Amazon Lambda - Alias specific environment variables

Question

I am using AWS Lambda and can use Alias feature to point to multiple code promotion stages that we have (e.g. dev, qa, prod etc). I have setup the alias the same name as stages. Most of these functions gets triggered from S3 or SNS which has a different instance for each stage.

How can I setup a alias based environment variable so the function can get the specific info. The env vars setup in the base function(typically dev) gets carried over to all alias which does not work for deployment.

I know how to use stage variables in API gateway but the current use is not via gateway.

Answer 1

I don't believe there is a way to achieve what you are trying to. You would need to publish three versions of your Lambda function each with the correct environment variables and point each of your aliases to the correct version of the function.

You could use the description fields to help describe the versions before you point the aliases to them too, to make the changes more understandable.

Answer 2

I also find it interesting this isn't part of the plan for aliases, however you do have the context available in your code - Context.InvokedFunctionArn

I think the MINDSET is that you may call, for example, a S3 bucket and have a prefix of TEST or DEV or PROD (based on context InvokedFunctionArn you know which alias). Given this context and security based on the ARN you can use bucket policy / IAM to restrict your TEST ARN can only reach TEST s3 prefix files. That solves security between environments.

NOTE: I disagree with this model and think environment variables should be in he aliases, and if not specified in the alias fall back to what is in the version.

Although this works, the complexity around extra conditions on prefix, etc. is something many times will be misconfigured - Having a separate bucket seems much safer and matches the Serverless Application Model documentation better.

Answer 3

EDIT I'll leave this answer here as it may help some people but note that I found AspNetCoreStartupMode.FirstRequest caused longer cold starts by a few seconds.

---

I added some code to the LambdaEntryPoint to get the alias at Startup which means you can use it to load environment specific config:

public class LambdaEntryPoint : Amazon.Lambda.AspNetCoreServer.APIGatewayProxyFunction
{
    private ILambdaContext LambdaContext;

    public LambdaEntryPoint()
        : base(AspNetCoreStartupMode.FirstRequest)
    {

    }

    [LambdaSerializer(typeof(Amazon.Lambda.Serialization.Json.JsonSerializer))]
    public override async Task<APIGatewayProxyResponse> FunctionHandlerAsync(APIGatewayProxyRequest request, ILambdaContext lambdaContext)
    {
        LambdaContext = lambdaContext;

        return await base.FunctionHandlerAsync(request, lambdaContext);
    }

    protected override void Init(IWebHostBuilder builder)
    {
        var alias = LambdaContext?.InvokedFunctionArn.Substring(LambdaContext.InvokedFunctionArn.LastIndexOf(":") + 1);

        // Do stuff based on the environment

        builder
            .ConfigureAppConfiguration((hostingContext, config) =>
            {
                config.AddJsonFile($"appsettings.{alias}.json", optional: true, reloadOnChange: true);
            })
            .UseStartup<Startup>();
    }
}

I've added a gist here: https://gist.github.com/secretorange/710375bc62bbc1f32e05822f55d4e8d3

Answer 4

The lambda context has invoked_function_arn – The Amazon Resource Name (ARN) that's used to invoke the function. Indicates if the invoker specified a version number or alias.

You can then use the alias to find the variables via Systems Manager parameter store, instead of environment variables.