and an EmailService reads it eventually, decoupling logic.
typescript
// Decoupled Microservice Architecture logic
class OrderService {
    processCheckout(orderData: any) {
        // ... Save order to DB ...
        
        // Push event to Kafka instead of calling EmailService directly
        kafkaProducer.send({
            topic: 'orders_topic',
            messages: [{ value: JSON.stringify({ event: 'ORDER_CREATED', data: orderData }) }]
        });
    }
}

2. Rate Limiting (Token Bucket)

To prevent DDoS attacks or API abuse, rate limiters are deployed. The most common algorithmic implementation is the Token Bucket.
typescript
class TokenBucket {
    private capacity: number;
    private tokens: number;
    private refillRatePerSecond: number;
    private lastRefillTimestamp: number;

    constructor(capacity: number, refillRate: number) {
        this.capacity = capacity;
        this.tokens = capacity;
        this.refillRatePerSecond = refillRate;
        this.lastRefillTimestamp = Date.now();
    }

    public allowRequest(): boolean {
        this.refillTokens();
        if (this.tokens > 0) {
            this.tokens--;
            return true;
        }
        return false; // Rate limit exceeded!
    }

    private refillTokens() {
        const now = Date.now();
        const secondsPassed = (now - this.lastRefillTimestamp) / 1000;
        const newTokens = secondsPassed * this.refillRatePerSecond;

        this.tokens = Math.min(this.capacity, this.tokens + newTokens);
        this.lastRefillTimestamp = now;
    }
}
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