DynamoDB ConditionalCheckFailedException：嵌套字段条件更新异常排查

现有DynamoDB条目 #

RequestId:'1234567890' // 分区键
LastScanDateTime: '2023-01-12T11:00:00.111Z'
SubjectUpdateCounter: {
  Maths: 1,
  English: 1
}

执行的更新请求 #

{
    "TableName": "EmpDetails",
    "Key": {
        "RequestId": {
            "S": "1234567890"
        }
    },
    "ConditionExpression": "(#subjectUpdateCounter > :subjectUpdateCounterLimit)",
    "UpdateExpression": "ADD #subjectUpdateCounter :dec  SET LastScanDateTime = :LastScanDateTime,",
    "ExpressionAttributeValues": {
        ":dec": {
            "N": "-1"
        },
        ":LastScanDateTime": {
            "S": "2023-02-13T18:14:52.143Z"
        },
        ":subjectUpdateCounterLimit": {
            "N": "0"
        }
    },
    "ReturnValues": "NONE",
    "ExpressionAttributeNames": {
        "#subjectUpdateCounter": "SubjectUpdateCounter.Maths"
    }
}

触发的错误信息 #

ConditionalCheckFailedException: The conditional request failed
  ....
  ....
  '$fault': 'client',
  '$metadata': {
    httpStatusCode: 400,
    requestId: '12345ygfsdfagagdf',
    extendedRequestId: undefined,
    cfId: undefined,
    attempts: 1,
    totalRetryDelay: 0
  },
  __type: 'com.amazonaws.dynamodb.v20120810#ConditionalCheckFailedException'
}

问题 #

当前SubjectUpdateCounter.Maths的值为1，大于0，符合条件表达式要求，但请求仍触发异常，请问原因是什么？

问题原因分析 #

有两个关键问题导致了这个异常：

1. UpdateExpression存在语法错误
   你的UpdateExpression末尾多了一个多余的逗号：SET LastScanDateTime = :LastScanDateTime,，这个语法错误会导致请求解析失败，进而触发客户端错误，表现为条件检查失败的异常。

2. 嵌套属性的引用方式错误
   在ExpressionAttributeNames中，你把#subjectUpdateCounter映射为"SubjectUpdateCounter.Maths"，但DynamoDB会将这个值视为单个完整的属性名，而非嵌套属性路径。也就是说，条件表达式实际在检查根级别是否存在名为SubjectUpdateCounter.Maths的属性，而不是SubjectUpdateCounter对象下的Maths字段。由于这个根属性不存在，条件判定不满足，最终触发ConditionalCheckFailedException。

修正后的请求示例 #

{
    "TableName": "EmpDetails",
    "Key": {
        "RequestId": {
            "S": "1234567890"
        }
    },
    "ConditionExpression": "(#parent.#maths > :subjectUpdateCounterLimit)",
    "UpdateExpression": "ADD #parent.#maths :dec SET LastScanDateTime = :LastScanDateTime",
    "ExpressionAttributeValues": {
        ":dec": {
            "N": "-1"
        },
        ":LastScanDateTime": {
            "S": "2023-02-13T18:14:52.143Z"
        },
        ":subjectUpdateCounterLimit": {
            "N": "0"
        }
    },
    "ReturnValues": "NONE",
    "ExpressionAttributeNames": {
        "#parent": "SubjectUpdateCounter",
        "#maths": "Maths"
    }
}

内容的提问来源于stack exchange，提问作者Krishna