Working with Global Secondary Indexes: .NET

Topics

Create a Table With a Global Secondary Index
Describe a Table With a Global Secondary Index
Query a Global Secondary Index
Example: Global Secondary Indexes Using the AWS SDK for .NET Low-Level API

You can use the AWS SDK for .NET low-level API to create a table with one or more global secondary indexes, describe the indexes on the table, and perform queries using the indexes. These operations map to the corresponding DynamoDB operations. For more information, see the Amazon DynamoDB API Reference.

The following are the common steps for table operations using the .NET low-level API.

Create an instance of the AmazonDynamoDBClient class.
Provide the required and optional parameters for the operation by creating the corresponding request objects.

For example, create a CreateTableRequest object to create a table and QueryRequest object to query a table or an index.
Execute the appropriate method provided by the client that you created in the preceding step.

Create a Table With a Global Secondary Index

You can create global secondary indexes at the same time that you create a table. To do this, use CreateTable and provide your specifications for one or more global secondary indexes. The following C# code snippet creates a table to hold information about weather data. The partition key is Location and the sort key is Date. A global secondary index named PrecipIndex allows fast access to precipitation data for various locations.

The following are the steps to create a table with a global secondary index, using the .NET low-level API.

Create an instance of the AmazonDynamoDBClient class.
Create an instance of the CreateTableRequest class to provide the request information.

You must provide the table name, its primary key, and the provisioned throughput values. For the global secondary index, you must provide the index name, its provisioned throughput settings, the attribute definitions for the index sort key, the key schema for the index, and the attribute projection.
Execute the CreateTable method by providing the request object as a parameter.

The following C# code snippet demonstrates the preceding steps. The snippet creates a table (WeatherData) with a global secondary index (PrecipIndex). The index partition key is Date and its sort key is Precipitation. All of the table attributes are projected into the index. Users can query this index to obtain weather data for a particular date, optionally sorting the data by precipitation amount.

Note that since Precipitation is not a key attribute for the table, it is not required; however, WeatherData items without Precipitation will not appear in PrecipIndex.


client = new AmazonDynamoDBClient();
string tableName = "WeatherData";

// Attribute definitions
var attributeDefinitions = new List<AttributeDefinition>() 
{ 
    {new AttributeDefinition{
        AttributeName = "Location",
        AttributeType = "S"}},
    {new AttributeDefinition{
        AttributeName = "Date",
        AttributeType = "S"}},
    {new AttributeDefinition(){
        AttributeName = "Precipitation",
        AttributeType = "N"}
    }
};

// Table key schema
var tableKeySchema = new List<KeySchemaElement>() 
{
    {new KeySchemaElement {
        AttributeName = "Location",
        KeyType = "HASH"}},  //Partition key
    {new KeySchemaElement {
        AttributeName = "Date",
        KeyType = "RANGE"}  //Sort key
    }
};

// PrecipIndex
var precipIndex = new GlobalSecondaryIndex
{
    IndexName = "PrecipIndex",
    ProvisionedThroughput = new ProvisionedThroughput
    {
        ReadCapacityUnits = (long)10,
        WriteCapacityUnits = (long)1
    },
    Projection = new Projection { ProjectionType = "ALL" }
};

var indexKeySchema = new List<KeySchemaElement> {
    {new KeySchemaElement { AttributeName = "Date", KeyType = "HASH"}},  //Partition key
    {new KeySchemaElement{AttributeName = "Precipitation",KeyType = "RANGE"}}  //Sort key
};

precipIndex.KeySchema = indexKeySchema;

CreateTableRequest createTableRequest = new CreateTableRequest
{
    TableName = tableName,
    ProvisionedThroughput = new ProvisionedThroughput
    {
        ReadCapacityUnits = (long)5,
        WriteCapacityUnits = (long)1
    },
    AttributeDefinitions = attributeDefinitions,
    KeySchema = tableKeySchema,
    GlobalSecondaryIndexes = { precipIndex }
};

CreateTableResponse response = client.CreateTable(createTableRequest);
Console.WriteLine(response.CreateTableResult.TableDescription.TableName);
Console.WriteLine(response.CreateTableResult.TableDescription.TableStatus);

You must wait until DynamoDB creates the table and sets the table status to ACTIVE. After that, you can begin putting data items into the table.

Describe a Table With a Global Secondary Index

To get information about global secondary indexes on a table, use DescribeTable. For each index, you can access its name, key schema, and projected attributes.

The following are the steps to access global secondary index information a table using the .NET low-level API.

Create an instance of the AmazonDynamoDBClient class.
Create an instance of the DescribeTableRequest class to provide the request information. You must provide the table name.
Execute the describeTable method by providing the request object as a parameter.

The following C# code snippet demonstrates the preceding steps.

Example


client = new AmazonDynamoDBClient();
string tableName = "WeatherData";

DescribeTableResponse response = client.DescribeTable(new DescribeTableRequest { TableName = tableName});

List<GlobalSecondaryIndexDescription> globalSecondaryIndexes = 
response.DescribeTableResult.Table.GlobalSecondaryIndexes;

// This code snippet will work for multiple indexes, even though
// there is only one index in this example.

foreach (GlobalSecondaryIndexDescription gsiDescription in globalSecondaryIndexes) {
     Console.WriteLine("Info for index " + gsiDescription.IndexName + ":");

     foreach (KeySchemaElement kse in gsiDescription.KeySchema) {
          Console.WriteLine("\t" + kse.AttributeName + ": key type is " + kse.KeyType);
     }

      Projection projection = gsiDescription.Projection;
      Console.WriteLine("\tThe projection type is: " + projection.ProjectionType);

      if (projection.ProjectionType.ToString().Equals("INCLUDE")) {
           Console.WriteLine("\t\tThe non-key projected attributes are: "
                + projection.NonKeyAttributes);
      }
}

Query a Global Secondary Index

You can use Query on a global secondary index, in much the same way you Query a table. You need to specify the index name, the query criteria for the index partition key and sort key (if present), and the attributes that you want to return. In this example, the index is PrecipIndex, which has a partition key of Date and a sort key of Precipitation. The index query returns all of the weather data for a particular date, where the precipitation is greater than zero.

The following are the steps to query a global secondary index using the .NET low-level API.

Create an instance of the AmazonDynamoDBClient class.
Create an instance of the QueryRequest class to provide the request information.
Execute the query method by providing the request object as a parameter.

The attribute name Date is a DynamoDB reserved word. Therefore, we must use an expression attribute name as a placeholder in the KeyConditionExpression.

The following C# code snippet demonstrates the preceding steps.

Example



client = new AmazonDynamoDBClient();

QueryRequest queryRequest = new QueryRequest
{
    TableName = "WeatherData",
    IndexName = "PrecipIndex",
    KeyConditionExpression = "#dt = :v_date and Precipitation > :v_precip",
    ExpressionAttributeNames = new Dictionary<String, String> {
        {"#dt", "Date"}
    },
    ExpressionAttributeValues = new Dictionary<string, AttributeValue> {
        {":v_date", new AttributeValue { S =  "2013-08-01" }},
        {":v_precip", new AttributeValue { N =  "0" }}
    },
    ScanIndexForward = true
};

var result = client.Query(queryRequest);

var items = result.Items;
foreach (var currentItem in items)
{
    foreach (string attr in currentItem.Keys)
    {
        Console.Write(attr + "---> ");
        if (attr == "Precipitation")
        {
            Console.WriteLine(currentItem[attr].N);
    }
    else
    {
        Console.WriteLine(currentItem[attr].S);
    }

         }
     Console.WriteLine();
}

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