Module 8 - API Gateway (Spring Cloud)

Lecture: API Gateway Patterns in Microservices Architecture

1. Introduction to API Gateway

1.1 What is an API Gateway?

An API Gateway is a server that acts as an entry point into a microservices-based application. It sits between client applications and your microservices, providing:

  • A single entry point for clients
  • Request routing to appropriate services
  • Cross-cutting concerns implementation (authentication, logging, etc.)
  • Response aggregation from multiple services

1.2 Why Use an API Gateway?

  • Simplifies Client Interaction: Clients only need to know about one endpoint
  • Decouples Clients from Services: Allows services to evolve independently
  • Centralized Cross-Cutting Concerns: Authentication, monitoring, and rate limiting in one place
  • Load Balancing: Distribution of traffic across service instances
  • Circuit Breaking: Preventing cascading failures

1.3 API Gateway Challenges

  • Single Point of Failure: Requires high availability implementation
  • Performance Overhead: Additional network hop
  • Maintenance Complexity: Needs careful management

2. Spring Cloud Gateway Overview

2.1 Spring Cloud Gateway Architecture

  • Non-blocking and reactive: Built on Spring WebFlux and Project Reactor
  • Route-based architecture: Routes requests based on predicates and filters
  • Integration with service discovery: Works seamlessly with Eureka

2.2 Core Components

  • Routes: Basic building blocks composed of:
    • ID: Unique identifier
    • Destination URI: Where to send the request
    • Predicates: Conditions to match the route
    • Filters: Modify requests and responses

2.3 Predicates

Predicates determine if a route matches based on:

  • Path
  • Host
  • Method
  • Headers
  • Query parameters
  • Cookies
  • Time-based conditions

2.4 Filters

Filters can modify requests before forwarding and responses before returning:

  • Path rewriting
  • Adding/removing headers
  • Rate limiting
  • Authentication/Authorization
  • Request size limiting
  • Circuit breaking

3. Circuit Breaking Patterns

3.1 The Circuit Breaker Pattern

  • Purpose: Prevent cascading failures across services
  • States: Closed, Open, Half-Open
  • Functionality: Detect failures and stop calling failing services

3.2 Circuit Breaker with Resilience4j

  • Integration: Spring Cloud Gateway + Resilience4j
  • Configuration: Setting thresholds, timeouts, and fallbacks
  • Monitoring: Circuit state visualization

3.3 Fallback Mechanisms

  • Default Responses: When services are unavailable
  • Cache Integration: Using cached data during outages
  • Degraded Functionality: Providing limited service

4. Advanced Gateway Features

4.1 Request Rate Limiting

  • Purpose: Protect services from overload
  • Implementation: Token bucket algorithm
  • Configuration: Per-client or global limits

4.2 Load Balancing Strategies

  • Round Robin: Default strategy
  • Weighted Round Robin: Preferences for certain instances
  • Least Connections: Route to less busy instances

4.3 Request Aggregation

  • Purpose: Combine responses from multiple services
  • Implementation: Using WebClient for reactive aggregation
  • Performance Considerations: Parallel vs. sequential requests

4.4 Metrics and Monitoring

  • Gateway-specific Metrics: Request counts, latencies
  • Circuit Breaker Metrics: Success/failure rates, open/closed status
  • Integration with Monitoring Systems: Prometheus, Grafana

Quiz: API Gateway and Circuit Breaking

  1. What is the primary purpose of an API Gateway in a microservices architecture? a) To store service configurations b) To provide a single entry point for all client requests c) To replace the service discovery mechanism d) To implement service-side load balancing

  2. Which of the following is NOT a typical function of an API Gateway? a) Request routing b) Authentication and authorization c) Service implementation d) Response transformation

  3. In the circuit breaker pattern, what happens in the “Open” state? a) All requests are forwarded to the service b) Requests are rejected immediately without calling the service c) Requests are queued until the service recovers d) Only a percentage of requests are forwarded to test service health

  4. Which component in Spring Cloud Gateway determines if a route should be taken? a) Filters b) Predicates c) Handlers d) Matchers

  5. What is the benefit of using a reactive API Gateway like Spring Cloud Gateway? a) It uses less memory for handling connections b) It automatically scales services c) It provides better security features d) It implements service discovery

  6. In the context of API Gateways, what is “request collapsing”? a) Rejecting duplicate requests b) Combining multiple client requests into a single backend request c) Compressing request payloads d) Simplifying complex request structures

  7. Which circuit breaker state allows a limited number of test requests through to check if the service has recovered? a) Closed b) Open c) Half-Open d) Testing

  8. What is rate limiting used for in an API Gateway? a) To prioritize certain types of requests b) To protect services from being overwhelmed c) To reduce network bandwidth usage d) To implement fair usage policies

  9. Which of the following is a valid predicate in Spring Cloud Gateway? a) After=2023-01-01T00:00:00Z b) RequestSize=1MB c) ClientIP=192.168.1.1 d) Priority=High

  10. What happens when a request matches multiple routes in Spring Cloud Gateway? a) All matching routes process the request in parallel b) An error is thrown c) The first matching route in order of declaration is chosen d) The most specific route is chosen based on predicate complexity

Answers to Quiz

  1. b) To provide a single entry point for all client requests
  2. c) Service implementation
  3. b) Requests are rejected immediately without calling the service
  4. b) Predicates
  5. a) It uses less memory for handling connections
  6. b) Combining multiple client requests into a single backend request
  7. c) Half-Open
  8. b) To protect services from being overwhelmed
  9. a) After=2023-01-01T00:00:00Z
  10. c) The first matching route in order of declaration is chosen

By Wahid Hamdi