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Lambda Synchronous Invocation

Links: 102 AWS DVA Index

Synchronous Invocation

  • We wait for the results of the invocation.
  • Results are returned right away
  • Error handling must happen client side (retries, exponential backoff, etc...)
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  • Services which can do synchronous invocation
    • User Invoked:
      • Elastic Load Balancing (Application Load Balancer)
      • Amazon API Gateway
      • Amazon CloudFront (Lambda@Edge)
      • Amazon S3 Batch
    • Service Invoked:
      • Cognito
      • AWS Step Functions
      • Kinesis Data Firehose
    • CLI/SDK can be used for both synchronous and asynchronous invocations

Lambda with ALB

  • To expose a Lambda function as an HTTPS endpoint we can use the Application Load Balancer or an API Gateway.
  • The Lambda function must be registered in a target group
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  • ALB can invoke Lambda with the help of the resource based policy of Lambda.


How does ALB convert an HTTP request to a lambda invocation?
  • From ALB to Lambda the HTTP request gets transformed to a JSON document.
  • We get all the information present in the HTTP request in the event parameter of the function handler.
  • Like Query String Parameters as Key/Value pairs, Headers as Key/Value pairs, Body (for POST, PUT...) is Base64Encoded.

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  • The JSON document returned by lambda is converted to an HTTP request by the ALB.
    • In lambdas response we get status code & description, headers as key value pairs and body which is base64 encoded.
    • The lambda must return these fields if we want the response to be displayed in the browser. Like if content-type header is not present then our browser won't know how to display the content and will download the response instead.
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ALB Multi Header Values

  • ALB can support multi header values (ALB setting)
  • When you enable multi-value headers, HTTP headers and query string parameters that are sent with multiple values are shown as arrays within the AWS Lambda event and response objects.
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Last updated: 2022-06-08