Enabling cancellationToken on NET6 Lambda (REST API Gateway)

Unfortunately, Lambda doesn't provide a CancellationToken (to my knowledge). It can, however, tell you the amount of time remaining before it gives up, which you can build a CancellationToken around. For example:

public async Task<Stream> Process(Stream input, ILambdaContext lambdaContext)
{
    var timeout = lambdaContext.RemainingTime.Subtract(TimeSpan.FromSeconds(5d));
    using var cts = new CancellationTokenSource(timeout);

    await my.Work(cts.Token);

    return Stream.Null;
}

When the RemainingTime has been reached, Lambda will abruptly shut everything down. There is seemingly no nice way to handle that. Since cancellation is mutual, you'll want to pad the timeout with a few seconds to shut down gracefully. I have arbitrarily selected 5 seconds, but your needs might be different.

When you use the Lambda to ASP.NET Core integration, this process is handled for you automatically and you can just add CancellationToken as a parameter to your action methods as normal.