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Scanning Tables Using the AWS SDK for .NET Low-Level API
The Scan method scans the entire table and you should therefore use queries
to retrieve information. To learn more about performance related to scan and query operations,
see Query and Scan Operations.
The following are the steps to scan a table using the AWS SDK for NET low-level API:
Create an instance of the
AmazonDynamoDBClientclass and provide your credentials.Create an instance of the
ScanRequestclass and provide scan operation parameters.The only required parameter is the table name.
Execute the
Scanmethod and provide theQueryRequestobject that you created in the preceding step.
The following Reply table stores replies for forum threads.
Reply ( Id, ReplyDateTime, Message, PostedBy )
The table maintains all the replies for various forum threads. Therefore, the primary
key is composed of both the Id (hash attribute) and ReplyDateTime (range attribute). The
following C# code snippet scans the entire table. The ScanRequest instance
specifies the name of the table to scan.
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);
}
Specifying Optional Parameters
The Scan method supports several optional parameters. For example, you can
optionally use a scan filter to filter the scan result. In a scan filter, you can specify a
condition and an attribute name on which you want the condition evaluated. For more
information about the parameters and the API, see Scan.
The following C# code scans the ProductCatalog table to find items that are priced less than 0. The sample specifies the following optional parameters:
A
ScanFilterparameter to retrieve only the items priced less than 0 (error condition).An
AttributesToGetparameter to specify a list of attributes to retrieve for items in the query results.
The following C# code snippet scans the ProductCatalog table to find all items priced less than 0.
var forumScanRequest = new ScanRequest
{
TableName = "ProductCatalog",
// Optional parameters.
ScanFilter = new Dictionary<string, Condition>()
{
{ "Price", new Condition
{
ComparisonOperator = "LT",
AttributeValueList = new List<AttributeValue>()
{
new AttributeValue { N = 0 }
}
}
}
},
AttributesToGet = new List<string> { "Id" }
};
You can also optionally limit the page size, or the number of items per page, by
adding the optional Limit parameter. Each time you execute the Scan
method, you get one page of results that has the specified number of items. To fetch the next
page, you execute the Scan method again by providing the primary key value of the
last item in the previous page so that the Scan method can return the next set of
items. You provide this information in the request by setting the
ExclusiveStartKey property. Initially, this property can be null. To retrieve
subsequent pages, you must update this property value to the primary key of the last item in
the preceding page.
The following C# code snippet scans the ProductCatalog table. In the request, it specifies
the Limit and ExclusiveStartKey optional parameters. The
do/while loop continues to scan one page at time until the
LastEvaluatedKey returns a null value.
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.ScanResult.Items)
{
PrintItem(item);
}
lastKeyEvaluated = response.ScanResult.LastEvaluatedKey;
} while (lastKeyEvaluated != null);
Additional AWS SDK for .NET APIs
The examples in this section use AWS SDK for .NET low-level API. The SDK also supports helper APIs that further simplify your application development. Additionally, the SDK provides Object Persistence Model API that enable you to map your client-side classes to your Amazon DynamoDB tables and perform scan operations on them. For more information, see Using the AWS SDK for .NET.
Example - Scan Using .NET
The following C# code example provides a working sample that scans the ProductCatalog table to find items priced less than 0.
For step-by-step instructions to test the following sample, see Using the AWS SDK for .NET.
using System;
using System.Collections.Generic;
using Amazon.DynamoDBv2;
using Amazon.DynamoDBv2.Model;
using Amazon.Runtime;
namespace Amazon.DynamoDBv2.Documentation
{
class Program
{
private static AmazonDynamoDBClient client;
static void Main(string[] args)
{
try
{
var config = new AmazonDynamoDBConfig();
config.ServiceURL = System.Configuration.ConfigurationManager.AppSettings["ServiceURL"];
client = new AmazonDynamoDBClient(config);
FindProductsForPriceLessThanZero();
Console.WriteLine("Example complete. To continue, press Enter");
Console.ReadLine();
}
catch (AmazonDynamoDBException e) { Console.WriteLine(e.Message); }
catch (AmazonServiceException e) { Console.WriteLine(e.Message); }
catch (Exception e) { Console.WriteLine(e.Message); }
}
private static void FindProductsForPriceLessThanZero()
{
Dictionary<string, AttributeValue> lastKeyEvaluated = null;
do
{
var request = new ScanRequest
{
TableName = "ProductCatalog",
Limit = 2,
ExclusiveStartKey = lastKeyEvaluated,
ScanFilter = new Dictionary<string, Condition>()
{
{ "Price",
new Condition
{
ComparisonOperator = "LT",
AttributeValueList = new List<AttributeValue>()
{
new AttributeValue { N = "0" }
}
}
}
},
AttributesToGet = new List<string> { "Id", "Title", "Price" }
};
var response = client.Scan(request);
foreach (Dictionary<string, AttributeValue> item
in response.ScanResult.Items)
{
Console.WriteLine("\nScanThreadTableUsePaging - printing.....");
PrintItem(item);
}
lastKeyEvaluated = response.ScanResult.LastEvaluatedKey;
} while (lastKeyEvaluated != null);
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("************************************************");
}
}
}Example - Parallel Scan Using .NET
The following C# code example demonstrates a parallel scan. The program deletes and then re-creates the ProductCatalog table, then loads the table with data. When the data load is finished, the program spawns multiple threads and issues parallel Scan requests. Finally, the program prints a summary of run time statistics.
For step-by-step instructions to test the following sample, see Using the AWS SDK for .NET.
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using Amazon.DynamoDBv2;
using Amazon.DynamoDBv2.Model;
using Amazon.Runtime;
namespace Amazon.DynamoDBv2.Documentation
{
class Program
{
private static string tableName = "ProductCatalog";
private static int exampleItemCount = 100;
private static int scanItemLimit = 10;
private static int totalSegments = 5;
private static AmazonDynamoDBClient client;
static void Main(string[] args)
{
// You need client to send requests.
var config = new AmazonDynamoDBConfig();
config.ServiceURL = System.Configuration.ConfigurationManager.AppSettings["ServiceURL"];
client = new AmazonDynamoDBClient(config);
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.ScanResult.LastEvaluatedKey;
totalScanRequestCount++;
totalScannedItemCount += response.ScanResult.ScannedCount;
foreach (var item in response.ScanResult.Items)
{
Console.WriteLine("Segment: {0}, Scanned Item with Title: {1}", segment, item["Title"].S);
}
} while (lastEvaluatedKey != null);
Console.WriteLine("*** Completed Scan Segment {0} of {1}. TotalScanRequestCount: {2}, TotalScannedItemCount: {3} ***", segment, tableName, totalScanRequestCount, totalScannedItemCount);
}
private static void UploadExampleData()
{
Console.WriteLine("\n*** Upoading {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 { N = "0" } }// 0 = 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"
}
},
ProvisionedThroughput = new ProvisionedThroughput
{
ReadCapacityUnits = 5,
WriteCapacityUnits = 6
},
TableName = tableName
};
var response = client.CreateTable(request);
var result = response.CreateTableResult;
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.DeleteTableResult;
Console.WriteLine("{0} is being deleted...", tableName);
WaitUntilTableDeleted(tableName);
}
catch (ResourceNotFoundException resouceNotFound)
{
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.DescribeTableResult.Table.TableName,
res.DescribeTableResult.Table.TableStatus);
status = res.DescribeTableResult.Table.TableStatus;
}
catch (ResourceNotFoundException resouceNotFound)
{
// 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.DescribeTableResult.Table.TableName,
res.DescribeTableResult.Table.TableStatus);
status = res.DescribeTableResult.Table.TableStatus;
}
catch (ResourceNotFoundException resouceNotFound)
{
Console.WriteLine("Table name: {0} is not found. It is deleted", tableName);
return;
}
} while (status == "DELETING");
}
}
}
