指定可选参数示例 – 使用 .NET 进行扫描示例 – 使用 .NET 进行并行扫描

扫描表和索引：.NET

Scan 操作读取 Amazon DynamoDB 中表或索引中的所有项目。

以下是使用 Amazon SDK for .NET 低级别 API 扫描表的步骤。

创建 AmazonDynamoDBClient 类的实例。
创建 ScanRequest 类的实例，并提供扫描操作参数。

唯一需要的参数是表名称。
执行 Scan 方法并提供您在之前步骤中创建的 ScanRequest 对象。

以下 Reply 表存储论坛主题的回复。

例


>Reply ( <emphasis role="underline">Id</emphasis>, <emphasis role="underline">ReplyDateTime</emphasis>, Message, PostedBy )

该表保存各种论坛主题的所有回复。因此，主键由 Id（分区键）和 ReplyDateTime（排序键）这两者组成。以下 C# 代码示例扫描整个表。ScanRequest 实例指定要扫描的表的名称。

例


AmazonDynamoDBClient client = new AmazonDynamoDBClient();

var request = new ScanRequest
{
    TableName = "Reply",
};

var response = client.Scan(request);
var result = response.ScanResult;

foreach (Dictionary<string, AttributeValue> item in response.ScanResult.Items)
{
  // Process the result.
  PrintItem(item);
}

指定可选参数

Scan 方法支持多个可选参数。例如，您可以选择使用扫描筛选器来筛选扫描结果。在扫描筛选器中，您可以指定条件，以及计算条件的属性名称。有关更多信息，请参阅 Scan。

以下 C# 代码扫描 ProductCatalog 表查找定价小于 0 的项目。该示例指定了以下可选参数：

FilterExpression参数，仅检索价格小于 0 的项目（错误条件）。
ProjectionExpression 参数，指定要为查询结果中的项目检索的属性。

以下 C# 示例扫描 ProductCatalog 表查找定价小于 0 的所有项目。

例


var forumScanRequest = new ScanRequest
 {
   TableName = "ProductCatalog",
   // Optional parameters.
   ExpressionAttributeValues = new Dictionary<string,AttributeValue> {
        {":val", new AttributeValue { N = "0" }}
   },
   FilterExpression = "Price < :val",
   ProjectionExpression = "Id"
 };

您还可以选择通过添加可选的 Limit 参数来限制页面大小（每页的项目数量）。每次执行 Scan 方法时，您都会获得一页结果，其中包含指定数量的项目。要抓取下一页结果，您可以提供上一页中最后一个项目的主键值再次执行 Scan 方法，这样 Scan 方法就会返回下一组项目。您可以通过设置 ExclusiveStartKey 属性在请求中提供此信息。最初，此属性可以为空。要检索后面的页面，您必须将此属性值更新为上一页中最后一个项目的主键。

以下 C# 代码示例扫描 ProductCatalog 表。它在请求中指定了 Limit 和 ExclusiveStartKey 可选参数。do/while 循环将不断执行一次扫描一页的操作，直到 LastEvaluatedKey 返回空值。

例


Dictionary<string, AttributeValue> lastKeyEvaluated = null;
do
{
    var request = new ScanRequest
    {
        TableName = "ProductCatalog",
        Limit = 10,
        ExclusiveStartKey = lastKeyEvaluated
    };

    var response = client.Scan(request);

    foreach (Dictionary<string, AttributeValue> item
      in response.Items)
    {
        PrintItem(item);
    }
    lastKeyEvaluated = response.LastEvaluatedKey;

} while (lastKeyEvaluated != null && lastKeyEvaluated.Count != 0);

示例 – 使用 .NET 进行扫描

下面的 C# 代码提供了一个工作示例，该示例扫描 ProductCatalog 表查找价格小于 0 的项目。

有关测试以下示例的分步说明，请参阅 .NET 代码示例。


using System;
using System.Collections.Generic;
using Amazon.DynamoDBv2;
using Amazon.DynamoDBv2.Model;
using Amazon.Runtime;

namespace com.amazonaws.codesamples
{
    class LowLevelScan
    {
        private static AmazonDynamoDBClient client = new AmazonDynamoDBClient();

        static void Main(string[] args)
        {
            try
            {
                FindProductsForPriceLessThanZero();

                Console.WriteLine("Example complete. To continue, press Enter");
                Console.ReadLine();
            }
            catch (Exception e)
            {
                Console.WriteLine(e.Message);
                Console.WriteLine("To continue, press Enter");
                Console.ReadLine();
            }
        }

        private static void FindProductsForPriceLessThanZero()
        {
            Dictionary<string, AttributeValue> lastKeyEvaluated = null;
            do
            {
                var request = new ScanRequest
                {
                    TableName = "ProductCatalog",
                    Limit = 2,
                    ExclusiveStartKey = lastKeyEvaluated,
                    ExpressionAttributeValues = new Dictionary<string, AttributeValue> {
                    {":val", new AttributeValue {
                         N = "0"
                     }}
                },
                    FilterExpression = "Price < :val",

                    ProjectionExpression = "Id, Title, Price"
                };

                var response = client.Scan(request);

                foreach (Dictionary<string, AttributeValue> item
                     in response.Items)
                {
                    Console.WriteLine("\nScanThreadTableUsePaging - printing.....");
                    PrintItem(item);
                }
                lastKeyEvaluated = response.LastEvaluatedKey;
            } while (lastKeyEvaluated != null && lastKeyEvaluated.Count != 0);

            Console.WriteLine("To continue, press Enter");
            Console.ReadLine();
        }

        private static void PrintItem(
            Dictionary<string, AttributeValue> attributeList)
        {
            foreach (KeyValuePair<string, AttributeValue> kvp in attributeList)
            {
                string attributeName = kvp.Key;
                AttributeValue value = kvp.Value;

                Console.WriteLine(
                    attributeName + " " +
                    (value.S == null ? "" : "S=[" + value.S + "]") +
                    (value.N == null ? "" : "N=[" + value.N + "]") +
                    (value.SS == null ? "" : "SS=[" + string.Join(",", value.SS.ToArray()) + "]") +
                    (value.NS == null ? "" : "NS=[" + string.Join(",", value.NS.ToArray()) + "]")
                    );
            }
            Console.WriteLine("************************************************");
        }
    }
}

示例 – 使用 .NET 进行并行扫描

以下 C# 代码示例演示了并行扫描。该程序删除并重新创建 ProductCatalog 表，然后加载数据。当数据加载完成时，程序会生成多个线程，发出并行 Scan 请求。最后，该程序输出运行时统计信息的摘要。

有关测试以下示例的分步说明，请参阅 .NET 代码示例。


using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using Amazon.DynamoDBv2;
using Amazon.DynamoDBv2.Model;
using Amazon.Runtime;

namespace com.amazonaws.codesamples
{
    class LowLevelParallelScan
    {
        private static AmazonDynamoDBClient client = new AmazonDynamoDBClient();
        private static string tableName = "ProductCatalog";
        private static int exampleItemCount = 100;
        private static int scanItemLimit = 10;
        private static int totalSegments = 5;


        static void Main(string[] args)
        {
            try
            {
                DeleteExampleTable();
                CreateExampleTable();
                UploadExampleData();
                ParallelScanExampleTable();
            }
            catch (AmazonDynamoDBException e) { Console.WriteLine(e.Message); }
            catch (AmazonServiceException e) { Console.WriteLine(e.Message); }
            catch (Exception e) { Console.WriteLine(e.Message); }

            Console.WriteLine("To continue, press Enter");
            Console.ReadLine();
        }

        private static void ParallelScanExampleTable()
        {
            Console.WriteLine("\n*** Creating {0} Parallel Scan Tasks to scan {1}", totalSegments, tableName);
            Task[] tasks = new Task[totalSegments];
            for (int segment = 0; segment < totalSegments; segment++)
            {
                int tmpSegment = segment;
                Task task = Task.Factory.StartNew(() =>
                                  {
                                      ScanSegment(totalSegments, tmpSegment);
                                  });

                tasks[segment] = task;
            }

            Console.WriteLine("All scan tasks are created, waiting for them to complete.");
            Task.WaitAll(tasks);

            Console.WriteLine("All scan tasks are completed.");
        }

        private static void ScanSegment(int totalSegments, int segment)
        {
            Console.WriteLine("*** Starting to Scan Segment {0} of {1} out of {2} total segments ***", segment, tableName, totalSegments);
            Dictionary<string, AttributeValue> lastEvaluatedKey = null;
            int totalScannedItemCount = 0;
            int totalScanRequestCount = 0;
            do
            {
                var request = new ScanRequest
                {
                    TableName = tableName,
                    Limit = scanItemLimit,
                    ExclusiveStartKey = lastEvaluatedKey,
                    Segment = segment,
                    TotalSegments = totalSegments
                };

                var response = client.Scan(request);
                lastEvaluatedKey = response.LastEvaluatedKey;
                totalScanRequestCount++;
                totalScannedItemCount += response.ScannedCount;
                foreach (var item in response.Items)
                {
                    Console.WriteLine("Segment: {0}, Scanned Item with Title: {1}", segment, item["Title"].S);
                }
            } while (lastEvaluatedKey.Count != 0);

            Console.WriteLine("*** Completed Scan Segment {0} of {1}. TotalScanRequestCount: {2}, TotalScannedItemCount: {3} ***", segment, tableName, totalScanRequestCount, totalScannedItemCount);
        }

        private static void UploadExampleData()
        {
            Console.WriteLine("\n*** Uploading {0} Example Items to {1} Table***", exampleItemCount, tableName);
            Console.Write("Uploading Items: ");
            for (int itemIndex = 0; itemIndex < exampleItemCount; itemIndex++)
            {
                Console.Write("{0}, ", itemIndex);
                CreateItem(itemIndex.ToString());
            }
            Console.WriteLine();
        }

        private static void CreateItem(string itemIndex)
        {
            var request = new PutItemRequest
            {
                TableName = tableName,
                Item = new Dictionary<string, AttributeValue>()
            {
                { "Id", new AttributeValue {
                      N = itemIndex
                  }},
                { "Title", new AttributeValue {
                      S = "Book " + itemIndex + " Title"
                  }},
                { "ISBN", new AttributeValue {
                      S = "11-11-11-11"
                  }},
                { "Authors", new AttributeValue {
                      SS = new List<string>{"Author1", "Author2" }
                  }},
                { "Price", new AttributeValue {
                      N = "20.00"
                  }},
                { "Dimensions", new AttributeValue {
                      S = "8.5x11.0x.75"
                  }},
                { "InPublication", new AttributeValue {
                      BOOL = false
                  } }
            }
            };
            client.PutItem(request);
        }

        private static void CreateExampleTable()
        {
            Console.WriteLine("\n*** Creating {0} Table ***", tableName);
            var request = new CreateTableRequest
            {
                AttributeDefinitions = new List<AttributeDefinition>()
            {
                new AttributeDefinition
                {
                    AttributeName = "Id",
                    AttributeType = "N"
                }
            },
                KeySchema = new List<KeySchemaElement>
            {
                new KeySchemaElement
                {
                    AttributeName = "Id",
                    KeyType = "HASH" //Partition key
                }
            },
                ProvisionedThroughput = new ProvisionedThroughput
                {
                    ReadCapacityUnits = 5,
                    WriteCapacityUnits = 6
                },
                TableName = tableName
            };

            var response = client.CreateTable(request);

            var result = response;
            var tableDescription = result.TableDescription;
            Console.WriteLine("{1}: {0} \t ReadsPerSec: {2} \t WritesPerSec: {3}",
                      tableDescription.TableStatus,
                      tableDescription.TableName,
                      tableDescription.ProvisionedThroughput.ReadCapacityUnits,
                      tableDescription.ProvisionedThroughput.WriteCapacityUnits);

            string status = tableDescription.TableStatus;
            Console.WriteLine(tableName + " - " + status);

            WaitUntilTableReady(tableName);
        }

        private static void DeleteExampleTable()
        {
            try
            {
                Console.WriteLine("\n*** Deleting {0} Table ***", tableName);
                var request = new DeleteTableRequest
                {
                    TableName = tableName
                };

                var response = client.DeleteTable(request);
                var result = response;
                Console.WriteLine("{0} is being deleted...", tableName);
                WaitUntilTableDeleted(tableName);
            }
            catch (ResourceNotFoundException)
            {
                Console.WriteLine("{0} Table delete failed: Table does not exist", tableName);
            }
        }

        private static void WaitUntilTableReady(string tableName)
        {
            string status = null;
            // Let us wait until table is created. Call DescribeTable.
            do
            {
                System.Threading.Thread.Sleep(5000); // Wait 5 seconds.
                try
                {
                    var res = client.DescribeTable(new DescribeTableRequest
                    {
                        TableName = tableName
                    });

                    Console.WriteLine("Table name: {0}, status: {1}",
                              res.Table.TableName,
                              res.Table.TableStatus);
                    status = res.Table.TableStatus;
                }
                catch (ResourceNotFoundException)
                {
                    // DescribeTable is eventually consistent. So you might
                    // get resource not found. So we handle the potential exception.
                }
            } while (status != "ACTIVE");
        }

        private static void WaitUntilTableDeleted(string tableName)
        {
            string status = null;
            // Let us wait until table is deleted. Call DescribeTable.
            do
            {
                System.Threading.Thread.Sleep(5000); // Wait 5 seconds.
                try
                {
                    var res = client.DescribeTable(new DescribeTableRequest
                    {
                        TableName = tableName
                    });

                    Console.WriteLine("Table name: {0}, status: {1}",
                              res.Table.TableName,
                              res.Table.TableStatus);
                    status = res.Table.TableStatus;
                }
                catch (ResourceNotFoundException)
                {
                    Console.WriteLine("Table name: {0} is not found. It is deleted", tableName);
                    return;
                }
            } while (status == "DELETING");
        }
    }
}

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