掃描表格和索引：。 NET - Amazon DynamoDB
指定選用參數範例-掃描使用. NET 範例-parallel 掃描使用. NET

本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

掃描表格和索引：。 NET

此 Scan 操作會讀取 Amazon DynamoDB 中資料表或索引中的所有項目。

以下是使用 AWS SDK for .NET 低級別API掃描表的步驟：

  1. 建立 AmazonDynamoDBClient 類別的執行個體。

  2. 建立 ScanRequest 類別的執行個體，並提供掃描操作參數。

    唯一所需的參數是資料表名稱。

  3. 執行 Scan 方法，並提供您在上一個步驟中建立的 ScanRequest 物件。

以下 Reply 資料表會存放論壇主題的回覆。

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

此資料表會維護各個論壇主題的所有回覆。因此，主索引鍵同時包含 Id (分割區索引鍵) 和 ReplyDateTime (排序索引鍵)。下列 C# 程式碼範例會掃描整個資料表。此 ScanRequest 執行個體會指定要掃描的資料表名稱。

範例 
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);
}

指定選用參數

Scan 方法支援數個選用的參數。例如，您可以選擇使用掃描篩選條件來篩選掃描結果。在掃描篩選條件中，您可以指定條件以及要據以評估條件的屬性名稱。如需詳細資訊，請參閱掃描。

以下 C# 程式碼會掃描 ProductCatalog 資料表，從中尋找價格低於 0 的項目。範例會指定下列選用參數：

  • 只擷取價格低於 0 的項目的 FilterExpression 參數 (錯誤條件)。

  • 指定在查詢結果中要擷取的項目 ProjectionExpression 參數。

下列 C# 範例會掃描 ProductCatalog 資料表，從中尋找價格低於 0 的所有項目。

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

您也可以透過新增選用的 Limit 參數，選擇性地限制頁面大小或是每一頁面的項目數目。每次您執行 Scan 方法時，就會擷取一頁具有指定項目數的結果。若要擷取下一頁面，您可以再次執行 Scan 方法，並提供先前頁面最後一個項目的主索引鍵值，讓 Scan 方法傳回下一組項目。您可以透過設定 ExclusiveStartKey 屬性，在請求中提供此資訊。一開始，此屬性可為 Null。若要擷取後續頁面，您必須將此屬性值更新為先前頁面最後一個項目的主索引鍵。

下列 C# 程式碼範例會掃描 ProductCatalog 資料表。在請求中，它會指定 Limit 和 ExclusiveStartKey 選用參數。do/while 迴圈會繼續一次掃描一個頁面，直到 LastEvaluatedKey 傳回 Null 數值為止。

範例 
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);

範例-掃描使用. NET

下列 C# 程式碼提供的工作範例會掃描 ProductCatalog 資料表，從中尋找價格低於 0 的項目。

如需測試下列範例的 step-by-step 指示，請參閱。 NET程式碼範例。

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("************************************************");
        }
    }
}

範例-parallel 掃描使用. NET

下列 C# 程式碼範例示範平行掃描。此程式會刪除再重新建立 ProductCatalog 資料表，然後載入含有資料的資料表。當資料載入完成時，此程式會產生多個執行緒並發出平行 Scan 請求。最後，此程式會列印執行時間統計數字的摘要。

如需測試下列範例的 step-by-step 指示，請參閱。 NET程式碼範例。

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");
        }
    }
}