Referencing TableStreamArn in Amazon CDK-CodePudding

I have the following configuration:

    //DynamoDB Table
    const table = new dynamodb.Table(this, `DataStore`, {
      tableName: `${StackConfiguration.appName}-data-${StackConfiguration.environmentKey}`,
      partitionKey: { name: 'scope', type: dynamodb.AttributeType.STRING },
      sortKey: { name: 'id', type: dynamodb.AttributeType.STRING },
      readCapacity: 5,
      writeCapacity: 5,
      removalPolicy: cdk.RemovalPolicy.DESTROY,
      stream: StreamViewType.NEW_IMAGE
      
    });

    // Define Call Scaler Lambda
    const callScalerLambda = new lambda.Function(this, 'CallLambdaHandler', {
      description: 'Call Scaler Document Aggregate Lambda',
      functionName: `${StackConfiguration.appName}-call-scaler-${StackConfiguration.environmentKey}`,
      runtime: lambda.Runtime.PYTHON_3_8, // execution environment
      handler: 'call-scaler/call_lambda.lambda_handler', // file is myLambda, function is lambda_handler
      code: lambda.Code.fromAsset('lambda'), // code loaded from the "lambda" directory
      role: myIAMRole,
      environment: {
        ENVIRONMENT: StackConfiguration.environmentKey,
        SCALERENV: StackConfiguration.scalerEnvironment,
        SYSTEMUSERNAME: StackConfiguration.systemUserName,
        SYSTEMUSERPASS: StackConfiguration.systemUserPass
        
      },
    });

    const DocumentAggregateCallScalerEventTrigger = new lambda.EventSourceMapping(this, 'DocumentEventTrigger', {
      target:callScalerLambda,
    
      // the properties below are optional
      batchSize: 1,
      enabled: true,
      eventSourceArn: table.tableStreamArn,
      startingPosition: lambda.StartingPosition.LATEST
    });

I am trying to simply define an EventSourceTrigger via the DocumentAggregateCallScalerEventTrigger and define the eventSourceArn as the table's streamArn. However, I continue to get the following errors:

error   13-Dec-2021 15:23:39    FAIL test/stack.test.ts
error   13-Dec-2021 15:23:39      ● Test suite failed to run
error   13-Dec-2021 15:23:39    
error   13-Dec-2021 15:23:39        [96mlib/stack.ts[0m:[93m206[0m:[93m7[0m - [91merror[0m[90m TS2322: [0mType 'string | undefined' is not assignable to type 'string'.
error   13-Dec-2021 15:23:39          Type 'undefined' is not assignable to type 'string'.
error   13-Dec-2021 15:23:39    
error   13-Dec-2021 15:23:39        [7m206[0m       eventSourceArn: table.tableStreamArn,
error   13-Dec-2021 15:23:39        [7m   [0m [91m      ~~~~~~~~~~~~~~[0m
error   13-Dec-2021 15:23:39    
error   13-Dec-2021 15:23:39          [96mnode_modules/@aws-cdk/aws-lambda/lib/event-source-mapping.d.ts[0m:[93m9[0m:[93m14[0m
error   13-Dec-2021 15:23:39            [7m9[0m     readonly eventSourceArn: string;
error   13-Dec-2021 15:23:39            [7m [0m [96m             ~~~~~~~~~~~~~~[0m
error   13-Dec-2021 15:23:39            The expected type comes from property 'eventSourceArn' which is declared here on type 'EventSourceMappingProps'

How do I properly reference the table's StreamArn in the CDK?

CodePudding user response：

As far a Typescript is concerned, table.tableStreamArn is an optional string (string | undefined), so it's not letting you assign it to a required eventSourceArn prop. In reality, table.tableStreamArn will resolve to a string, because you have set the stream prop on your table. But Typescript has no way of figuring this out on its own.

There are various ways to make Typescript happy, some more elegant than others: You can assert non-null with table.tableStreamArn!, for instance, or force to a string table?.tableStreamArn ?? 'SHOULD NOT HAPPEN'. Or throw an error if it is undefined.

Another option would be add the event source right on the lambda, which avoids the whole issue:

import * as evtsrc from '@aws-cdk/aws-lambda-event-sources';

callScalerLambda.addEventSource(
  new evtsrc.DynamoEventSource(table, {
    batchSize: 1,
    enabled: true,
    startingPosition: lambda.StartingPosition.LATEST,
  })
);