README.md

Amazon.Lambda.AspNetCoreServer

This package makes it easy to run ASP.NET Core Web API applications as a Lambda function with API Gateway or an ELB Application Load Balancer. This allows .NET Core developers to create "serverless" applications using the ASP.NET Core Web API framework.

The function takes a request from an API Gateway Proxy or from an Application Load Balancer and converts that request into the classes the ASP.NET Core framework expects and then converts the response from the ASP.NET Core framework into the response body that API Gateway Proxy or Application Load Balancer understands.

Example Lambda Function

In the ASP.NET Core application add a class that extends from APIGatewayProxyFunction and implement the Init method.

Here is an example implementation of the Lamba function in an ASP.NET Core Web API application.

using System.IO;

using Amazon.Lambda.AspNetCoreServer;
using Microsoft.AspNetCore.Hosting;

namespace TestWebApp
{
    public class LambdaFunction : APIGatewayProxyFunction
    {
        protected override void Init(IWebHostBuilder builder)
        {
            builder
                .UseStartup<Startup>();
        }
    }
}

The function handler for the Lambda function will be TestWebApp::TestWebApp.LambdaFunction::FunctionHandlerAsync.

Once the function is deployed configure API Gateway with a HTTP Proxy to call the Lambda Function. Refer to the API Gateway developer guide for more information.

Application Load Balancer Example

To be used with an Application Load Balancer from ELB the base class needs to be changed from APIGatewayProxyFunction to ApplicationLoadBalancerFunction.

using System.IO;

using Amazon.Lambda.AspNetCoreServer;
using Microsoft.AspNetCore.Hosting;

namespace TestWebApp
{
    public class LambdaFunction : ApplicationLoadBalancerFunction
    {
        protected override void Init(IWebHostBuilder builder)
        {
            builder
                .UseStartup<Startup>();
        }
    }
}

Web App Path Base

By default this package configure the path base for incoming requests to be root of the API Gateway Stage or Application Load Balancer.

If you want to treat a subresource in the resource path to be the path base you will need to modify how requests are marshalled into ASP.NET Core. For example if the listener of an Application Load Balancer points to a Lambda Target Group for requests starting with /webapp/* and you want to call a controller api/values ASP.NET Core will think the resource you want to access is /webapp/api/values which will return a 404 NotFound.

In the LambdaEntryPoint class you can override the PostMarshallRequestFeature method to add custom logic to how the path base is computed. In the example below it configures the path base to be /webapp/. When the Application Load balancer sends in a request with the resource path set to /webapp/api/values. This code configures the ASP.NET Core request to have the path base set to /webapp/ and the path to /api/values.

    public class LambdaEntryPoint : ApplicationLoadBalancerFunction
    {
        protected override void Init(IWebHostBuilder builder)
        {
            builder
                .UseStartup<Startup>();
        }

        protected override void PostMarshallRequestFeature(IHttpRequestFeature aspNetCoreRequestFeature, ApplicationLoadBalancerRequest lambdaRequest, ILambdaContext lambdaContext)
        {
            aspNetCoreRequestFeature.PathBase = "/webapp/";

            // The minus one is ensure path is always at least set to `/`
            aspNetCoreRequestFeature.Path = 
                aspNetCoreRequestFeature.Path.Substring(aspNetCoreRequestFeature.PathBase.Length - 1);
            lambdaContext.Logger.LogLine($"Path: {aspNetCoreRequestFeature.Path}, PathBase: {aspNetCoreRequestFeature.PathBase}");
        }
    }

Supporting Binary Response Content

The interface between the API Gateway/Application Load Balancer and Lambda assumes response content to be returned as a UTF-8 string. In order to return binary content it is necessary to encode the raw response content in Base64 and to set a flag in the response object that Base64-encoding has been applied.

In order to facilitate this mechanism, the base class maintains a registry of MIME content types and how they should be transformed before being returned to the calling API Gateway or Application Load Balancer. For any binary content types that are returned by your application, you should register them for Base64 tranformation and then the framework will take care of intercepting any such responses and making an necessary transformations to preserve the binary content. For example:

using System.IO;

using Amazon.Lambda.AspNetCoreServer;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;

namespace TestWebApp
{
    public class LambdaFunction : APIGatewayProxyFunction
    {
        protected override void Init(IWebHostBuilder builder)
        {
            // Register any MIME content types you want treated as binary
            RegisterResponseContentEncodingForContentType("application/octet-stream",
                    ResponseContentEncoding.Base64);
            
            // ...
        }
    }

    // In your controller actions, be sure to provide a Content Type for your responses
    public class LambdaController : Controller
    {
        public IActionResult GetBinary()
        {
            var binData = new byte[] { 0x00, 0x01, 0x02, 0x03 };
 
            return base.File(binData, "application/octet-stream");
        }
    }
}

IMPORTANT - Registering Binary Response with API Gateway

In order to use this mechanism to return binary response content, in addition to registering any binary MIME content types that will be returned by your application, it also necessary to register those same content types with the API Gateway using either the console or the REST interface.

For Application Load Balancer this step is not necessary.

Default Registered Content Types

By default several commonly used MIME types that are typically used with Web API services are already pre-registered. You can make use of these content types without any further changes in your code, however, for any binary content types, you do still need to make the necessary adjustments in the API Gateway as described above.

MIME Content Type	Response Content Encoding
text/plain	Default (UTF-8)
text/xml	Default (UTF-8)
application/xml	Default (UTF-8)
application/json	Default (UTF-8)
text/html	Default (UTF-8)
text/css	Default (UTF-8)
text/javascript	Default (UTF-8)
text/ecmascript	Default (UTF-8)
text/markdown	Default (UTF-8)
text/csv	Default (UTF-8)
application/octet-stream	Base64
image/png	Base64
image/gif	Base64
image/jpeg	Base64
application/zip	Base64
application/pdf	Base64