AWS API GATEWAY configuration to return binary pdf file from lambda

user_669670 0
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I want to return a pdf from a AWS Lambda function, and I use API Gateway for calling it from a any browser.

I have a aws lambda function in c# that returns an API Gateway Response which body is a pdf in base64 encoded string.

So far the endpoint returns a file with .pdf extension but not in binary. Instead, is a text file with the base64 string.

The headers i'm returning from the c# code are:

  var headersDic = new Dictionary<string, string>();
      headersDic.Add("Content-type", "application/pdf");
      headersDic.Add("Content-disposition", "attachment; filename=file.pdf");

I converted manually the base64 string to binary file and opened it as a pdf, and it works, I mean, the base64 string is correct so I assume the problem is being the API Gateway.

In the integration response console of the API Gateway, I got this:

enter image description here

But i'm not able to make it work.

I also have binary media types enabled.

enter image description here

aws-api-gateway amazon-web-services base64 binary pdf
4 Answers

Actually, you do not have to disable Lambda Proxy Integration, nor do you have to specify "*/*" as a Binary Media Type, nor do you have to have your Lambda code convert the output to Base64, nor do you have to modify LambdaEntryPoint in order to get this to work.

Although some sources indicate that the "*/*" entry is necessary, the answer lies in the comment accompanying the Binary Media Types setting:

"API Gateway will look at the Content-Type and Accept HTTP headers to decide how to handle the body."

In other words, for some odd reason, rather than just paying attention to the returned Content-Type in applying the Binary Media Types list, the gateway also takes into account the requesting client's Accept header. Most clients default to Accept:*/*, thus the origin of the workaround, and the most common explanation as to why this list doesn't seem to work. Not only does the client's Accept header seem to be given more credence than the returned Content-Type value here, there is no semantic assessment of "*/*". It is not evaluated as a wildcard, which should match any content type. Thus, an exact match of said value, literally "*/*", winds up being needed in the Binary Media Types list.

A simple solution (one which works for me) is to:

  • Have the client, instead of specifying "*/*" in its Accept header, specify the actual expected MIME type. (Do not specify multiple Accept types, as only the first listed type is evaluated.)
  • Include the same MIME type in the Binary Media Types list. Do not specify "*/*".
  • Have the Lambda function return a non-Base64-encoded binary file, specifying its actual MIME type in the Content-Type header. For instance, in C#, by returning a FileContentResult from the ControllerBase.File() method, specifying the appropriate MIME content type, as in:
return File(pdfContents, "application/pdf"); //pdfContents contains raw binary data

API Gateway will match the data type to its Binary Media Types list and regard the returned data as binary. So far, I have not needed to insert RegisterResponseContentEncodingForContentType() in LambdaEntryPoint.Init().

user_193168980

I don't know exactly what I did, but I deleted the 'ALL' method, and created the 'GET' method with this config and now it works.

enter image description here

user_6696700

You also need to declare the content type like the following in your LambdaEntryPoint.cs

See this documentation: https://github.com/aws/aws-lambda-dotnet/blob/master/Libraries/src/Amazon.Lambda.AspNetCoreServer/README.md

You have to tell Lambda you're going to return something besides strings.

public class LambdaEntryPoint : Amazon.Lambda.AspNetCoreServer.APIGatewayProxyFunction
{

    protected override void Init(IWebHostBuilder builder)
    {
        builder
            .UseStartup<Startup>();

        ****IMPORTANT PART HERE****
        RegisterResponseContentEncodingForContentType("application/pdf",
            ResponseContentEncoding.Base64);
    }
}
user_13018630

Response Headers

'Content-Type': 'application/pdf'
'Content-disposition', 'attachment; filename=fileName.pdf'

Response to Client

{
 statusCode: 200
 headers: responseHeader,
 body: pdfContent.toString(base64),
 isBase64Encoded: true,
}

API Gateway Binary Media Types

enter image description here

Hope it will help

user_83170830
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