Added API Gateway notes

Author: rishabkumar7

CDA.md

@@ -1348,3 +1348,57 @@ about the change
 
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+# API Gateway
+## API Gateway -- Integrations High Level
+- Lambda Function
+  - Invoke Lambda function
+  - Easy way to expose REST API backed by AWS Lambda
+- HTTP 
+  - Expose HTTP endpoints in the backend
+  - Example: internal HTTP API on premise, Application Load Balancer... 
+  - Why? Add rate limiting, caching, user authentications, API keys, etc... 
+- AWS Service 
+  - Expose any AWS API through the API Gateway? 
+  - Example: start an AWS Step Function workflow, post a message to SQS 
+  - Why? Add authentication, deploy publicly, rate control...
+  
+## API Gateway - Endpoint Types 
+- Edge-Optimized (default): For global clients - Requests are routed through the CloudFront Edge locations (improves latency) - The API Gateway still lives in only one region 
+- Regional: - For clients within the same region - Could manually combine with CloudFront (more control over the caching strategies and the distribution) 
+- Private: - Can only be accessed from your VPC using an interface VPC endpoint (ENI) - Use a resource policy to define access
+
+## API Gateway -- Deployment Stages 
+- Making changes in the API Gateway does not mean they're effective 
+- You need to make a "deployment" for them to be in effect 
+- It's a common source of confusion 
+- Changes are deployed to "Stages" (as many as you want) 
+- Use the naming you like for stages (dev, test, prod) 
+- Each stage has its own configuration parameters 
+- Stages can be rolled back as a history of deployments is kept
+
+## API Gateway -- Stage Variables 
+- Stage variables are like environment variables for API Gateway 
+- Use them to change often changing configuration values 
+- They can be used in: 
+  - Lambda function ARN 
+  - HTTP Endpoint 
+  - Parameter mapping templates 
+- Use cases: 
+  - Configure HTTP endpoints your stages talk to (dev, test, prod...) 
+  - Pass configuration parameters to AWS Lambda through mapping templates 
+- Stage variables are passed to the "context" object in AWS Lambda
+
+## API Gateway - Integration Types 
+- Integration Type MOCK 
+  - API Gateway returns a response without sending the request to the backend 
+- Integration Type HTTP / AWS (Lambda & AWS Services) 
+  - you must configure both the integration request and integration response 
+  - Setup data mapping using mapping templates for the request & response
+- Integration Type AWS_PROXY (Lambda Proxy): 
+  - incoming request from the client is the input to Lambda 
+  - The function is responsible for the logic of request / response 
+  - No mapping template, headers, query string parameters... are passed as arguments
+- Integration Type HTTP_PROXY 
+  - No mapping template 
+  - The HTTP request is passed to the backend 
+  - The HTTP response from the backend is forwarded by API Gateway