code: function invokeLambda(bucket, key, nextFileNum, lastFileNum, arn, retries) {
      // figure out if there's a next file to process
      if (nextFileNum && arn) {
        const stepfunctions = new AWS.StepFunctions()
        const params = {
          stateMachineArn: arn,
          input: JSON.stringify({ bucket, key, currentFileNum: nextFileNum, lastFileNum, arn, retries}),
          name: `ingest_${nextFileNum}_${Date.now()}`
        }
        stepfunctions.startExecution(params, function(err, data) {
          if (err) {
            console.log(err, err.stack)
          } else {
            console.log(`launched ${JSON.stringify(params)}`)
          }
        }) 
      }
  }
docs: kick off processing next CSV file