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I established a authentication flow with Facebook Login and AWS Cognito on the client site. Works fine. But now I need a reference of the user with its facebook id in a dynambodb table. Of course I could just call a AWS lambda function exposed via AWS API gateway, but how can I verify that the API call actually has a valid facebook id and that this facebook id matches the AWS Cognito Id. Maybe I am missing something here, I hope you guys can point me in the right direction ;) thanks!
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Create an IAM Role for your Lambda Function Access your IAM Management console and select Roles from the left menu. Click Create role and select the AWS Service Lambda role. Once both are highlighted, click Next: Permissions. Name your role whatever you want, as long as it's recognizable to you, and click Create role.
With DynamoDB Streams, you can trigger a Lambda function to perform additional work each time a DynamoDB table is updated. Lambda reads records from the stream and invokes your function synchronously with an event that contains stream records.
To allow users to run Lambda with their Amazon Cognito permissions, follow these steps: Use the API Gateway console to establish your Amazon Cognito user pool as an authorizer. Then, assign the Amazon Cognito user pool as the authorizer for the method of your API.
If you can key your ddb table by cognito id instead of facebook id, you can invoke api gateway with cognito credentials. If you use callee credentials when calling lambda you can access the cognito id via the token $context.identity.cognitoIdentityId. This ensures the call was made by the owner of this id. You can further check that $context.identity.cognitoAuthenticationProvider is graph.facebook.com to ensure they authed via Facebook. Unfortunately, the facebook id is not passed in the credentials, so if you need it you will need a lookup table mapping cognito id to facebook id. For more details on the available tokens see here.
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