Task Statement 2.1: Design scalable and loosely coupled architectures.

📘AWS Certified Solutions Architect – (SAA-C03)

1. What Is an API?

An API (Application Programming Interface) allows one software application to communicate with another.

In IT environments:

A web application may call an API to get user data.
A mobile app may call an API to upload files.
A backend service may call another internal service using an API.

An API acts as a bridge between systems.

For the SAA-C03 exam, you must understand:

What APIs are
Why APIs are important for scalable systems
How AWS manages APIs using Amazon API Gateway

2. What Is a REST API?

A REST API (Representational State Transfer API) is a common type of API that:

Uses HTTP methods (GET, POST, PUT, DELETE)
Uses URLs to access resources
Is stateless (each request is independent)

Common HTTP Methods

Method	Purpose
GET	Retrieve data
POST	Create new data
PUT	Update data
DELETE	Remove data

Example (IT Environment)

A frontend web app calls:

GET /users

The backend returns a list of users in JSON format.

Each request contains everything needed (no session stored on server). That is called stateless design, which helps scalability.

3. Why APIs Are Important for Scalable and Loosely Coupled Architectures

Loose Coupling

Loose coupling means:

Services work independently
One service can change without breaking others

With APIs:

Frontend does not need to know how backend is built
Backend can change database without affecting frontend
Services can scale independently

Scalability

APIs help scalability because:

Multiple backend servers can handle requests
API layer can throttle traffic
API layer can cache responses

APIs allow horizontal scaling (adding more servers).

4. Amazon API Gateway

The main AWS service for API creation and management is:

Amazon API Gateway

It is a fully managed service that allows you to:

Create APIs
Secure APIs
Monitor APIs
Throttle requests
Scale automatically

You do not manage servers.

5. Types of APIs in API Gateway (Important for Exam)

API Gateway supports three types:

1️⃣ REST API

Most feature-rich
Supports API keys
Supports usage plans
Supports request validation
Supports caching
Higher cost

Used for enterprise-grade APIs.

2️⃣ HTTP API

Lower cost
Simpler
Lower latency
Good for microservices

Limited features compared to REST API.

Exam Tip:
If question mentions:

Cost-sensitive
Simple microservices
Low latency
→ Choose HTTP API

3️⃣ WebSocket API

For real-time communication
Two-way communication
Used in chat apps or live updates

6. How API Gateway Works (Architecture Flow)

Typical Architecture:

Client → API Gateway → Backend Service

Backend can be:

AWS Lambda
Amazon EC2
Amazon ECS
AWS Fargate
Any HTTP endpoint

API Gateway acts as a front door.

7. Integration Types (Very Important for Exam)

1️⃣ Lambda Integration

API directly invokes Lambda function.

Best for:

Serverless architecture
Event-driven systems

Common exam answer for scalable systems.

2️⃣ HTTP Integration

API calls:

Internal HTTP service
Application Load Balancer
EC2 application

3️⃣ Mock Integration

Returns fixed response (used for testing).

8. Security in API Gateway

Security is heavily tested in SAA-C03.

1️⃣ IAM Authorization

Use AWS Identity and Access Management.

Used when:

Internal services
AWS users only

2️⃣ Cognito Authorizer

Uses:

Amazon Cognito

Provides:

User authentication
Token validation

Used for:

Web/mobile app authentication

3️⃣ Lambda Authorizer

Custom authentication logic.

4️⃣ API Keys

Identify clients
Not for authentication
Used with usage plans

5️⃣ Resource Policies

Control who can access API:

By IP
By VPC
By AWS account

9. Throttling and Rate Limiting

Very important for scalability.

API Gateway can:

Limit requests per second
Prevent backend overload

You can configure:

Account-level throttling
Method-level throttling
Usage plan throttling

Protects backend systems.

10. Caching

REST API supports caching.

Benefits:

Reduces backend load
Improves response time
Reduces cost

If exam says:

Improve performance
Reduce backend calls
Reduce latency
→ Enable API Gateway caching

11. Monitoring and Logging

Integrated with:

Amazon CloudWatch

Provides:

Metrics (requests, errors, latency)
Logs
Alarms

You can:

Monitor 4XX errors (client errors)
Monitor 5XX errors (server errors)

12. API Deployment Concepts

Important terms:

API

The entire API configuration.

Resource

URL path (e.g., /users)

Method

GET, POST, etc.

Stage

Environment version (dev, test, prod)

Each stage can:

Have different throttling
Have different variables
Have logging enabled

13. Custom Domain Names

You can use your own domain name instead of default AWS domain.

Requires:

AWS Certificate Manager

Used for:

HTTPS certificates
Secure communication

14. Private APIs

API Gateway can create:

Public APIs (internet accessible)
Private APIs (inside VPC)

Private APIs use:

VPC endpoints
Resource policies

Used for internal enterprise systems.

15. API Gateway + Microservices

Common scalable architecture for exam:

API Gateway
→ Lambda
→ DynamoDB

API Gateway
→ ECS/Fargate
→ RDS

API Gateway helps:

Route traffic
Secure endpoints
Scale automatically

16. Designing for Loose Coupling (Exam Focus)

To design loosely coupled systems:

Use API Gateway in front of services
Use Lambda for stateless execution
Avoid direct service-to-service tight integration
Use event-driven patterns when possible

Loose coupling benefits:

Independent deployment
Independent scaling
Fault isolation

17. High Availability

API Gateway is:

Fully managed
Multi-AZ by default
Automatically scales

No need to configure load balancer.

18. Cost Considerations (Exam Tips)

You are charged for:

API calls
Data transfer
Caching (if enabled)

HTTP API is cheaper than REST API.

If exam question says:

Low cost
Basic API
High request volume
→ Choose HTTP API

19. Common Exam Scenarios

Scenario 1:

Need serverless, auto-scaling backend.
→ API Gateway + Lambda

Scenario 2:

Need authentication for mobile users.
→ API Gateway + Cognito

Scenario 3:

Need to protect backend from overload.
→ Throttling + Caching

Scenario 4:

Need internal-only API.
→ Private API + VPC endpoint

Scenario 5:

Need custom authentication logic.
→ Lambda Authorizer

20. Important Differences to Remember

Feature	REST API	HTTP API
Cost	Higher	Lower
Features	Full	Limited
Caching	Yes	No
Best for	Enterprise	Simple microservices

Final Exam Checklist (Must Know)

You must understand:

What REST APIs are
Stateless communication
Loose coupling benefits
API Gateway types
Lambda integration
Security options (IAM, Cognito, Lambda Authorizer)
Throttling and caching
Private vs public APIs
Monitoring with CloudWatch
Cost differences (REST vs HTTP API)

Final Summary

APIs allow systems to communicate in a structured way.

Amazon API Gateway:

Creates APIs
Secures APIs
Scales automatically
Protects backend services
Supports serverless architectures

For SAA-C03:

API Gateway is a key service for scalable architectures
Lambda integration is very common
HTTP API is cost-effective
Security and throttling are critical topics

If you understand these concepts clearly, you will be well prepared for this section of the exam.