DynamoDB扫描返回多个扫描结果

Question

所以我写了下面的函数。这个版本有点删节，我已经匿名的数据，但关键的组成部分在那里。

该函数基本上接受来自API调用的参数列表，查询每个参数的db，然后返回结果。

我发现扫描在一个参数下运行得很好，但是当超过一个参数被调用时返回重复的数据。从日志中我可以看到，当传递多个参数时，扫描会多次运行。

例如，使用一个param函数日志返回

2020-03-19 20:27:42.974 Starting the 0 scan with 3 as the id 
2020-03-19 20:27:43.047 The 0 scan has completed successfully

对于两个参数，日志是

2020-03-19 20:28:42.189 Starting the 0 scan with 2 as the id
2020-03-19 20:28:42.261 The 0 scan has completed successfully
2020-03-19 20:28:42.262 Starting the 1 scan with 3 as the id
2020-03-19 20:28:42.267 The 0 scan has completed successfully
2020-03-19 20:28:42.293 The 1 scan has completed successfully

对于3个params，原木是

2020-03-19 20:29:49.209 Starting the 0 scan with 1 as the id
2020-03-19 20:29:49.323 The 0 scan has completed successfully
2020-03-19 20:29:49.325 Starting the 1 scan with 2 as the id
2020-03-19 20:29:49.329 The 0 scan has completed successfully
2020-03-19 20:29:49.380 The 1 scan has completed successfully
2020-03-19 20:29:49.381 Starting the 2 scan with 3 as the id
2020-03-19 20:29:49.385 The 1 scan has completed successfully
2020-03-19 20:29:49.437 The 2 scan has completed successfully

下面是运行for循环和扫描的代码。我对参数进行了硬编码，排除了一些无关的内容。

     const params = ['1','2','3'];
     for (let i = 0; i < params.length; i++) {
      console.log("Starting the " + i + " scan with " + params[i] + " as the scan parameter")
      const scanParams = {
      TableName: "Dynamo_Table",
      FilterExpression: "Org = :Org",
      ExpressionAttributeValues: { ":Org": params[i] },
      ProjectionExpression: "User_ID, Org, first_name, last_name"
     };
     await dynamoClient.scan(scanParams, function(err, data) {
      if (err) {
        console.log("data retrival failed, error logged is :" + err);
        return err;
      }
      else {
        console.log("The " + i +" scan has completed successfully")
        //console.log("data retrival successful: " + JSON.stringify(data));
        userData = userData.concat(data.Items)
        //console.log("partial data structure is " + data)
      }
    }).promise();
  }
      responseData = JSON.stringify(userData)
      console.log("Complete response is " + responseData)
      console.log("data after execution scan is " + data)

我试图通过定义等待并使用AWS的.promise()函数来迫使程序等待扫描的竞争。然而，它们似乎并没有阻止线程的执行。我不太清楚为什么它会启动多次扫描。for循环运行的次数不超过应该运行的次数，那么为什么要调用搜索函数呢？

Answer 1

下面是一个示例，说明如何使用DynamoDB DocumentClient按分区键查询多个项并收集结果。这使用了query()调用的Promisified变体，并等待使用Promise.all()实现所有查询承诺。

var AWS = require('aws-sdk');
AWS.config.update({ region: 'us-east-1' });

const dc = new AWS.DynamoDB.DocumentClient();

// Array of organization IDs we want to query
const orgs = ['1', '2', '3'];

// Async function to query for one specific organization ID
const queryOrg = async org => {
  const params = {
    TableName: 'orgs',
    KeyConditionExpression: 'org = :o1',
    ExpressionAttributeValues: { ':o1': org, },
  };

  return dc.query(params).promise();
}

// Async IIFE because you cannot use await outside of an async function
(async () => {
  // Array of promises representing async organization queries made
  const promises = orgs.map(org => queryOrg(org));

  // Wait for all queries to complete and collect the results in an array
  const items = await Promise.all(promises);

  // Results are present in the same order that the queries were mapped
  for (const item of items) {
    console.log('Item:', item.Items[0]);
  }
})();

Answer 2

每当您想在DynamoDB数据库中搜索某些内容时，建议您使用查询选项，而不是Scan

这是因为扫描读取数据库的每一项，而查询只查找提到的Hask键(主键)。

如果您想要查找具有特定“属性”的数据，可以使用Global备用索引，其中可以将“属性”设置为Hash键，同时可以选择一种您选择的键。这可能解决表多次返回答案的问题。