API Gateway

API Gateway is a single entry point for all client requests, handling cross-cutting concerns.

API Gateway Responsibilities

class APIGateway {
  private rateLimiter: RateLimiter;
  private authService: AuthService;
  private serviceRegistry: ServiceRegistry;

  async handleRequest(req: Request): Promise {
    // 1. Authentication
    const user = await this.authenticate(req);
    if (!user) return this.unauthorized();

    // 2. Rate Limiting
    if (!await this.rateLimiter.allowRequest(user.id)) {
      return this.tooManyRequests();
    }

    // 3. Route to appropriate service
    const service = this.routeRequest(req.path);

    // 4. Transform request if needed
    const transformedReq = this.transformRequest(req, user);

    // 5. Forward to backend service
    const response = await this.forwardRequest(service, transformedReq);

    // 6. Transform response
    return this.transformResponse(response);
  }

  private routeRequest(path: string): ServiceEndpoint {
    const routes: Record = {
      '/api/users': 'user-service',
      '/api/orders': 'order-service',
      '/api/products': 'product-service'
    };

    const serviceName = Object.entries(routes)
      .find(([prefix]) => path.startsWith(prefix))?.[1];

    return this.serviceRegistry.getEndpoint(serviceName);
  }
}

Rate Limiting Algorithms

// Token Bucket Algorithm
class TokenBucketRateLimiter {
  private buckets: Map = new Map();
  private maxTokens: number;
  private refillRate: number; // tokens per second

  constructor(maxTokens: number, refillRate: number) {
    this.maxTokens = maxTokens;
    this.refillRate = refillRate;
  }

  allowRequest(userId: string): boolean {
    const now = Date.now();
    let bucket = this.buckets.get(userId);

    if (!bucket) {
      bucket = { tokens: this.maxTokens, lastRefill: now };
      this.buckets.set(userId, bucket);
    }

    // Refill tokens based on time elapsed
    const elapsed = (now - bucket.lastRefill) / 1000;
    bucket.tokens = Math.min(
      this.maxTokens,
      bucket.tokens + elapsed * this.refillRate
    );
    bucket.lastRefill = now;

    if (bucket.tokens >= 1) {
      bucket.tokens -= 1;
      return true;
    }

    return false;
  }
}

Service Mesh (Istio/Linkerd)

# Istio Virtual Service for traffic splitting
apiVersion: networking.istio.io/v1beta1
kind: VirtualService
metadata:
  name: order-service
spec:
  hosts:
    - order-service
  http:
    - route:
        - destination:
            host: order-service
            subset: v1
          weight: 90
        - destination:
            host: order-service
            subset: v2
          weight: 10

---
# Circuit Breaker Configuration
apiVersion: networking.istio.io/v1beta1
kind: DestinationRule
metadata:
  name: order-service
spec:
  host: order-service
  trafficPolicy:
    connectionPool:
      tcp:
        maxConnections: 100
    outlierDetection:
      consecutive5xxErrors: 5
      interval: 30s
      baseEjectionTime: 30s

API Gateway & Service Mesh

API Gateway & Service Mesh

API Gateway

API Gateway Responsibilities

Rate Limiting Algorithms

Service Mesh (Istio/Linkerd)