[lambda] expose log group and support metric filter

🚀 Feature Request

General Information

• 👋 I may be able to implement this feature request

• ⚠️ This feature might incur a breaking change

Description

Currently it’s impossible to add MetricFilter for Lambda logs, because Lambda does not return log group and log group will be created only after any invocations of function. Manually created log groups (in CDK) might not work because of possible race conditions.

Addition of MetricFilter to logs is similar to LogGroup retention setting as discussed here https://github.com/aws/aws-cdk/issues/667#issuecomment-474002870 and implemented in PR #2067

Proposed Solution

Environment

• CDK CLI Version: all
• Module Version: all
• OS: all
• Language: all

Other information

[mariusGundersen]

There seems to be two ways to do this:

• create a new logGroup()
• get an existing logGroup using LogGroup.fromLogGroupArn()

The first method only works if the log group doesn’t already exist, but the second one only works if the log group already exists. In my scenario I had made the lambda (and the log group) in a previous deploy, and then I wanted to add a subscription to the log group in a second deploy, so I had to use the second variant. But then I tried to deploy this construct in a new stack, and now it failed, since the log group didn’t exist yet. So Unless you know the state of the stack you are deploying to you can’t really use these.

[AdonousTech]

@nija-at I take that back, I can actually create the log group. I’m not sure what I did wrong last time when I received the naming error.

After working through this, it seems that I now have an acceptable flow. In my TS code I simply 1) declare the LogGroup after the lambda.Function and follow the standard naming convention /aws/lambda/' + this.functionName. 2) I reference the log group on the MetricFilter by simply using this.logGroup. This order seems to work (e.g. lambda function first, log group second).

Problem solved. Thanks! For anyone else struggling with this:

      // first declare lambda function
      this.lambdaFunction = new lambda.Function(this, Vars.lambda.functionName, {
        runtime: lambda.Runtime.NODEJS_12_X,
        handler: 'index.handler',
        tracing: lambda.Tracing.ACTIVE,
        role: this.executionRole,
        code: lambda.Code.fromAsset(path.join(__dirname, '../../lambda/code/skill_lambda.zip'))
      });

      // declare log group using standard naming convention
      this.logGroup = new LogGroup(this, 'LogGroup', {
        logGroupName: '/aws/lambda/' + this.lambdaFunction.functionName
      })

      // declare other dependencies on log group such as metric filter, alarms, etc
            this.metricFilter = new logs.MetricFilter(this, 'MetricFilter', {
        filterPattern: {logPatternString: '{ $.eventType = "CriticalError" }'},
        logGroup: this.logGroup,
        metricNamespace: this.metric.namespace,
        metricName: this.metric.metricName,
        defaultValue: 0,
        metricValue: '1'
      });