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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 events

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

public class Function
{
    public async Task<APIGatewayProxyResponse> FunctionHandler(APIGatewayProxyRequest request)
    {
        var id = request.PathParameters["id"];
        
        return new APIGatewayProxyResponse 
        {
            StatusCode = (int)HttpStatusCode.OK,
            Body = JsonSerializer.Serialize(id)
        };
    }
}

.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.

Properties\launchSettings.json

{
  "profiles": {
    "Mock Lambda Test Tool": {
      "commandName": "Executable",
      "commandLineArgs": "--port 5050",
      "executablePath": "<home-directory>\\.dotnet\\tools\\dotnet-lambda-test-tool-6.0.exe",
      "workingDirectory": ".\\bin\\Debug\\net6.0"
    }
  }
}

Deployment

Lambda function handler in C#

For class libraries: ASSEMBLY::TYPE::METHOD

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);
}