ScanRequest Class | AWS SDK for .NET V2

Container for the parameters to the Scan operation. The Scan operation returns one or more items and item attributes by accessing every item in a table or a secondary index. To have DynamoDB return fewer items, you can provide a ScanFilter operation.

If the total number of scanned items exceeds the maximum data set size limit of 1 MB, the scan stops and results are returned to the user as a LastEvaluatedKey value to continue the scan in a subsequent operation. The results also include the number of items exceeding the limit. A scan can result in no table data meeting the filter criteria.

By default, Scan operations proceed sequentially; however, for faster performance on a large table or secondary index, applications can request a parallel Scan operation by providing the Segment and TotalSegments parameters. For more information, see Parallel Scan in the Amazon DynamoDB Developer Guide.

By default, Scan uses eventually consistent reads when accessing the data in a table; therefore, the result set might not include the changes to data in the table immediately before the operation began. If you need a consistent copy of the data, as of the time that the Scan begins, you can set the ConsistentRead parameter to true.

Inheritance Hierarchy

System.Object
  Amazon.Runtime.AmazonWebServiceRequest
    Amazon.DynamoDBv2.AmazonDynamoDBRequest
      Amazon.DynamoDBv2.Model.ScanRequest

Namespace: Amazon.DynamoDBv2.Model
Assembly: AWSSDK.dll
Version: (assembly version)

Syntax

public class ScanRequest : AmazonDynamoDBRequest
         IRequestEvents

The ScanRequest type exposes the following members

Constructors

	Name	Description
	ScanRequest()	Empty constructor used to set properties independently even when a simple constructor is available
	ScanRequest(string)	Instantiates ScanRequest with the parameterized properties

Properties

Name	Type	Description
AttributesToGet	System.Collections.Generic.List<System.String>	Gets and sets the property AttributesToGet. This is a legacy parameter, for backward compatibility. New applications should use ProjectionExpression instead. Do not combine legacy parameters and expression parameters in a single API call; otherwise, DynamoDB will return a ValidationException exception. This parameter allows you to retrieve attributes of type List or Map; however, it cannot retrieve individual elements within a List or a Map. The names of one or more attributes to retrieve. If no attribute names are provided, then all attributes will be returned. If any of the requested attributes are not found, they will not appear in the result. Note that AttributesToGet has no effect on provisioned throughput consumption. DynamoDB determines capacity units consumed based on item size, not on the amount of data that is returned to an application.
ConditionalOperator	Amazon.DynamoDBv2.ConditionalOperator	Gets and sets the property ConditionalOperator. This is a legacy parameter, for backward compatibility. New applications should use FilterExpression instead. Do not combine legacy parameters and expression parameters in a single API call; otherwise, DynamoDB will return a ValidationException exception. A logical operator to apply to the conditions in a ScanFilter map: `AND` - If all of the conditions evaluate to true, then the entire map evaluates to true. `OR` - If at least one of the conditions evaluate to true, then the entire map evaluates to true. If you omit ConditionalOperator, then `AND` is the default. The operation will succeed only if the entire map evaluates to true. This parameter does not support attributes of type List or Map.
ConsistentRead	System.Boolean	Gets and sets the property ConsistentRead. A Boolean value that determines the read consistency model during the scan: If ConsistentRead is `false`, then the data returned from Scan might not contain the results from other recently completed write operations (PutItem, UpdateItem or DeleteItem). If ConsistentRead is `true`, then all of the write operations that completed before the Scan began are guaranteed to be contained in the Scan response. The default setting for ConsistentRead is `false`. The ConsistentRead parameter is not supported on global secondary indexes. If you scan a global secondary index with ConsistentRead set to true, you will receive a ValidationException.
ExclusiveStartKey	System.Collections.Generic.Dictionary<System.String, Amazon.DynamoDBv2.Model.AttributeValue>	Gets and sets the property ExclusiveStartKey. The primary key of the first item that this operation will evaluate. Use the value that was returned for LastEvaluatedKey in the previous operation. The data type for ExclusiveStartKey must be String, Number or Binary. No set data types are allowed. In a parallel scan, a Scan request that includes ExclusiveStartKey must specify the same segment whose previous Scan returned the corresponding value of LastEvaluatedKey.
ExpressionAttributeNames	System.Collections.Generic.Dictionary<System.String, System.String>	Gets and sets the property ExpressionAttributeNames. One or more substitution tokens for attribute names in an expression. The following are some use cases for using ExpressionAttributeNames: To access an attribute whose name conflicts with a DynamoDB reserved word. To create a placeholder for repeating occurrences of an attribute name in an expression. To prevent special characters in an attribute name from being misinterpreted in an expression. Use the # character in an expression to dereference an attribute name. For example, consider the following attribute name: `Percentile` The name of this attribute conflicts with a reserved word, so it cannot be used directly in an expression. (For the complete list of reserved words, see Reserved Words in the Amazon DynamoDB Developer Guide). To work around this, you could specify the following for ExpressionAttributeNames: `{"#P":"Percentile"}` You could then use this substitution in an expression, as in this example: `#P = :val` Tokens that begin with the : character are expression attribute values, which are placeholders for the actual value at runtime. For more information on expression attribute names, see Accessing Item Attributes in the Amazon DynamoDB Developer Guide.
ExpressionAttributeValues	System.Collections.Generic.Dictionary<System.String, Amazon.DynamoDBv2.Model.AttributeValue>	Gets and sets the property ExpressionAttributeValues. One or more values that can be substituted in an expression. Use the : (colon) character in an expression to dereference an attribute value. For example, suppose that you wanted to check whether the value of the ProductStatus attribute was one of the following: `Available \| Backordered \| Discontinued` You would first need to specify ExpressionAttributeValues as follows: { ":avail":{"S":"Available"}, ":back":{"S":"Backordered"}, ":disc":{"S":"Discontinued"} } You could then use these values in an expression, such as this: `ProductStatus IN (:avail, :back, :disc)` For more information on expression attribute values, see Specifying Conditions in the Amazon DynamoDB Developer Guide.
FilterExpression	System.String	Gets and sets the property FilterExpression. A string that contains conditions that DynamoDB applies after the Scan operation, but before the data is returned to you. Items that do not satisfy the FilterExpression criteria are not returned. A FilterExpression is applied after the items have already been read; the process of filtering does not consume any additional read capacity units. For more information, see Filter Expressions in the Amazon DynamoDB Developer Guide. FilterExpression replaces the legacy ScanFilter and ConditionalOperator parameters.
IndexName	System.String	Gets and sets the property IndexName. The name of a secondary index to scan. This index can be any local secondary index or global secondary index. Note that if you use the `IndexName` parameter, you must also provide `TableName`.
Limit	System.Int32	Gets and sets the property Limit. The maximum number of items to evaluate (not necessarily the number of matching items). If DynamoDB processes the number of items up to the limit while processing the results, it stops the operation and returns the matching values up to that point, and a key in LastEvaluatedKey to apply in a subsequent operation, so that you can pick up where you left off. Also, if the processed data set size exceeds 1 MB before DynamoDB reaches this limit, it stops the operation and returns the matching values up to the limit, and a key in LastEvaluatedKey to apply in a subsequent operation to continue the operation. For more information, see Query and Scan in the Amazon DynamoDB Developer Guide.
ProjectionExpression	System.String	Gets and sets the property ProjectionExpression. A string that identifies one or more attributes to retrieve from the specified table or index. These attributes can include scalars, sets, or elements of a JSON document. The attributes in the expression must be separated by commas. If no attribute names are specified, then all attributes will be returned. If any of the requested attributes are not found, they will not appear in the result. For more information, see Accessing Item Attributes in the Amazon DynamoDB Developer Guide. ProjectionExpression replaces the legacy AttributesToGet parameter.
ReturnConsumedCapacity	Amazon.DynamoDBv2.ReturnConsumedCapacity	Gets and sets the property ReturnConsumedCapacity.
ScanFilter	System.Collections.Generic.Dictionary<System.String, Amazon.DynamoDBv2.Model.Condition>	Gets and sets the property ScanFilter. This is a legacy parameter, for backward compatibility. New applications should use FilterExpression instead. Do not combine legacy parameters and expression parameters in a single API call; otherwise, DynamoDB will return a ValidationException exception. A condition that evaluates the scan results and returns only the desired values. This parameter does not support attributes of type List or Map. If you specify more than one condition in the ScanFilter map, then by default all of the conditions must evaluate to true. In other words, the conditions are ANDed together. (You can use the ConditionalOperator parameter to OR the conditions instead. If you do this, then at least one of the conditions must evaluate to true, rather than all of them.) Each ScanFilter element consists of an attribute name to compare, along with the following: AttributeValueList - One or more values to evaluate against the supplied attribute. The number of values in the list depends on the operator specified in ComparisonOperator . For type Number, value comparisons are numeric. String value comparisons for greater than, equals, or less than are based on ASCII character code values. For example, `a` is greater than `A`, and `a` is greater than `B`. For a list of code values, see http://en.wikipedia.org/wiki/ASCII#ASCII_printable_characters. For Binary, DynamoDB treats each byte of the binary data as unsigned when it compares binary values. For information on specifying data types in JSON, see JSON Data Format in the Amazon DynamoDB Developer Guide. ComparisonOperator - A comparator for evaluating attributes. For example, equals, greater than, less than, etc. The following comparison operators are available: EQ \| NE \| LE \| LT \| GE \| GT \| NOT_NULL \| NULL \| CONTAINS \| NOT_CONTAINS \| BEGINS_WITH \| IN \| BETWEEN For complete descriptions of all comparison operators, see Condition.
Segment	System.Int32	Gets and sets the property Segment. For a parallel Scan request, Segment identifies an individual segment to be scanned by an application worker. Segment IDs are zero-based, so the first segment is always 0. For example, if you want to use four application threads to scan a table or an index, then the first thread specifies a Segment value of 0, the second thread specifies 1, and so on. The value of LastEvaluatedKey returned from a parallel Scan request must be used as ExclusiveStartKey with the same segment ID in a subsequent Scan operation. The value for Segment must be greater than or equal to 0, and less than the value provided for TotalSegments. If you provide Segment, you must also provide TotalSegments.
Select	Amazon.DynamoDBv2.Select	Gets and sets the property Select. The attributes to be returned in the result. You can retrieve all item attributes, specific item attributes, or the count of matching items. `ALL_ATTRIBUTES` - Returns all of the item attributes. `COUNT` - Returns the number of matching items, rather than the matching items themselves. `SPECIFIC_ATTRIBUTES` - Returns only the attributes listed in AttributesToGet. This return value is equivalent to specifying AttributesToGet without specifying any value for Select. If neither Select nor AttributesToGet are specified, DynamoDB defaults to `ALL_ATTRIBUTES`. You cannot use both AttributesToGet and Select together in a single request, unless the value for Select is `SPECIFIC_ATTRIBUTES`. (This usage is equivalent to specifying AttributesToGet without any value for Select.)
TableName	System.String	Gets and sets the property TableName. The name of the table containing the requested items; or, if you provide `IndexName`, the name of the table to which that index belongs.
TotalSegments	System.Int32	Gets and sets the property TotalSegments. For a parallel Scan request, TotalSegments represents the total number of segments into which the Scan operation will be divided. The value of TotalSegments corresponds to the number of application workers that will perform the parallel scan. For example, if you want to use four application threads to scan a table or an index, specify a TotalSegments value of 4. The value for TotalSegments must be greater than or equal to 1, and less than or equal to 1000000. If you specify a TotalSegments value of 1, the Scan operation will be sequential rather than parallel. If you specify TotalSegments, you must also specify Segment.

Examples

The following example shows how to scan items in a table.
Note: the Scan operation goes through every item in the table to check if the item matches all the scan conditions. This makes the Scan operation particularly slow and expensive (in terms of provisioned throughput).
We will now retrieve all items where the Pages attribute is greater than the numerical value "200" and where the Title attribute contains the string "Adventures".

Scan sample


// Create a client
AmazonDynamoDBClient client = new AmazonDynamoDBClient();

// Define scan conditions
Dictionary<string, Condition> conditions = new Dictionary<string, Condition>();

// Title attribute should contain the string "Adventures"
Condition titleCondition = new Condition();
titleCondition.ComparisonOperator = ComparisonOperator.CONTAINS;
titleCondition.AttributeValueList.Add(new AttributeValue { S = "Adventures" });
conditions["Title"] = titleCondition;

// Pages attributes must be greater-than the numeric value "200"
Condition pagesCondition = new Condition();
pagesCondition.ComparisonOperator = ComparisonOperator.GT;;
pagesCondition.AttributeValueList.Add(new AttributeValue { N = "200" });
conditions["Pages"] = pagesCondition;


// Define marker variable
Dictionary<string, AttributeValue> startKey = null;

do
{
    // Create Scan request
    ScanRequest request = new ScanRequest
    {
        TableName = "SampleTable",
        ExclusiveStartKey = startKey,
        ScanFilter = conditions
    };

    // Issue request
    ScanResult result = client.Scan(request);

    // View all returned items
    List<Dictionary<string, AttributeValue>> items = result.Items;
    foreach (Dictionary<string, AttributeValue> item in items)
    {
        Console.WriteLine("Item:");
        foreach (var keyValuePair in item)
        {
            Console.WriteLine("{0} : S={1}, N={2}, SS=[{3}], NS=[{4}]",
                keyValuePair.Key,
                keyValuePair.Value.S,
                keyValuePair.Value.N,
                string.Join(", ", keyValuePair.Value.SS ?? new List<string>()),
                string.Join(", ", keyValuePair.Value.NS ?? new List<string>()));
        }
    }

    // Set marker variable
    startKey = result.LastEvaluatedKey;
} while (startKey != null && startKey.Count > 0);

The following example shows how we can utilize parallel scan to partition a table into 10 segments and scan each segment in a separate thread.
To avoid resource contention between threads, the results will be written into 10 separate files. Each segment will have a file of its own.

Parallel scan sample


// Create a client
AmazonDynamoDBClient client = new AmazonDynamoDBClient();

// Define scan conditions
Dictionary<string, Condition> conditions = new Dictionary<string, Condition>();

// Pages attributes must be greater-than the numeric value "200"
Condition pagesCondition = new Condition();
pagesCondition.ComparisonOperator = ComparisonOperator.GT;
pagesCondition.AttributeValueList.Add(new AttributeValue { N = "200" });
conditions["Pages"] = pagesCondition;

// Setup 10 simultaneous threads, each thread calling Scan operation
// with its own segment value.
int totalSegments = 10;
Parallel.For(0, totalSegments, segment =>
{
    // Define marker variable
    Dictionary<string, AttributeValue> startKey = null;

    do
    {
        // Create Scan request
        ScanRequest request = new ScanRequest
        {
            TableName = "SampleTable",
            ExclusiveStartKey = startKey,
            ScanFilter = conditions,
            // Total segments to split the table into
            TotalSegments = totalSegments,
            // Current segment to scan
            Segment = segment
        };

        // Issue request
        var result = client.Scan(request);

        // Write returned items to file
        string path = string.Format("ParallelScan-{0}-of-{1}.txt", totalSegments, segment);
        List<Dictionary<string, AttributeValue>> items = result.Items;
        using (Stream stream = File.OpenWrite(path))
        using (StreamWriter writer = new StreamWriter(stream))
        {
            foreach (Dictionary<string, AttributeValue> item in items)
            {
                writer.WriteLine("Item:");
                foreach (var keyValuePair in item)
                {
                    writer.WriteLine("{0} : S={1}, N={2}, SS=[{3}], NS=[{4}]",
                        keyValuePair.Key,
                        keyValuePair.Value.S,
                        keyValuePair.Value.N,
                        string.Join(", ", keyValuePair.Value.SS ?? new List<string>()),
                        string.Join(", ", keyValuePair.Value.NS ?? new List<string>()));
                }
            }
        }

        // Set marker variable
        startKey = result.LastEvaluatedKey;
    } while (startKey != null && startKey.Count > 0);
});

Version Information

.NET Framework:
Supported in: 4.5, 4.0, 3.5

.NET for Windows Store apps:
Supported in: Windows 8.1, Windows 8

.NET for Windows Phone:
Supported in: Windows Phone 8.1, Windows Phone 8