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API Gateway

Links: 101 AWS SAA Index


  • We use an API Gateway for creating REST APIs. The client will talk to the API gateway and then the API gateway will proxy the request to the lambda.
  • We use API gateway as it provides us with a lot of features. These features are not present when you use an ALB.
  • API Gateway creates RESTful APIs that:

    • Are HTTP-based.
    • Enable stateless client-server communication.
    • Implement standard HTTP methods such as GET, POST, PUT, PATCH, and DELETE.
  • API Gateway creates WebSocket APIs that:

    • Adhere to the WebSocket protocol, which enables stateful, full-duplex communication between client and server. Route incoming messages based on message content.
  • So API Gateway supports stateless RESTful APIs as well as stateful WebSocket APIs


  • API Gateway + Lambda = No infrastructure to manage.
  • You pay only for the API calls you receive and the amount of data transferred out.
  • We can handle API versioning (v1,v2..) without breaking the clients.
  • Handle different environments (test, dev, prod)
  • We can handle security (authentication and authorisation) using API Gateway.

  • We can cache API responses.

    • API caching is enabled for a stage (like prod, test etc) and not a method.
  • Transform and validate requests and responses


  • We can have per client throttling limits using the API keys.

    • Like having different throttling limits for the basic and premium users.
  • We can also have per method throttling limits.

  • We can create API keys and do request throttling to protect backend systems.

    • Any request over the limit will receive a 429 HTTP (Too many requests) response.
    • The requests are failed.

API Gateway Integrations

  • Lambda functions: for full serverless applications.
  • HTTP:
    • Expose HTTP endpoints in the backend. Example: internal HTTP API on premise
    • To add rate limiting, caching, user authentications, API keys, etc...
  • AWS Services:
    • Expose any AWS API through the API Gateway
    • To add authentication, deploy publicly, rate control etc

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



  • Create an IAM policy authorisation and attach to User / Role
  • Great for users / roles already within your AWS account
    • attachments/Pasted image 20220427095642.jpg
  • Authentication = IAM | Authorisation = IAM Policy
  • Good to provide access within AWS (EC2, Lambda, lAM users...)
  • Handles authentication + authorisation
  • Leverages Sig v4 capability where IAM credential are in headers

Lambda/Custom Authoriser

  • Token-based authoriser (bearer token) ex JWT (SON Web Token) or Oauth
  • Authentication = External | Authorisation = Lambda function
  • Uses AWS Lambda to validate the token in header being passed

    • A request parameter-based Lambda authoriser (headers, query string, stage var)
    • Retrieves the token from an external provider, uses the lambda authoriser to validate the token with the external provider and then generate the IAM policy.
    • attachments/Pasted image 20220527093730.jpg
  • Helps to use OAuth / SAML / 3rd party type of authentication

  • Lambda must return an IAM policy for the user. Very flexible in what IAM policy is returned.

Cognito User Pools

  • Cognito fully manages user lifecycle
  • First the call is to cognito user pools to authenticate and get the token. Once the token is returned it is sent along with the REST API request.

    • Cognito has a direct integration with API Gateway so the token sent with the REST request is evaluated.
    • attachments/Pasted image 20220427095926.jpg
  • No custom implementation required

  • Authentication = Cognito User Pools | Authorisation = API Gateway Methods
    • Cognito only helps with authentication, not authorisation.

Last updated: 2022-05-27