┌─────────────────────────────────────────────────────────────┐
│                    Monolithic Application                    │
├─────────────────────────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐         │
│  │  Web UI     │  │  API Layer  │  │  Admin UI   │         │
│  └──────┬──────┘  └──────┬──────┘  └──────┬──────┘         │
│         │                │                │                 │
│         └────────────────┼────────────────┘                 │
│                          │                                  │
│  ┌───────────────────────┴───────────────────────────┐     │
│  │              Business Logic Layer                  │     │
│  │  ┌──────────┐  ┌──────────┐  ┌──────────┐        │     │
│  │  │ Orders   │  │ Users    │  │ Products │        │     │
│  │  └──────────┘  └──────────┘  └──────────┘        │     │
│  └───────────────────────┬───────────────────────────┘     │
│                          │                                  │
│  ┌───────────────────────┴───────────────────────────┐     │
│  │              Data Access Layer                     │     │
│  └───────────────────────┬───────────────────────────┘     │
│                          │                                  │
└──────────────────────────┼──────────────────────────────────┘
                           │
                    ┌──────┴──────┐
                    │  Database   │
                    └─────────────┘

When to Use:

Small team (< 10 developers)
Simple domain
Rapid iteration needed
Limited infrastructure expertise

Trade-offs:

Pros	Cons
Simple deployment	Limited scalability
Easy debugging	Large codebase to manage
Single codebase	Technology lock-in
Fast development initially	Team coupling
Transactional consistency	Full redeploy for changes

Microservices Architecture

Independently deployable services organized around business capabilities.

┌────────┐   ┌────────┐   ┌────────┐   ┌────────┐
│ Web UI │   │Mobile  │   │ Admin  │   │External│
└───┬────┘   └───┬────┘   └───┬────┘   └───┬────┘
    │            │            │            │
    └────────────┴────────────┴────────────┘
                       │
              ┌────────┴────────┐
              │   API Gateway   │
              └────────┬────────┘
                       │
    ┌──────────────────┼──────────────────┐
    │                  │                  │
┌───┴───┐         ┌────┴───┐         ┌───┴───┐
│ Order │         │ User   │         │Product│
│Service│         │Service │         │Service│
├───────┤         ├────────┤         ├───────┤
│  DB   │         │   DB   │         │  DB   │
└───────┘         └────────┘         └───────┘
    │                  │                  │
    └──────────────────┴──────────────────┘
                       │
              ┌────────┴────────┐
              │  Message Bus    │
              └─────────────────┘

When to Use:

Large team (> 20 developers)
Complex, evolving domain
Independent scaling needed
Different tech stacks for different services
High availability requirements

Trade-offs:

Pros	Cons
Independent deployment	Operational complexity
Technology flexibility	Network latency
Team autonomy	Distributed debugging
Targeted scaling	Data consistency challenges
Fault isolation	More infrastructure

Event-Driven Architecture

Services communicate through events rather than direct calls.

┌─────────────────────────────────────────────────────────────┐
│                      Event Bus / Broker                      │
├─────────────────────────────────────────────────────────────┤
│                                                             │
│   OrderPlaced    UserCreated    PaymentReceived             │
│                                                             │
└──────┬──────────────┬──────────────┬───────────────────────┘
       │              │              │
       ▼              ▼              ▼
┌─────────────┐ ┌─────────────┐ ┌─────────────┐
│   Order     │ │   User      │ │  Payment    │
│   Service   │ │   Service   │ │   Service   │
│             │ │             │ │             │
│ Publishes:  │ │ Publishes:  │ │ Publishes:  │
│ OrderPlaced │ │ UserCreated │ │ PaymentRcvd │
│             │ │             │ │             │
│ Subscribes: │ │ Subscribes: │ │ Subscribes: │
│ PaymentRcvd │ │ OrderPlaced │ │ OrderPlaced │
└─────────────┘ └─────────────┘ └─────────────┘

When to Use:

Loose coupling required
Asynchronous processing acceptable
Complex workflows spanning multiple services
Audit trail needed
Event sourcing scenarios

Trade-offs:

Pros	Cons
Temporal decoupling	Eventual consistency
Natural audit log	Complex debugging
Scalability	Message ordering challenges
Extensibility	Infrastructure requirements
Resilience	Learning curve

Serverless Architecture

Functions executed on-demand without managing servers.

┌────────────────────────────────────────────────────────────┐
│                         Client                              │
└────────────────────────────┬───────────────────────────────┘
                             │
┌────────────────────────────┴───────────────────────────────┐
│                      API Gateway                            │
└────────────────────────────┬───────────────────────────────┘
                             │
    ┌────────────────────────┼────────────────────────┐
    │                        │                        │
    ▼                        ▼                        ▼
┌──────────┐          ┌──────────┐          ┌──────────┐
│ Function │          │ Function │          │ Function │
│ GetUser  │          │CreateOrder│         │ SendEmail│
└────┬─────┘          └────┬─────┘          └────┬─────┘
     │                     │                     │
     ▼                     ▼                     ▼
┌──────────┐          ┌──────────┐          ┌──────────┐
│ Database │          │  Queue   │          │  Email   │
│          │          │          │          │ Service  │
└──────────┘          └──────────┘          └──────────┘

When to Use:

Variable/unpredictable workloads
Event-triggered processing
Cost optimization for low traffic
Rapid development needed
Short-running operations

Trade-offs:

Pros	Cons
No server management	Cold start latency
Pay-per-use	Execution time limits
Auto-scaling	Vendor lock-in
Rapid deployment	Complex local development
Reduced ops burden	Stateless constraints

Pattern Selection Guide

Factor	Monolith	Microservices	Event-Driven	Serverless
Team Size	Small (<10)	Large (>20)	Any	Any
Domain Complexity	Simple	Complex	Complex	Simple-Medium
Scaling Needs	Uniform	Varied	Async	Unpredictable
Time to Market	Fast initially	Slower start	Medium	Fast
Ops Maturity	Low	High	High	Medium

C4 Model

Hierarchical way to document architecture at multiple levels of detail.

Level 1: System Context

Shows system in its environment with external actors and systems.

┌──────────────────────────────────────────────────────────────┐
│                     System Context Diagram                    │
├──────────────────────────────────────────────────────────────┤
│                                                              │
│   ┌──────────┐                           ┌──────────┐       │
│   │ Customer │                           │  Admin   │       │
│   │  [User]  │                           │  [User]  │       │
│   └────┬─────┘                           └────┬─────┘       │
│        │                                      │              │
│        │     Places orders                    │ Manages      │
│        │                                      │ products     │
│        ▼                                      ▼              │
│   ┌────────────────────────────────────────────────┐        │
│   │              E-Commerce System                  │        │
│   │                [Software System]               │        │
│   └───────────┬─────────────────┬──────────────────┘        │
│               │                 │                            │
│               │                 │                            │
│               ▼                 ▼                            │
│   ┌───────────────┐    ┌───────────────┐                    │
│   │ Payment       │    │ Email         │                    │
│   │ Gateway       │    │ Provider      │                    │
│   │ [External]    │    │ [External]    │                    │
│   └───────────────┘    └───────────────┘                    │
│                                                              │
└──────────────────────────────────────────────────────────────┘

Level 2: Container

Shows the high-level technology choices and how containers communicate.

┌──────────────────────────────────────────────────────────────┐
│                      Container Diagram                        │
├──────────────────────────────────────────────────────────────┤
│                                                              │
│   ┌──────────────────┐        ┌──────────────────┐          │
│   │    Web App       │        │   Mobile App     │          │
│   │  [React SPA]     │        │  [React Native]  │          │
│   └────────┬─────────┘        └────────┬─────────┘          │
│            │                           │                     │
│            │         HTTPS             │                     │
│            └───────────┬───────────────┘                     │
│                        ▼                                     │
│            ┌───────────────────────┐                         │
│            │      API Gateway      │                         │
│            │       [Kong]          │                         │
│            └───────────┬───────────┘                         │
│                        │                                     │
│       ┌────────────────┼────────────────┐                   │
│       │                │                │                    │
│       ▼                ▼                ▼                    │
│  ┌─────────┐     ┌─────────┐     ┌─────────┐               │
│  │ Order   │     │ User    │     │ Product │               │
│  │ Service │     │ Service │     │ Service │               │
│  │ [Node]  │     │ [Node]  │     │ [Go]    │               │
│  └────┬────┘     └────┬────┘     └────┬────┘               │
│       │               │               │                     │
│       ▼               ▼               ▼                     │
│  ┌─────────┐     ┌─────────┐     ┌─────────┐               │
│  │ Orders  │     │ Users   │     │Products │               │
│  │   DB    │     │   DB    │     │   DB    │               │
│  │[Postgres│     │[Postgres│     │ [Mongo] │               │
│  └─────────┘     └─────────┘     └─────────┘               │
│                                                              │
└──────────────────────────────────────────────────────────────┘

Level 3: Component

Shows internal structure of a container.

┌──────────────────────────────────────────────────────────────┐
│                Component Diagram: Order Service               │
├──────────────────────────────────────────────────────────────┤
│                                                              │
│  ┌───────────────────────────────────────────────────────┐  │
│  │                     API Layer                          │  │
│  │  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐   │  │
│  │  │ OrdersCtrl  │  │ HealthCtrl  │  │ MetricsCtrl │   │  │
│  │  └──────┬──────┘  └─────────────┘  └─────────────┘   │  │
│  └─────────┼─────────────────────────────────────────────┘  │
│            │                                                 │
│  ┌─────────┼─────────────────────────────────────────────┐  │
│  │         ▼           Domain Layer                       │  │
│  │  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐   │  │
│  │  │OrderService │  │ OrderCalc   │  │ Validators  │   │  │
│  │  └──────┬──────┘  └─────────────┘  └─────────────┘   │  │
│  └─────────┼─────────────────────────────────────────────┘  │
│            │                                                 │
│  ┌─────────┼─────────────────────────────────────────────┐  │
│  │         ▼       Infrastructure Layer                   │  │
│  │  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐   │  │
│  │  │ OrderRepo   │  │PaymentClient│  │ EventPub    │   │  │
│  │  └─────────────┘  └─────────────┘  └─────────────┘   │  │
│  └───────────────────────────────────────────────────────┘  │
│                                                              │
└──────────────────────────────────────────────────────────────┘

Level 4: Code

Shows implementation details (class diagrams, sequence diagrams).

Use standard UML when needed at this level.

Scalability Patterns

Horizontal Scaling

Add more instances of the same component.

                    Load Balancer
                         │
         ┌───────────────┼───────────────┐
         │               │               │
         ▼               ▼               ▼
    ┌─────────┐     ┌─────────┐     ┌─────────┐
    │Instance │     │Instance │     │Instance │
    │    1    │     │    2    │     │    3    │
    └─────────┘     └─────────┘     └─────────┘

Requirements:
- Stateless services
- Shared session storage
- Database can handle connections

Caching

Reduce load on slow resources.

┌─────────────────────────────────────────────────────┐
│                  Caching Layers                      │
├─────────────────────────────────────────────────────┤
│                                                     │
│  Browser Cache → CDN → App Cache → Database Cache  │
│                                                     │
│  Examples:                                          │
│  - Browser: Static assets, API responses           │
│  - CDN: Static content, cached API responses       │
│  - App: Redis/Memcached for sessions, computed data│
│  - Database: Query cache, connection pooling       │
│                                                     │
└─────────────────────────────────────────────────────┘

Cache Invalidation Strategies:
- TTL (Time to Live): Simplest, eventual consistency
- Write-through: Update cache on write
- Write-behind: Async update for performance
- Cache-aside: App manages cache explicitly

Database Scaling

Strategy	Use Case	Trade-off
Read Replicas	Read-heavy workloads	Replication lag
Sharding	Large datasets	Query complexity
Partitioning	Time-series data	Partition management
CQRS	Different read/write patterns	System complexity

Reliability Patterns

Pattern	Purpose	Implementation
Circuit Breaker	Prevent cascade failures	Fail fast after threshold
Bulkhead	Isolate failures	Separate thread pools
Retry	Handle transient failures	Exponential backoff
Timeout	Bound wait times	Don't wait forever
Rate Limiting	Prevent overload	Throttle requests

Circuit Breaker States:

    ┌────────┐
    │ CLOSED │ ──── Failure Threshold ──► ┌────────┐
    │(normal)│                            │  OPEN  │
    └────────┘                            │(failing│
         ▲                                └────┬───┘
         │                                     │
    Success                              Timeout
    Threshold                                  │
         │                                     ▼
    ┌────┴────┐                          ┌─────────┐
    │HALF-OPEN│ ◄─── Test Request ────── │         │
    └─────────┘                          └─────────┘

Technology Selection

Selection Criteria

Criterion	Questions
Fit	Does it solve the actual problem?
Maturity	Production-proven? Community size?
Team Skills	Can the team use it effectively?
Performance	Meets requirements? Benchmarks?
Operations	How hard to deploy, monitor, debug?
Cost	License, infrastructure, learning curve?
Lock-in	Exit strategy? Standards compliance?
Security	Track record? Compliance certifications?

Evaluation Matrix

| Technology | Fit | Maturity | Skills | Perf | Ops | Cost | Score |
|------------|-----|----------|--------|------|-----|------|-------|
| Option A   | 4   | 5        | 3      | 4    | 4   | 3    | 3.8   |
| Option B   | 5   | 3        | 4      | 5    | 2   | 4    | 3.8   |
| Option C   | 3   | 4        | 5      | 3    | 5   | 5    | 4.2   |

Weights: Fit(25%), Maturity(15%), Skills(20%), Perf(15%), Ops(15%), Cost(10%)

Architecture Decision Records (ADRs)

ADR Template

# ADR-[NUMBER]: [TITLE]

## Status
[Proposed | Accepted | Deprecated | Superseded by ADR-XXX]

## Context
[What is the issue we're facing? What decision needs to be made?]

## Decision
[What is the change we're proposing/making?]

## Consequences
### Positive
- [Benefit 1]
- [Benefit 2]

### Negative
- [Trade-off 1]
- [Trade-off 2]

### Neutral
- [Observation]

## Alternatives Considered
### Alternative 1: [Name]
- Pros: [...]
- Cons: [...]
- Why rejected: [...]

Anti-Patterns

Anti-Pattern	Problem	Solution
Big Ball of Mud	No clear architecture	Establish bounded contexts
Distributed Monolith	Microservices without independence	True service boundaries
Resume-Driven	Choosing tech for experience	Match tech to requirements
Premature Optimization	Scaling before needed	Start simple, measure, scale
Ivory Tower	Architecture divorced from reality	Evolutionary architecture
Golden Hammer	Same solution for every problem	Evaluate each case

References

Pattern Examples - Detailed implementations
ADR Repository - Example decision records

architecture-selection

Architecture Pattern Selection

When to Activate

Architecture Patterns

Monolithic Architecture