🚪 What Is AWS API Gateway?

AWS API Gateway is a fully managed service that allows you to create, publish, monitor, and secure REST, HTTP, and WebSocket APIs.

✅ Use Cases:

• Front-end to Lambda functions

• Backend for mobile/web apps

• Proxy to internal services

• Integrate with AWS services like S3, DynamoDB

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🧱 Architecture: API Gateway + Lambda

plaintext
Client (Browser / App / Curl)
    ↓ (HTTP)
API Gateway (REST API)
    ↓
AWS Lambda (Business Logic)
    ↓
(Optional) DynamoDB / S3 / RDS

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🧪 Example Goal

Endpoint	Method	Functionality
/hello	GET	Returns greeting message
/submit	POST	Accepts JSON and stores it

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🔧 Step-by-Step Setup

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✅ Step 1: Create Lambda Functions

📍 Lambda 1: HelloFunction (for GET)

python
def lambda_handler(event, context):
    return {
        "statusCode": 200,
        "body": "Hello from Lambda!"
    }

📍 Lambda 2: SubmitFunction (for POST)

python
import json

def lambda_handler(event, context):
    body = json.loads(event["body"])
    name = body.get("name", "Guest")
    
    return {
        "statusCode": 200,
        "body": json.dumps({"message": f"Hello, {name}!"})
    }

🔼 Deploy these Lambda functions in AWS Console or CLI

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✅ Step 2: Create a REST API in API Gateway

1. Open API Gateway → Create API → Select REST API (not HTTP API)

2. Choose:

   • Name: MyRestAPI

   • Endpoint Type: Regional

   • Click Create API

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✅ Step 3: Create Resources and Methods

📍 Create /hello resource

1. Select your API → Resources

2. Click Actions > Create Resource

3. Resource Name: hello, Resource Path: /hello

📍 Add GET Method to /hello

1. Select /hello → Actions → Create Method → Select GET

2. Integration type: Lambda Function

3. Region: Your region

4. Function name: HelloFunction

5. Save → Allow permissions

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📍 Create /submit resource

1. Actions → Create Resource

2. Resource Path: /submit

📍 Add POST Method to /submit

1. Select /submit → Create Method → POST

2. Integration: Lambda Function

3. Function name: SubmitFunction

4. Save → Allow permissions

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✅ Step 4: Enable CORS (if needed)

1. For both /hello and /submit

2. Actions → Enable CORS

3. Save and Deploy again

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✅ Step 5: Deploy the API

1. Click Actions > Deploy API

2. Deployment stage: prod (or create a new one)

3. Click Deploy

You’ll get an endpoint like:

bash
https://abcd1234.execute-api.us-east-1.amazonaws.com/prod

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✅ Step 6: Access the API

🔹 Test GET endpoint:

bash
curl https://abcd1234.execute-api.us-east-1.amazonaws.com/prod/hello

Output:

json
Hello from Lambda!

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🔹 Test POST endpoint:

bash
curl -X POST \
  -H "Content-Type: application/json" \
  -d '{"name": "Mitesh"}' \
  https://abcd1234.execute-api.us-east-1.amazonaws.com/prod/submit

Output:

json
{"message": "Hello, Mitesh!"}

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✅ Optional: Secure with IAM/Key/Auth

Method	Use case
API Key	Rate limiting and usage plans
IAM	Internal access via roles
Cognito Authorizer	User-based authentication
Lambda Authorizer	Custom auth logic

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📦 Summary

Step	Description
Create Lambda	Business logic (GET/POST)
Create REST API	In API Gateway
Add Resources & Methods	/hello (GET), /submit (POST)
Integrate with Lambda	Select your function and region
Deploy API	Use prod or any stage
Access & Test	Via browser, Postman, or curl