Get AWS Lambda Function Name or ARN from API Gateway Method ARN in Custom Authorizer

Question

I get the following methodArn from the event payload when I invoke my Lambda Authorizer in my API Gateway:

arn:aws:execute-api:us-east-1:account-id:api-id/stage/GET/entity

I would like to get the name of the associated Lambda or, preferably, the ARN of the Lambda associated with that API Gateway method.

How can I do it?

Answer 1

Retreiving the Lambda can only be done through the AWS SDK. You need to go through the following steps to get to the implementation URI of the Lambda.

Steps:

• Get the API resources for your API
• Get the integration for that resource
• Extract the Lambda ARN from the target URI

In Code, this would look something like this.

var aws = require('aws-sdk');
var apigateway = new aws.APIGateway();
exports.handler = (event, context, callback) => {
  //sample methodArn: 'arn:aws:execute-api:eu-west-1:1234567890:p2llm1cs/prod/GET/blah/gerfea'
  var section = event.methodArn.substring(event.methodArn.lastIndexOf(':') + 1);
  var restId = section.split('\/')[0];
  var method = section.split('\/')[2];
  var path = section.substring(section.indexOf(method) + method.length);

  //getting the resources for the given API
  var params = {
    restApiId: restId,
  };
  apigateway.getResources(params, function(err, data) {
    //iterate through all attached resources
    for (var idx = 0; idx < data.items.length; idx++) {
      var item = data.items[idx];
      if (item.path == path) {
        //get the integration for the matching resource.
        var params = {
          httpMethod: method,
          resourceId: item.id,
          restApiId: restId
        };
        apigateway.getIntegration(params, function(err, data) {
          //sample data.uri arn:aws:apigateway:eu-west-1:lambda:path/2015-03-31/functions/arn:aws:lambda:eu-west-1:1234567890:function:echo/invocations
          console.log(data.uri);
          callback(null, 'Hello from Lambda');
        });
      }
    }
  });
};