Amazon DynamoDB
开发人员指南 (API Version 2012-08-10)
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扫描表和索引:.NET

Scan 操作读取表或索引中的所有项目。

以下介绍使用适用于 .NET 的 AWS 开发工具包低级 API 扫描表的步骤:

  1. 创建 AmazonDynamoDBClient 类的实例。

  2. 创建 ScanRequest 类的实例,并提供扫描操作参数。

    唯一必需的参数是表名称。

  3. 执行 Scan 方法并提供您在之前步骤中创建的 QueryRequest 数据元。

以下 Reply 表存储论坛话题的回复。

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

该表存储各种论坛话题的所有回复。因此,主键由 Id (分区键) 和 ReplyDateTime (排序键) 这两者组成。以下 C# 代码段扫描整个表。ScanRequest 实例指定要扫描的表的名称。

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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条件来筛选扫描结果。在扫描筛选条件中,您可以指定一个条件和一个属性名称 (根据此属性评估条件)。有关更多信息,请参阅Scan

以下 C# 代码扫描 ProductCatalog 表,以查找价格低于 0 的项目。该示例指定了以下可选参数:

  • FilterExpression 参数,只检索价格低于 0 (错误条件) 的项目。

  • ProjectionExpression 参数,指定要为查询结果中的项目检索的属性。

以下 C# 代码段扫描 ProductCatalog 表,以查找价格低于 0 的所有项目。

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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# 代码段扫描 ProductCatalog 表。它在请求中指定了 LimitExclusiveStartKey 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。

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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

以下 C# 代码示例提供了扫描 ProductCatalog 表,以查找价格低于 0 的项目的使用实例。

有关测试以下示例的分步说明,请参阅.NET 代码示例

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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。该程序先删除再重新创建 ProductCatalog 表,然后在表中加载数据。数据加载完后,该程序会生成多个线程并发出并行 Scan 请求。最终,该程序会打印运行时统计信息汇总。

有关测试以下示例的分步说明,请参阅.NET 代码示例

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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"); } } }