AWS CDK Step Functions问题：Lambda任务返回Payload始终为Null

问题 #

使用AWS CDK构建Step Functions工作流时，第一个Lambda调用正常，第二个Lambda的输入符合预期，但Lambda任务输出的Payload始终为null。期望Lambda返回的output键中数据能传递给下一个任务。

Lambda处理代码 #

export const bulkSummaryHandler = (event) => {
  try {
    console.log('LAMBA SUMMARY!', event);
    return { output: { status: 'finished' } };
  } catch (error) {
    return handleError(error);
  }
};

CDK代码 #

const getUserCsvFileTask = new tasks.LambdaInvoke(ctx.stack, 'getUserCsvFileTask', {
  lambdaFunction: getUserCsvFileFn,
  comment: 'fetch user uploaded csv from csv-integration-service',
  inputPath: '$',
  resultPath: '$.taskResult',
  outputPath: '$.taskResult.Payload'
});

const bulkSummaryTask = new tasks.LambdaInvoke(ctx.stack, 'bulkProcessingSummaryTask', {
  lambdaFunction: bulkSummaryFn,
  comment: 'summarise bulk processing',
  inputPath: '$'
});

const definition = stepfunctions.Chain.start(getUserCsvFileTask).next(bulkSummaryTask).next(nextLambdaTask);

响应示例 #

{
  "ExecutedVersion": "$LATEST",
  "Payload": null,
  "SdkHttpMetadata": {
    "AllHttpHeaders": {
      "X-Amz-Executed-Version": [
        "$LATEST"
      ],
      "x-amzn-Remapped-Content-Length": [
        "0"
      ],
      "Connection": [
        "keep-alive"
      ],
      "x-amzn-RequestId": [
        "fed8b1bd-d188-4425-ade7-ce2723aef4c8"
      ],
      "Content-Length": [
        "4"
      ],
      "Date": [
        "Wed, 21 Sep 2022 22:54:00 GMT"
      ],
      "X-Amzn-Trace-Id": [
        "root=1-632b9607-0e451e4c5dd4c21c7a3eaa8b;sampled=1"
      ],
      "Content-Type": [
        "application/json"
      ]
    },
    "HttpHeaders": {
      "Connection": "keep-alive",
      "Content-Length": "4",
      "Content-Type": "application/json",
      "Date": "Wed, 21 Sep 2022 22:54:00 GMT",
      "X-Amz-Executed-Version": "$LATEST",
      "x-amzn-Remapped-Content-Length": "0",
      "x-amzn-RequestId": "fed8b1bd-d188-4425-ade7-ce2723aef4c8",
      "X-Amzn-Trace-Id": "root=1-632b9607-0e451e4c5dd4c21c7a3eaa8b;sampled=1"
    },
    "HttpStatusCode": 200
  },
  "SdkResponseMetadata": {
    "RequestId": "fed8b1bd-d188-4425-ade7-ce2723aef4c8"
  },
  "StatusCode": 200
}

解决思路 #

• 验证Lambda实际返回值：查看Lambda的CloudWatch日志，确认函数是否真的返回了{ output: { status: 'finished' } }。重点检查handleError函数的返回值，如果它返回null，会直接导致Payload为null。
• 调整Lambda函数写法：将Lambda改为异步函数测试，Node.js环境下异步写法的返回值序列化更稳定：

  export const bulkSummaryHandler = async (event) => {
    try {
      console.log('LAMBA SUMMARY!', event);
      return { output: { status: 'finished' } };
    } catch (error) {
      return handleError(error);
    }
  };
  

• 配置Step Functions的结果路径：给第二个LambdaInvoke任务添加resultPath和outputPath，确保返回的output能传递给下一个任务：

  const bulkSummaryTask = new tasks.LambdaInvoke(ctx.stack, 'bulkProcessingSummaryTask', {
    lambdaFunction: bulkSummaryFn,
    comment: 'summarise bulk processing',
    inputPath: '$',
    resultPath: '$.bulkSummaryResult', // 保留原有工作流数据，新增当前任务结果
    outputPath: '$.bulkSummaryResult.Payload.output' // 仅传递Lambda返回的output内容给下一个任务
  });
  

• 检查Lambda执行权限与运行环境：确认Lambda函数拥有足够权限（比如CloudWatch日志写入权限），同时验证Node.js版本与代码兼容，且部署的是最新版本的函数代码。

内容的提问来源于stack exchange，提问作者Outto320