Microservices Architecture

Comprehensive guide for designing and implementing microservices-based systems.

Microservices Fundamentals

What are Microservices?

MICROSERVICES = Independently deployable services
               that do one thing well

Characteristics:
┌─────────────────────────────────────────┐
│ ✓ Single responsibility                 │
│ ✓ Own their data                        │
│ ✓ Independently deployable              │
│ ✓ Communicate via APIs                  │
│ ✓ Technology agnostic                   │
│ ✓ Owned by small teams                  │
└─────────────────────────────────────────┘

Monolith vs Microservices

MONOLITH:
┌─────────────────────────────────────────┐
│           Single Application            │
│  ┌──────┬──────┬──────┬──────┐         │
│  │ Users│Orders│ Cart │Search│         │
│  └──────┴──────┴──────┴──────┘         │
│           Single Database               │
└─────────────────────────────────────────┘

MICROSERVICES:
┌──────┐  ┌──────┐  ┌──────┐  ┌──────┐
│Users │  │Orders│  │ Cart │  │Search│
│ DB   │  │ DB   │  │ DB   │  │ DB   │
└──────┘  └──────┘  └──────┘  └──────┘
    │         │         │         │
    └─────────┴─────────┴─────────┘
              API Gateway

When to Use Microservices

USE WHEN:
✓ Large, complex domain
✓ Need independent scaling
✓ Multiple teams working in parallel
✓ Different technology needs per service
✓ Fault isolation is critical
✓ Frequent, independent deployments

DON'T USE WHEN:
✗ Small team/application
✗ Simple domain
✗ Tight latency requirements
✗ Limited DevOps maturity
✗ Unclear domain boundaries

---

Service Design

Domain-Driven Design

BOUNDED CONTEXTS:
┌─────────────────┐  ┌─────────────────┐
│   Orders        │  │   Shipping      │
│  Context        │  │   Context       │
│ ┌─────────────┐ │  │ ┌─────────────┐ │
│ │ Order       │ │  │ │ Shipment    │ │
│ │ LineItem    │ │  │ │ Carrier     │ │
│ │ Customer(ID)│ │  │ │ Address     │ │
│ └─────────────┘ │  │ └─────────────┘ │
└─────────────────┘  └─────────────────┘

Each context has its own:
- Ubiquitous language
- Data model
- Business rules

Service Boundaries

GOOD boundaries follow:
┌─────────────────────────────────────────┐
│ Business Capability                     │
│ - What the business does                │
│ - e.g., Payment Processing, Inventory   │
├─────────────────────────────────────────┤
│ Subdomain                               │
│ - Area of expertise                     │
│ - e.g., Pricing, Catalog, Customer      │
├─────────────────────────────────────────┤
│ Single Responsibility                   │
│ - Does one thing well                   │
│ - Can explain in one sentence           │
└─────────────────────────────────────────┘

BAD boundaries:
✗ Technical layers (UI service, DB service)
✗ CRUD operations (User CRUD service)
✗ Too granular (EmailSender service)

Service Size Guidelines

Right-sized service:
- 2-pizza team can own it (5-8 people)
- Rewrite in 2-4 weeks if needed
- Clear, single business purpose
- Minimal external dependencies
- Own its data completely

Too big: Multiple teams needed, mixed concerns
Too small: Can't function independently

---

Communication Patterns

Synchronous (Request/Response)

REST:
┌──────┐  HTTP GET /users/123  ┌──────┐
│Client│ ─────────────────────>│Server│
│      │<───────────────────── │      │
└──────┘  { "name": "John" }   └──────┘

gRPC:
┌──────┐  Binary/Protobuf      ┌──────┐
│Client│ ─────────────────────>│Server│
│      │<───────────────────── │      │
└──────┘  Strongly typed       └──────┘

GraphQL:
┌──────┐  POST /graphql        ┌──────┐
│Client│ ─────────────────────>│Server│
│      │<───────────────────── │      │
└──────┘  Flexible queries     └──────┘

Asynchronous (Event-Driven)

MESSAGE QUEUE:
┌────────┐      ┌─────────┐      ┌────────┐
│Producer│ ───> │  Queue  │ ───> │Consumer│
└────────┘      │(RabbitMQ│      └────────┘
                │ SQS)    │
                └─────────┘

EVENT STREAMING:
┌────────┐      ┌─────────┐      ┌────────┐
│Producer│ ───> │  Topic  │ ───> │Consumer│
└────────┘      │ (Kafka) │ ───> │Consumer│
                └─────────┘ ───> │Consumer│
                                 └────────┘

Communication Comparison

Pattern	Use Case	Trade-offs
REST	CRUD, simple queries	Simple, but chatty
gRPC	High performance, internal	Fast, but complex
GraphQL	Flexible client needs	Flexible, but overhead
Message Queue	Task processing	Decoupled, but delay
Event Streaming	Event sourcing, analytics	Scalable, but complex

---

Data Management

Database per Service

SEPARATE DATABASES:
┌────────┐  ┌────────┐  ┌────────┐
│Users   │  │Orders  │  │Products│
│Service │  │Service │  │Service │
├────────┤  ├────────┤  ├────────┤
│PostgreSQL│ │MongoDB │  │MySQL   │
└────────┘  └────────┘  └────────┘

Benefits:
✓ Independent scaling
✓ Technology freedom
✓ No shared schema coupling
✓ Fault isolation

Challenges:
- No joins across services
- Eventual consistency
- Data duplication

Data Consistency Patterns

SAGA PATTERN (Choreography):
┌──────┐      ┌──────┐      ┌──────┐
│Order │─────>│Payment│─────>│Ship  │
│Create│      │Process│      │Order │
└──────┘      └──────┘      └──────┘
    │             │             │
    └─────────────┴─────────────┘
    Each service publishes events
    that trigger next step

SAGA PATTERN (Orchestration):
           ┌────────────┐
           │Orchestrator│
           └────────────┘
          /      │      \
         ↓       ↓       ↓
    ┌──────┐ ┌──────┐ ┌──────┐
    │Order │ │Payment│ │Ship  │
    └──────┘ └──────┘ └──────┘
    Central coordinator manages flow

Event Sourcing

Instead of storing current state:
┌────────────────────────────┐
│ Account: $500              │
└────────────────────────────┘

Store all events:
┌────────────────────────────┐
│ 1. AccountCreated $0       │
│ 2. Deposited $1000         │
│ 3. Withdrawn $300          │
│ 4. Withdrawn $200          │
│ Current: $500              │
└────────────────────────────┘

Benefits:
✓ Complete audit trail
✓ Temporal queries
✓ Event replay
✓ Natural fit for CQRS

---

API Gateway

Gateway Pattern

               ┌─────────────┐
               │ API Gateway │
               └──────┬──────┘
        ┌──────────┬──┴──┬──────────┐
        ↓          ↓     ↓          ↓
   ┌────────┐ ┌────────┐ ┌────────┐ ┌────────┐
   │Users   │ │Orders  │ │Products│ │Auth    │
   │Service │ │Service │ │Service │ │Service │
   └────────┘ └────────┘ └────────┘ └────────┘

Gateway responsibilities:
- Request routing
- Authentication/Authorization
- Rate limiting
- Load balancing
- Request/Response transformation
- Caching
- Monitoring

BFF (Backend for Frontend)

              Mobile App        Web App
                  │                │
                  ↓                ↓
          ┌────────────┐   ┌────────────┐
          │ Mobile BFF │   │  Web BFF   │
          └─────┬──────┘   └─────┬──────┘
                │                │
         ┌──────┴────────────────┴──────┐
         │         Internal APIs         │
         └───────────────────────────────┘

Each BFF:
- Optimized for its client
- Aggregates multiple services
- Handles client-specific logic

---

Service Discovery

Client-Side Discovery

┌──────┐     ┌──────────────┐
│Client│────>│Service       │────> Service A (192.168.1.10)
│      │     │Registry      │────> Service A (192.168.1.11)
│      │<────│(Consul, etcd)│────> Service A (192.168.1.12)
└──────┘     └──────────────┘

Client queries registry, then calls service directly.
Client handles load balancing.

Server-Side Discovery

┌──────┐     ┌─────────────┐     ┌──────────────┐
│Client│────>│Load Balancer│────>│Service       │
│      │     │             │     │Registry      │
└──────┘     └─────────────┘     └──────────────┘
                                        │
                               ┌────────┴────────┐
                               ↓        ↓        ↓
                            Service  Service  Service

Load balancer handles discovery and routing.
Simpler for clients.

---

Resilience Patterns

Circuit Breaker

States:
┌────────┐    Failures    ┌────────┐
│ CLOSED │───────────────>│  OPEN  │
│(normal)│                │(reject)│
└────────┘                └────────┘
    ↑                          │
    │      Timeout             │
    │   ┌────────────┐         │
    └───│HALF-OPEN   │<────────┘
        │(test)      │
        └────────────┘

Implementation:
- Track failure count
- Open circuit after threshold
- Reject calls while open
- Periodically test with half-open
- Close circuit on success

Retry Pattern

async function withRetry<T>(
  fn: () => Promise<T>,
  maxAttempts: number = 3,
  backoff: number = 1000,
): Promise<T> {
  for (let attempt = 1; attempt <= maxAttempts; attempt++) {
    try {
      return await fn();
    } catch (error) {
      if (attempt === maxAttempts) throw error;

      // Exponential backoff with jitter
      const delay = backoff * Math.pow(2, attempt - 1);
      const jitter = delay * 0.1 * Math.random();
      await sleep(delay + jitter);
    }
  }
}

Bulkhead Pattern

Isolate resources to prevent cascade:

┌─────────────────────────────────────┐
│           Service A                 │
│  ┌─────────┐  ┌─────────┐          │
│  │Thread   │  │Thread   │          │
│  │Pool 1   │  │Pool 2   │          │
│  │(Service │  │(Service │          │
│  │   B)    │  │   C)    │          │
│  └─────────┘  └─────────┘          │
└─────────────────────────────────────┘

If Service C is slow, only Pool 2 is affected.
Service B calls continue normally.

Timeout Pattern

async function withTimeout<T>(fn: () => Promise<T>, ms: number): Promise<T> {
  return Promise.race([
    fn(),
    new Promise<T>((_, reject) =>
      setTimeout(() => reject(new Error("Timeout")), ms),
    ),
  ]);
}

// Always set timeouts on external calls
const user = await withTimeout(
  () => userService.getUser(id),
  5000, // 5 second timeout
);

---

Observability

The Three Pillars

LOGS:
┌─────────────────────────────────────────┐
│ 2024-01-15 10:30:45 [INFO] OrderService │
│ Order created: { id: 123, user: 456 }   │
└─────────────────────────────────────────┘

METRICS:
┌─────────────────────────────────────────┐
│ order_created_total: 1523               │
│ order_processing_seconds: 0.234         │
│ active_connections: 45                  │
└─────────────────────────────────────────┘

TRACES:
┌─────────────────────────────────────────┐
│ [Request ID: abc123]                    │
│ ├─ Gateway: 2ms                         │
│ ├─ OrderService: 150ms                  │
│ │  ├─ UserService: 45ms                 │
│ │  └─ InventoryService: 80ms            │
│ └─ Total: 152ms                         │
└─────────────────────────────────────────┘

Distributed Tracing

Trace Context Propagation:
┌──────┐  X-Trace-ID: abc  ┌──────┐
│API   │─────────────────>│Order │
│GW    │                   │Svc   │
└──────┘                   └──────┘
                              │
              X-Trace-ID: abc │
                              ↓
                           ┌──────┐
                           │User  │
                           │Svc   │
                           └──────┘

Tools: Jaeger, Zipkin, AWS X-Ray, Datadog

Health Checks

// Liveness: Is the service running?
app.get("/health/live", (req, res) => {
  res.status(200).json({ status: "alive" });
});

// Readiness: Is the service ready to handle traffic?
app.get("/health/ready", async (req, res) => {
  const dbHealthy = await checkDatabase();
  const cacheHealthy = await checkCache();

  if (dbHealthy && cacheHealthy) {
    res.status(200).json({ status: "ready" });
  } else {
    res.status(503).json({
      status: "not ready",
      checks: { database: dbHealthy, cache: cacheHealthy },
    });
  }
});

---

Deployment

Containerization

# Dockerfile
FROM node:18-alpine

WORKDIR /app

COPY package*.json ./
RUN npm ci --only=production

COPY dist ./dist

USER node

EXPOSE 3000

CMD ["node", "dist/main.js"]

Kubernetes Basics

# deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: order-service
spec:
  replicas: 3
  selector:
    matchLabels:
      app: order-service
  template:
    metadata:
      labels:
        app: order-service
    spec:
      containers:
        - name: order-service
          image: order-service:1.0.0
          ports:
            - containerPort: 3000
          resources:
            limits:
              cpu: "500m"
              memory: "256Mi"
          livenessProbe:
            httpGet:
              path: /health/live
              port: 3000
          readinessProbe:
            httpGet:
              path: /health/ready
              port: 3000

---

Testing Strategies

Test Pyramid for Microservices

              /\
             /  \  E2E Tests
            /────\  (Few, slow, brittle)
           /      \
          /────────\  Contract Tests
         /          \  (Service boundaries)
        /────────────\
       /              \  Integration Tests
      /────────────────\  (With dependencies)
     /                  \
    /────────────────────\  Unit Tests
   /                      \  (Many, fast, isolated)
  /________________________\

Contract Testing

// Consumer test (Order Service)
describe("User Service Contract", () => {
  it("returns user by ID", async () => {
    // Define expected interaction
    await provider.addInteraction({
      state: "user 123 exists",
      uponReceiving: "a request for user 123",
      withRequest: {
        method: "GET",
        path: "/users/123",
      },
      willRespondWith: {
        status: 200,
        body: {
          id: "123",
          name: like("John"),
          email: like("john@example.com"),
        },
      },
    });

    // Test passes if consumer expectations match
  });
});

---

Best Practices

DO:

• Start with a monolith, extract services later
• Define clear service boundaries
• Use asynchronous communication where possible
• Implement circuit breakers
• Centralize logging and monitoring
• Automate everything
• Design for failure
• Version your APIs

DON'T:

• Create too many, too small services
• Share databases between services
• Make synchronous chains too deep
• Ignore distributed system complexities
• Couple services through shared libraries
• Skip contract testing
• Deploy without monitoring

---

Migration Checklist

Monolith to Microservices

• Map domain boundaries clearly
• Identify candidate services (start small)
• Establish CI/CD for new services
• Implement API gateway
• Set up service discovery
• Add distributed tracing
• Implement circuit breakers
• Extract first service (strangler fig pattern)
• Test extensively
• Monitor and iterate