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Running .NET 8 Lambda Functions on AWS using Custom Runtime & Lambda Internals

Description

Serverless service / Function as a service allowing to run code without having to worry about underlying hardware and OS.
Event driven: the lambda is triggered by an event.
Pay only for what you use: per request and based on the duration of the code execution.

Use cases

Data transformation (Kinesis Data Stream as input)
File processing (when uploaded to S3 bucket)
Website backend microservice
Scheduled tasks

Bad use cases

Long running processes (timeout after 15mn)
Constant workload (no scalability and high cost)
Large code base (needed to be loaded at startup)
State management (lambda are stateless)

Anti-patterns

Monolithic function
- increase package size
- hard to enforce least privilege permissions
- hard to upgrade, maintain and test
Recursion
- endless loop
Orchestration
- avoid complex workflow logic
- ties lambda with other systems
- instead consider AWS Step Functions or EventBridge
Chaining (synchronously invoke another lambda)
- instead use EventBridge or QueueService
Waiting (synchronously call services or databases)
- instead use asynchronous calls

Runtime

OS
Libraries
Programming language (.NET, Node.js, Python, Go, Ruby, Java)

Environnement variables

DOTNET_STARTUP_HOOKS	ex: path to an assembly to inject logging

Wrapper scripts

Execute the wrapper on top of the runtime and the lambda function.

run shell commands and binaries

Use AWS_LAMBDA_EXEC_WRAPPER to point to your wrapper script.

Custom runtime

Provide your custom runtime.

unsupported programming language

Handler (entry point)

Method responsible for processing input events.

synchronous execution	result returned to the calling app
asynchronous execution	result sent to the configured destination otherwise lost

C# executable assembly

Using the C# 9's top-level statements feature, you generate an executable assembly which will be run by the Lambda. You provide Lambda only with the name of the executable assembly to run.

var handler = async (string argument1, ILambdaContext context) => { };

// bootstrap the Lambda runtime and pass it the handler method
await LambdaBootstrapBuilder.Create(handler, new DefaultLambdaJsonSerializer()).Build().RunAsync();

C# class library

You provide Lambda with information about your function's handler in the form of a handler string: ASSEMBLY::TYPE::METHOD

ASSEMBLY is the name of the .NET assembly file for your application. If you use the Amazon.Lambda.Tools CLI to build your application and you don't set the assembly name using the AssemblyName property in the .csproj file, then ASSEMBLY is simply the name of your .csproj file.
TYPE is the full name of the handler type, which consists of the Namespace and the ClassName.
METHOD is the name of the function handler method in your code.

ex: MyProject::MyNamespace.MyClass::MyFunctionHandler

[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.CamelCaseLambdaJsonSerializer))]
namespace MyNamespace;

public class MyClass
{
    public APIGatewayProxyResponse Handle(APIGatewayProxyRequest request, ILambdaContext context)
    {
        var id = request.PathParameters["id"];

        var function = $"{context.FunctionName} {context.FunctionVersion}";
        var cognitoIdentity = context.Identity;
        context.Logger.LogLine("Message");
        
        return new APIGatewayProxyResponse 
        {
            StatusCode = (int)HttpStatusCode.OK,
            Body = JsonSerializer.Serialize(id)
        };
    }

    public void Handle(SQSEvent sqsEvent, ILambdaContext context) { }
}

Setting up your .NET development environment

# install the VS project template
dotnet new install Amazon.Lambda.Templates

# install the command line tools
dotnet tool install -g Amazon.Lambda.Tools

.NET 8

scenario	description
Native AOT
Custom Runtime Function	to use .NET 8
Container Image	lambda function package as a container image

Debug locally

On VS, if the extension AWS Toolkit is installed you have the AWS .NET Mock Lambda Test Tool available.
A debug configuration is created for you lambda. If not, create it.

Properties\launchSettings.json

{
  "profiles": {
    "Mock Lambda Test Tool": {
      "commandName": "Executable",
      "commandLineArgs": "--port 5050",
      "executablePath": "%USERPROFILE%\\.dotnet\\tools\\dotnet-lambda-test-tool-8.0.exe",
      "workingDirectory": ".\\bin\\$(Configuration)\\net8.0",
      "environmentVariables": {
        "AWS_LAMBDA_RUNTIME_API": "localhost:5050",
        "AWS_PROFILE": "MyProfile",
        "AWS_REGION": "us-east-1"
      }
    }
  }
}

Deployment

dotnet lambda deploy-function [AssemblyName] --profile [Profile]

Call a lambda from code

var jsonSerializerOptions = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    Converters = { new JsonStringEnumConverter() }
};

var amazonLambdaClient = new AmazonLambdaClient();

var request = new InvokeRequest
{
    FunctionName = functionName,
    Payload = JsonSerializer.Serialize(myObject, jsonSerializerOptions),
    LogType = LogType.Tail
};
var response = await this.amazonLambdaClient.InvokeAsync(request);

if(response.HttpStatusCode == System.Net.HttpStatusCode.OK)
{
    var payload = Encoding.ASCII.GetString(response.Payload.ToArray()); // to debug only
    var result = JsonSerializer.Deserialize<AwsJobResult<LambdaJob>>(response.Payload, jsonSerializerOptions);
}

Errors

Could not find the specified handler assembly with the file name LambdaTest

The Lambda on AWS has wrongly set the handler to LambdaTest.
AWS - Lambda - Functions - select your function - Code tab - Runtime settings - Edit - change the Handler

Lambda

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