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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();

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

C# class library

Function.cs

// Assembly attribute to enable the Lambda function's JSON input to be converted into a .NET class.
[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

namespace MyProject;

public class Function
{
    public string FunctionHandler(string input, ILambdaContext context)
    {
        return input.ToUpper();
    }
}

aws-lambda-tools-defaults.json

{
  "Information": [ ],
  "profile": "MyAwsProfile",
  "region": "eu-central-1",
  "configuration": "Release",
  "function-architecture": "x86_64",
  "function-runtime": "dotnet8",
  "function-memory-size": 512,
  "function-timeout": 30,
  "function-handler": "MyProject::MyProject.Function::FunctionHandler"
}

Properties/launchSettings.json

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

Debug locally

C# class library

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.

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"
    }
  }
}

C# executable assembly

You have to create the debug configuration file for you lambda.

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