Integration testing AWS Lambda C# Functions with Lambda Test Server

Using Lambda Test Server to integration test your C# AWS Lambda functions for .NET Core locally when using a custom runtime.

11 November 2019 by Martin Costello |
11 November 2019 by Martin Costello

Lambda Test Server is a .NET Core 3.0 library available from NuGet which builds on top of the TestServer class in the Microsoft.AspNetCore.TestHost NuGet package to provide infrastructure to use with end-to-end/integration tests for .NET Core AWS Lambda Functions using a custom runtime.

The example below shows how Lambda Test Server can be used to write an xunit integration test for a simple C# Lambda function that reverses an array of integers:

public static async Task Function_Reverses_Numbers()
    // Arrange
    using var server = new LambdaTestServer();
    using var cancellationTokenSource = new CancellationTokenSource(TimeSpan.FromSeconds(1));

    await server.StartAsync(cancellationTokenSource.Token);

    int[] value = new[] { 1, 2, 3 };
    string json = JsonConvert.SerializeObject(value);

    LambdaTestContext context = await server.EnqueueAsync(json);

    using var httpClient = server.CreateClient();

    // Act
    await ReverseFunction.RunAsync(httpClient, cancellationTokenSource.Token);

    // Assert
    Assert.True(context.Response.TryRead(out LambdaTestResponse response));

    json = await response.ReadAsStringAsync();
    int[] actual = JsonConvert.DeserializeObject<int[]>(json);

    Assert.Equal(new[] { 3, 2, 1 }, actual);

The source is available in GitHub - pull requests are welcome!

Further samples for using the library with xunit are available in GitHub here:


Back in mid-October I received an email from AWS stating that my Lambda functions using Node.js 8.10 needed to be updated to version 10 as 8.10 was going to be deprecated at the start of 2020.

I have a few Lambda@Edge functions that are used for this blog that were easy enough to update, but I also have a published Alexa skill, London Travel, that was also using node 8.10 that isn't as simple.

While it was still very easy to migrate the function for Node.js 10, I thought it might be a good opportunity to take the time to rewrite the Lambda function that powers the skill in C# using .NET Core. I had some tasks coming up at work to migrate various .NET Core 3.0 worker services running on EC2 that my team maintains to be Lambda functions, so I figured this would be a good learning opportunity before tackling some high-throughput production workloads.

So first I rewrote the skill in C# targeting .NET Core 2.1 and deployed it, then I started looking into the upgrade to 3.0.

AWS Lambda has built-in support for .NET Core 2.1, which at the time of writing is the Long Term Service (LTS) version, but no runtime support for .NET Core 2.2 or the recently released .NET Core 3.0. Even though support for .NET Core 3.x is likely to be available in the next few months when .NET Core 3.1 is released (which will be the next LTS release), I didn't want to wait around to be able to benefit from .NET Core 3.0's various performance improvements.

AWS does however allow you to bring your own runtime for Lambda using custom runtimes and the LambdaBootstrap class from the Amazon.Lambda.RuntimeSupport NuGet package. Following the AWS blog post about .NET Core 3.0 as well as my colleague Zac Charles' recommendations for .NET Core 3.0 Lambdas, I updated the function to target .NET Core 3.0 and deployed it too.

It was at this point with the EC2 to Lambda migration work about to start that I started thinking about how to make it easier to integration test such a Lambda without having to deploy the code into the AWS Lambda runtime first. This wasn't about AWS hosting costs, but about helping to "shift left" the testing - decreasing the cycle time for iterating on the implementation of the Lambda functions as I wrote the code on my laptop.

After looking through the code for LambdaBootstrap and seeing what it did, I decided to write a test server that emulated the AWS Lambda runtime environment to allow running the code as close to a "black box" as possible in tests to validate the Lambda function.

How It Works

Under-the-hood, the AWS Lambda runtime works by exposing an HTTP API with four resources that are used to drive a message-pump that processes messages, one of which is an "input" with the other three being for "output".

The runtime code effectively runs an infinite while-loop which calls the GET /{LambdaVersion}/runtime/invocation/next resource and processes the content of the HTTP responses. The responses also contain metadata about the function in the headers, such as the AWS request Id and function ARN.

Once the function code has handled the request, the runtime either calls the POST /{LambdaVersion}/runtime/invocation/{AwsRequestId}/response resource for successfully processed requests or the POST /{LambdaVersion}/runtime/invocation/{AwsRequestId}/error resource to report errors. The fourth resource is used to handle failed function initialization: POST /{LambdaVersion}/runtime/init/error.

In theory this works like HTTP long-polling within your function, but in practice the AWS Lambda Runtime freezes the function process if there are no pending messages to invoke your code with.

Equipped with this knowledge and further reverse-engineering of the .NET Lambda runtime support, I started to implement Lambda Test Server, starting with a proof-of-concept which I committed directly into the repo for my Alexa skill.

It uses ASP.NET Core 3.0's TestServer and endpoint routing to implement an in-memory HTTP server exposed via HttpClient. Requests for the Lambda to process can be queued with the server and are delivered to the message pump sequentially over "HTTP" and are then passed to the Lambda function being tested, with the response being posted back into the runtime.

Here's an excerpt of the code (tweaked for brevity) that sets up the HTTP endpoints for the emulated Lambda runtime:

protected virtual void Configure(IApplicationBuilder app)
    app.UseEndpoints(endpoints =>
        endpoints.MapGet("/{Version}/runtime/invocation/next", OnNext);
        endpoints.MapPost("/{Version}/runtime/init/error", OnInitializationError);
        endpoints.MapPost("/{Version}/runtime/invocation/{RequestId}/error", OnInvocationError);
        endpoints.MapPost("/{Version}/runtime/invocation/{RequestId}/response", OnResponse);

The requests to pass to the function that are queued into the test server are routed through into a Channel<T> which provides a reader and writer that acts as a producer-consumer pair. Enqueueing a message places it into the ChannelWriter<T>, while the message pump consumes the ChannelReader<T>. You can read more about .NET's Channels in this post by fellow Microsoft MVP Steve Gordon.

Once I was happy with the basic concept and had it working with my Alexa skill's codebase, I started the process to make it its own open source repository and to publish it as a NuGet package. Once I'd shipped version 0.1.0 I then just needed to circle back around to my skill and delete the proof-of-concept and consume the library instead.


In the course of implementing the first version there were a few bits I wasn't entirely happy with.

The first was that I needed to use reflection to pass the HttpClient for the test server into LambdaBootstrap to get it wired-up. Not the end of the world, but not as neat as it could be.

The second was that the Lambda runtime loop, depending on timing, would always wait for at least one more new message to process before terminating, even if your test was completed and didn't need to queue anything new. I worked around this by having the test server deliver a "fake" message with empty content to "break" the loop.

These seemed like simple enough things to alter, and with the AWS SDK for Lambda being open source on GitHub, I submitted a Pull Request to refactor things a bit to allow the HttpClient to be injected and to have the message loop observe a thrown OperationCanceledException if the cancellation token for the loop was signalled.

Stuart Lang, an ex-colleague of mine I'd been discussing the test server with, reached out to AWS' Norm Johanson to have him look at the PR. Just under three days later the PR had been merged and a new version of the library pushed to!

Closing the loop on this the PR has been released as part of version 1.1.0 of Amazon.Lambda.RuntimeSupport

— Norm Johanson (@socketnorm) November 6, 2019

With the changes made available, all that was left was to remove the workarounds, publish an updated version of Lambda Test Server and update my Alexa skill's tests to use it.


Using Lambda Test Server in my Alexa skill, as well as the EC2 to Lambda migration at work, has allowed me to write a small number of acceptance-style integration tests for some Lambda functions while also providing high code coverage (>80%) and being able to target .NET Core 3.0 without the native Lambda runtime support.

At the same time it's also taught me some things about how the AWS Lambda runtime works internally, which I've found interesting, as well as some experience coding with channels and the lower-level endpoint routing in ASP.NET Core 3.0 without the weight of using MVC controllers.

With just some small refactoring of your function entrypoint to accept an HttpClient and CancellationToken, you can really boost the amount of code you can quickly test locally before committing your code to Continuous Integration and deploying it to your AWS account.

I hope you find it useful in your own .NET Core Lambda functions!