High-Level Architecture Patterns

This helps candidates see the big patterns they will repeatedly use in real products and interviews.

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Why Architecture Patterns Matter

Architecture patterns are reusable solutions to common problems:

• Speed: Don't reinvent the wheel
• Best practices: Patterns encapsulate lessons learned
• Communication: Common vocabulary for discussing designs
• Scalability: Patterns designed for scale

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Pattern 1: Event-Driven Architecture

What It Is

Event-driven architecture uses events to trigger and communicate between services:

• Services publish events
• Other services subscribe to events
• Loose coupling between services
• Asynchronous processing

Architecture

graph TB
    Service1[Service 1] --> EventBus[Event Bus]
    Service2[Service 2] --> EventBus
    Service3[Service 3] --> EventBus
    EventBus --> Service4[Service 4]
    EventBus --> Service5[Service 5]
    EventBus --> Service6[Service 6]

When to Use

• Real-time updates: Prices, notifications, feeds
• Decoupled services: Services don't need to know about each other
• High volume: Millions of events per second
• Event sourcing: Need to replay events

Example: E-commerce Order System

Events:

• OrderCreated → Triggers payment, inventory, notification
• PaymentProcessed → Triggers shipping, notification
• OrderShipped → Triggers notification, analytics

Benefits:

• Services are decoupled (order service doesn't know about shipping)
• Can add new services (analytics) without changing existing ones
• Handles high volume (millions of orders per day)

Trade-offs

• ✅ Loose coupling
• ✅ Scalability
• ✅ Flexibility
• ❌ Eventual consistency
• ❌ Complex debugging
• ❌ Event ordering challenges

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Pattern 2: Layered Architecture

What It Is

Layered architecture organizes code into layers:

• Presentation Layer: UI, API
• Business Layer: Business logic
• Data Layer: Database, cache
• Each layer depends on layers below

Architecture

graph TB
    Presentation[Presentation Layer] --> Business[Business Layer]
    Business --> Data[Data Layer]

When to Use

• Simple systems: Clear separation of concerns
• Small teams: Easy to understand
• Monoliths: Good for monoliths
• Traditional applications: Web apps, APIs

Example: E-commerce API

Layers:

• Presentation: REST API, request/response handling
• Business: Order processing, payment logic, inventory management
• Data: Database access, caching, file storage

Benefits:

• Clear separation of concerns
• Easy to understand
• Good for small teams

Trade-offs

• ✅ Simple and clear
• ✅ Easy to understand
• ✅ Good for monoliths
• ❌ Can become bloated
• ❌ Hard to scale layers independently
• ❌ Tight coupling between layers

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Pattern 3: CQRS (Command Query Responsibility Segregation)

What It Is

CQRS separates reads and writes:

• Command Side: Handles writes (mutations)
• Query Side: Handles reads (queries)
• Different models for reads and writes
• Can use different databases

Architecture

graph TB
    Client[Client] --> Command[Command Side]
    Client --> Query[Query Side]
    Command --> WriteDB[(Write Database)]
    Query --> ReadDB[(Read Database)]
    WriteDB --> Sync[Sync]
    Sync --> ReadDB

When to Use

• Read/write asymmetry: Many more reads than writes
• Different query patterns: Complex queries for reads, simple writes
• Scale reads independently: Need to scale reads separately
• Event sourcing: Want to replay events

Example: Social Media Feed

Command Side:

• Create post (write to database)
• Like post (update database)
• Simple writes

Query Side:

• Get feed (complex query, join multiple tables)
• Get user posts (filter, sort, paginate)
• Complex reads

Benefits:

• Scale reads independently (add read replicas)
• Optimize reads (denormalized data, materialized views)
• Optimize writes (simple, normalized data)

Trade-offs

• ✅ Scale reads/writes independently
• ✅ Optimize each side separately
• ✅ Handle read/write asymmetry
• ❌ Data synchronization complexity
• ❌ Eventual consistency
• ❌ More infrastructure

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Pattern 4: Microservices

What It Is

Microservices break application into small, independent services:

• Each service owns a domain
• Independent deployment
• Independent scaling
• Different technologies per service

Architecture

graph TB
    Client[Client] --> Gateway[API Gateway]
    Gateway --> Service1[User Service]
    Gateway --> Service2[Order Service]
    Gateway --> Service3[Payment Service]
    Gateway --> Service4[Shipping Service]
    Service1 --> DB1[(User DB)]
    Service2 --> DB2[(Order DB)]
    Service3 --> DB3[(Payment DB)]
    Service4 --> DB4[(Shipping DB)]

When to Use

• Large teams: Multiple teams work independently
• Different scaling needs: Services scale differently
• Technology diversity: Different stacks per service
• Fault isolation: Service failure doesn't affect others

Example: E-commerce Platform

Services:

• User Service: Authentication, profiles
• Product Service: Catalog, search
• Order Service: Order management
• Payment Service: Payment processing
• Shipping Service: Shipping, tracking

Benefits:

• Teams work independently
• Scale services based on demand
• Use different technologies
• Fault isolation

Trade-offs

• ✅ Independent scaling
• ✅ Technology freedom
• ✅ Team autonomy
• ✅ Fault isolation
• ❌ Increased complexity
• ❌ Network latency
• ❌ Distributed system challenges
• ❌ Operational overhead

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Pattern 5: Real-Time Systems

What It Is

Real-time systems process and respond to events immediately:

• WebSocket connections
• Server-sent events
• Message queues
• Low latency (< 100ms)

Architecture

graph TB
    Client[Client] --> WS[WebSocket Gateway]
    WS --> Queue[Message Queue]
    Queue --> Service[Real-Time Service]
    Service --> DB[(Database)]
    Service --> Cache[Cache]

When to Use

• Chat applications: Real-time messaging
• Collaborative editing: Google Docs, Figma
• Live updates: Stock prices, sports scores
• Gaming: Real-time multiplayer

Example: Chat Application

Components:

• WebSocket Gateway: Handles connections
• Message Queue: Routes messages
• Chat Service: Processes messages
• Presence Service: Tracks online/offline users

Benefits:

• Real-time communication
• Low latency
• Bidirectional communication

Trade-offs

• ✅ Real-time communication
• ✅ Low latency
• ✅ Bidirectional
• ❌ Connection management complexity
• ❌ Scale challenges (millions of connections)
• ❌ State management

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Pattern 6: Batch-Processing Systems

What It Is

Batch-processing systems process large volumes of data in batches:

• Scheduled jobs
• ETL pipelines
• Data warehouses
• Analytics processing

Architecture

graph TB
    Source[Data Source] --> Queue[Message Queue]
    Queue --> Worker1[Worker 1]
    Queue --> Worker2[Worker 2]
    Queue --> Worker3[Worker 3]
    Worker1 --> DB[(Database)]
    Worker2 --> DB
    Worker3 --> DB

When to Use

• ETL pipelines: Extract, transform, load
• Analytics: Process large datasets
• Reports: Generate daily/weekly reports
• Data migration: Move large amounts of data

Example: Analytics Pipeline

Components:

• Data Source: Application logs, events
• Message Queue: Kafka (buffers events)
• Workers: Process events in parallel
• Data Warehouse: Store processed data

Benefits:

• Process large volumes
• Parallel processing
• Fault tolerant (retry on failure)

Trade-offs

• ✅ Handle large volumes
• ✅ Parallel processing
• ✅ Fault tolerant
• ❌ Not real-time (batch processing)
• ❌ Latency (process in batches)
• ❌ Complex orchestration

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How to Choose a Pattern

Decision Framework

1. Understand requirements: What are you building?
2. Identify constraints: Scale, latency, team size
3. Consider trade-offs: Pros and cons of each pattern
4. Start simple: Don't over-engineer
5. Evolve: Patterns can evolve as system grows

Example: Choosing a Pattern

Problem: Build a social media feed

Requirements:

• Real-time updates
• High scale (millions of users)
• Complex queries (join, filter, sort)

Pattern Choice:

• Event-driven: For real-time updates (new posts trigger feed updates)
• CQRS: For read/write asymmetry (many reads, fewer writes)
• Microservices: For scale (feed service, post service, user service)

Combination: Use multiple patterns together

• Event-driven for updates
• CQRS for reads/writes
• Microservices for scale

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Thinking Aloud Like a Senior Engineer

Let me walk you through how I'd actually choose and combine architecture patterns for a real problem. This is the messy reasoning that happens before you settle on a design.

Problem: "Design a social media platform with feeds, messaging, and real-time notifications."

My first instinct: "I'll use microservices! That's what everyone does for complex systems, right?"

But wait—that's pattern-driven thinking. Let me step back and think about what I actually need.

What are the requirements?

• Social feeds (posts from users you follow)
• Messaging (1-on-1 and group chats)
• Real-time notifications
• Scale: 100M users, billions of posts

Let me think about patterns one by one:

For feeds: "I need to show posts from users you follow, sorted by time. This is read-heavy, complex queries."

My first thought: "I'll use CQRS. Separate read and write models. Pre-compute feeds for fast reads."

But is that necessary? "Actually, I could start with a simple database query. JOIN users and posts, sort by time. It works for MVP."

I'm thinking: "Let me start simple, then add CQRS if I need it. This avoids over-engineering."

For messaging: "I need real-time delivery. Users send messages, recipients get them immediately."

My first thought: "I'll use event-driven architecture. Message sent → event published → recipient receives."

Actually, that makes sense: "Messaging is inherently event-driven. A message is an event. Multiple services can react (notifications, analytics, etc.)."

I'm choosing event-driven for messaging: "Because messaging is event-based, and I need loose coupling between services."

For real-time notifications: "I need to push notifications to users in real-time."

My first thought: "I'll use WebSocket. Real-time, bidirectional communication."

But how do I scale WebSockets? "I can't have millions of connections on a single server. I need a WebSocket gateway pattern."

I'm thinking: "Multiple WebSocket servers, message queue to route notifications, presence service to track which server has which user."

Now, should I use microservices? "Let me think about the services:

• Feed service
• Messaging service
• Notification service
• User service"

My first instinct: "Yes! Separate services, independent scaling, team autonomy."

But wait—what's the team size? "If it's a small team (5 engineers), microservices add too much complexity. If it's a large team (50+ engineers), microservices make sense."

Assuming small team: "I'll start with a monolith, split into services later if needed. This is the trade-off: simpler now, potential refactoring later."

But for messaging and notifications, I need real-time: "These are inherently distributed. I can't do real-time in a monolith easily."

I'm thinking: "Maybe a hybrid approach. Monolith for feeds (can start simple), microservices for messaging and notifications (need real-time)."

This is the trade-off I'm making: Use different patterns for different parts. Monolith for simple parts, microservices for complex parts.

Final architecture:

• Feeds: Monolith (start simple), can evolve to CQRS if needed
• Messaging: Event-driven microservice (inherently event-based)
• Notifications: Real-time microservice with WebSocket gateway
• Users: Part of monolith initially, can split later

This is how I combine patterns: I don't use one pattern everywhere. I choose patterns based on requirements, not popularity.

Notice how I didn't jump to "microservices everywhere." I thought about each part separately, chose patterns based on needs, and combined them appropriately.

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How a Senior Engineer Uses Patterns

A senior engineer:

1. Knows patterns: Understands common patterns and when to use them
2. Combines patterns: Uses multiple patterns together
3. Adapts patterns: Modifies patterns to fit context
4. Starts simple: Doesn't over-engineer
5. Evolves: Patterns evolve as system grows

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Best Practices

1. Know common patterns: Understand when to use each
2. Combine patterns: Use multiple patterns together
3. Adapt to context: Modify patterns to fit your needs
4. Start simple: Don't over-engineer
5. Evolve: Patterns can evolve as system grows
6. Document: Explain why you chose a pattern

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Common Interview Questions

Beginner

Q: What is event-driven architecture?

A: Event-driven architecture uses events to trigger and communicate between services. Services publish events, and other services subscribe to events. This creates loose coupling and enables asynchronous processing.

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Intermediate

Q: When would you use CQRS?

A: I'd use CQRS when:

• Read/write asymmetry (many more reads than writes)
• Different query patterns (complex reads, simple writes)
• Need to scale reads independently
• Want to optimize reads and writes separately

Example: Social media feed (many reads, fewer writes, complex queries).

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Senior

Q: How would you combine multiple architecture patterns in a single system?

A: I'd combine patterns based on requirements:

Example: E-commerce Platform

• Microservices: Break into services (user, product, order, payment)
• Event-driven: Services communicate via events (order created → payment, shipping)
• CQRS: Separate reads/writes (complex product queries, simple order writes)
• Real-time: WebSocket for order updates
• Batch processing: Daily analytics, reports

Each pattern solves a specific problem, and they work together to create a complete system.

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Summary

Architecture patterns are reusable solutions to common problems:

• Event-Driven: Real-time updates, decoupled services
• Layered: Simple systems, clear separation
• CQRS: Read/write asymmetry, independent scaling
• Microservices: Large teams, independent scaling
• Real-Time: Chat, collaborative editing, live updates
• Batch Processing: ETL, analytics, reports

Key takeaways:

• Know common patterns and when to use them
• Combine patterns for complex systems
• Adapt patterns to fit your context
• Start simple, evolve as needed
• Document your pattern choices

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FAQs

Q: Do I need to know all patterns?

A: Not all, but know the common ones (event-driven, microservices, CQRS, real-time). Understand when to use each and how they work together.

Q: Can I use multiple patterns together?

A: Yes. Most systems use multiple patterns. For example, microservices with event-driven communication and CQRS for reads/writes.

Q: How do I choose a pattern?

A: Consider:

• Requirements (scale, latency, team size)
• Constraints (budget, timeline, expertise)
• Trade-offs (complexity vs benefits)
• Start simple, evolve as needed

Q: What if a pattern doesn't fit my problem?

A: Adapt it. Patterns are starting points, not rigid rules. Modify to fit your context.

Q: Should I use microservices from day 1?

A: Usually no. Start with a monolith, evolve to microservices when needed. Don't over-engineer.

Q: How do I learn patterns?

A:

• Study real-world systems (Instagram, Netflix, Uber)
• Read architecture books
• Practice designing systems
• Get feedback from peers

Q: Are patterns the same as design patterns?

A: Similar but different. Architecture patterns are high-level (system structure), while design patterns are low-level (code structure). Both are useful.