Microservices Architecture Patterns

Microservices questions dominate modern architect interviews. Understanding decomposition strategies, communication patterns, and trade-offs is essential.

Microservices vs Monolith

Decision Framework

Factor	Monolith Wins	Microservices Wins
Team Size	< 20 engineers	> 30 engineers
Domain Complexity	Single domain	Multiple bounded contexts
Deployment Cadence	Weekly releases	Daily/hourly releases
Scale Requirements	Uniform scaling	Component-specific scaling
Organizational Structure	Single team	Multiple autonomous teams

Interview Question: When to Use Microservices

Q: "A startup has 8 engineers and wants to adopt microservices. What's your advice?"

A: Generally advise against microservices for small teams:

Reasons:

1. Operational Overhead: Each service needs deployment, monitoring, logging
2. Network Complexity: Distributed tracing, latency, failure handling
3. Cognitive Load: 8 engineers can't deeply own many services
4. Development Speed: Monolith allows faster initial development

Recommendation:

• Start with a modular monolith (clear internal boundaries)
• Use dependency injection and interfaces
• Extract services only when specific scaling needs emerge
• First candidates: high-scale features, different tech requirements

Service Decomposition

Domain-Driven Design (DDD) Approach

Bounded Contexts:

E-commerce Platform:
├── Catalog Context (Product, Category, Search)
├── Order Context (Order, Cart, Checkout)
├── Payment Context (Payment, Refund)
├── Shipping Context (Shipment, Tracking)
└── User Context (User, Authentication, Profile)

Decomposition Heuristics:

• Single Responsibility: Each service does one thing well
• Team Ownership: One team can own and deploy independently
• Business Capability: Aligned with business function
• Data Ownership: Each service owns its data

Interview Question: Service Boundaries

Q: "You're designing an e-commerce platform. Where do you draw service boundaries?"

A: Apply decomposition principles:

Service: Order Service
Responsibilities:
  - Order lifecycle (create, update, cancel)
  - Cart management
  - Order validation

Does NOT include:
  - Payment processing (Payment Service)
  - Inventory check (Catalog Service - query only)
  - Shipping (Shipping Service - event-driven)

Communication:
  - Sync: Query Catalog for price/availability
  - Async: Emit OrderCreated event → Payment, Inventory, Shipping

Communication Patterns

Synchronous Communication

REST over HTTP:

• Simple, widely understood
• Good for CRUD operations
• Challenge: Cascading failures

gRPC:

• Binary protocol, faster than REST
• Strong typing with Protocol Buffers
• Bidirectional streaming
• Better for internal service-to-service

Asynchronous Communication

Message Queues (SQS, RabbitMQ):

Producer → Queue → Consumer

• Point-to-point communication
• Load leveling
• Guaranteed delivery

Event Streaming (Kafka, Kinesis):

Producer → Topic → Multiple Consumers

• Publish-subscribe pattern
• Event replay capability
• Real-time processing

Interview Question: Sync vs Async

Q: "When would you choose async over sync communication?"

A:

Use Case	Communication Style	Reason
Get product details	Sync (REST/gRPC)	User waiting, need immediate response
Place order	Async (Event)	Can background process, send confirmation
Send notification	Async (Queue)	User doesn't wait for email delivery
Payment processing	Sync (with timeout)	Need immediate result for checkout
Update search index	Async (Event)	Eventually consistent is acceptable

Saga Pattern for Distributed Transactions

The Problem

Microservices can't use traditional ACID transactions across services.

Choreography-Based Saga

Each service listens for events and acts:

Order Service → OrderCreated Event
    ↓
Payment Service → PaymentCompleted Event
    ↓
Inventory Service → InventoryReserved Event
    ↓
Shipping Service → ShipmentCreated Event

Pros: Decoupled, scalable Cons: Hard to track overall flow, complex failure handling

Orchestration-Based Saga

Central orchestrator coordinates:

Saga Orchestrator:
  1. Call Order Service → Create Order
  2. Call Payment Service → Process Payment
  3. Call Inventory Service → Reserve Items
  4. Call Shipping Service → Create Shipment

On Failure at step 3:
  - Compensate: Refund Payment
  - Compensate: Cancel Order

Pros: Clear flow, easier debugging Cons: Orchestrator is single point of complexity

Interview Question: Saga Compensation

Q: "What happens if the Shipping Service fails after payment succeeds?"

A: Implement compensating transactions:

Happy Path:
  Order → Payment → Inventory → Shipping ✓

Failure at Shipping:
  Order ✓ → Payment ✓ → Inventory ✓ → Shipping ✗

Compensation:
  1. Release Inventory (compensate reservation)
  2. Refund Payment (compensate charge)
  3. Cancel Order (or mark as failed)
  4. Notify User (explain failure)

Key Principles:

• Compensations must be idempotent
• Store saga state for recovery
• Consider timeouts and retries before compensation

API Gateway Pattern

Responsibilities

• Request routing
• Authentication/Authorization
• Rate limiting
• Request transformation
• Response aggregation
• Caching

API Gateway Options

Service	Cloud	Features
Amazon API Gateway	AWS	Lambda integration, throttling
Apigee	GCP	Analytics, developer portal
Azure API Management	Azure	Policies, developer portal
Kong	Multi-cloud	Plugin ecosystem, open source
AWS ALB	AWS	Simple routing, cost-effective

Interview Question: BFF Pattern

Q: "Explain the Backend for Frontend (BFF) pattern."

A: Create specialized API gateways per client type:

Mobile App → Mobile BFF → Microservices
Web App → Web BFF → Microservices
Partner API → Partner BFF → Microservices

Benefits:

• Optimized payloads per client (mobile gets less data)
• Client-specific auth flows
• Independent evolution

When to Use:

• Significantly different client requirements
• Multiple teams own different clients
• Performance optimization needs

Architecture Principle: Start with a single API Gateway, extract BFFs only when client divergence justifies the overhead.

Next, we'll explore event-driven architecture patterns. :::