(@aws-cdk/aws-lambda): Version.metric() function uses wrong metric options when the version is associated with an alias

[huyphan]

The Issue

When the Lambda function version is associated with an alias, its metric() function (or other metric helper functions such as metricErrors() or metricInvocations()) uses the wrong values of dimensions property for the metrics. Consider the snippet below where we have a Lambda function and an alias:

const lambdaFunction = new Function(this, "MyFunction", { 
  ...
  functionName: "MyFunctionName"
  ...
});
const alias = new Alias(this, "MyFunctionAlias", {
  aliasName: "live",
  version: lambdaFunction.currentVersion
});
return lambdaFunction.version.metricErrors();

That code will return a metric with a dimensions property like this:

{        
  FunctionName: "MyFunctionName",
  Resource: `arn:aws:lambda:us-west-2:123456789012:function:MyFunctionName:42`,
}

(42 is the function version)

In reality, the correct dimensions for function version's metrics should be like this:

{
  "FunctionName": "MyFunctionName",
  "Resource': "MyFunctionName:live"
  "ExecutedVersion": "42"
}

(live is the name of the function alias)

You can probably verify that by running using ListMetrics API of CloudWatch against AWS/Lambda namespace:

❯ aws cloudwatch list-metrics --namespace AWS/Lambda --metric-name Errors --region=us-west-2
{
    "Metrics": [
    <output redacted>
       {
            "Namespace": "AWS/Lambda",
            "Dimensions": [
                {
                    "Name": "FunctionName",
                    "Value": "MyFunctionName"
                },
                {
                    "Name": "ExecutedVersion",
                    "Value": "5"
                },
                {
                    "Name": "Resource",
                    "Value": "MyFunctionName:live"
                }
            ],
            "MetricName": "Errors"
        }
    ]
} 

---

References:

• Current implementation of metric() function

  public metric(metricName: string, props: cloudwatch.MetricOptions = {}): cloudwatch.Metric {
    // Metrics on Aliases need the "bare" function name, and the alias' ARN, this differs from the base behavior.
    return super.metric(metricName, {
      dimensions: {
        FunctionName: this.lambda.functionName,
        // construct the ARN from the underlying lambda so that alarms on an alias
        // don't cause a circular dependency with CodeDeploy
        // see: https://github.com/aws/aws-cdk/issues/2231
        Resource: `${this.lambda.functionArn}:${this.version}`,
      },
      ...props,
    });
  }

• AWS docs that briefly describes the ExecutedVersion dimension: https://docs.aws.amazon.com/lambda/latest/dg/monitoring-metrics.html

The proposal

Users should be able to:

1. Pass an alias to Version.metric*() functions
2. Pass a function version to Alias.metric*() functions

Either case, the functions would return metrics with the same dimensions described above.

Environment

• CDK CLI Version: 1.98.0 (build 79f4512)
• Module Version: 1.98
• Node.js Version: v10.0.0
• OS: all
• Language (Version): all

Other information

[peterwoodworth]

Hey @huyphan, sorry for the long response time.

As you can see in the implementation, the comment says that Metrics on Aliases need the "bare" function name, and the alias' ARN, this differs from the base behavior. This is presently intended behavior, so I'll mark this as a feature request as you're requesting for additional functionality to the metric() functions.

@nija-at what are your thoughts on this?

[nija-at]

I'm not super familiar with function metrics, so let me know if I say anything incorrect here.

It seems to me that we need to split this issue into two.

1. There seems to be a bug in the value assigned to the Resource dimension. We're setting the value to be an ARN, while what it expects is the function, version or alias' name. This should be a bug.

2. Currently, we don't support the ExecutedVersion dimension at all. This should be a feature request.

@huyphan - does that sound correct?

@peterwoodworth - could you apply these changes?

[huyphan]

Thanks @nija-at and @peterwoodworth!

What @nija-at described is correct. Let me know if you want me to create a separate feature request for ExecutedVersion dimension.

[peterwoodworth]

@huyphan I can't promise I can get to actually fixing these any time in the next month. If you'd like to see the bug fixed, you could implement it yourself and I'd be happy to take a look at the PR. Also, go ahead with creating the feature request for executed version. It would be good to separate those from each other.

[varju]

Has anybody had time to look at this issue? In theory I think it might be a simple change, but I'm struggling to get CDK built locally so I can verify the change:

diff --git a/packages/@aws-cdk/aws-lambda/lib/lambda-version.ts b/packages/@aws-cdk/aws-lambda/lib/lambda-version.ts
index 1249c48746..a2fdf25c60 100644
--- a/packages/@aws-cdk/aws-lambda/lib/lambda-version.ts
+++ b/packages/@aws-cdk/aws-lambda/lib/lambda-version.ts
@@ -229,15 +229,25 @@ export class Version extends QualifiedFunctionBase implements IVersion {
   }
 
   public metric(metricName: string, props: cloudwatch.MetricOptions = {}): cloudwatch.Metric {
-    // Metrics on Aliases need the "bare" function name, and the alias' ARN, this differs from the base behavior.
-    return super.metric(metricName, {
-      dimensions: {
+    let dimensions;
+    if (this.lambda instanceof Alias) {
+      // Metrics on Aliases need the "bare" function name, and the alias' ARN, this differs from the base behavior.
+      dimensions = {
+        FunctionName: this.lambda.functionName,
+        Resource: `${this.lambda.functionArn}:${this.lambda.aliasName}`,
+        ExecutedVersion: this.version,
+      };
+    } else {
+      dimensions = {
         FunctionName: this.lambda.functionName,
         // construct the ARN from the underlying lambda so that alarms on an alias
         // don't cause a circular dependency with CodeDeploy
         // see: https://github.com/aws/aws-cdk/issues/2231
         Resource: `${this.lambda.functionArn}:${this.version}`,
-      },
+      };
+    }
+    return super.metric(metricName, {
+      dimensions: dimensions,
       ...props,
     });
   }