IAmazonDynamoDB.BatchGetItem Method (BatchGetItemRequest)

The BatchGetItem operation returns the attributes of one or more items from one or more tables. You identify requested items by primary key.

A single operation can retrieve up to 1 MB of data, which can comprise as many as 100 items. BatchGetItem will return a partial result if the response size limit is exceeded, the table's provisioned throughput is exceeded, or an internal processing failure occurs. If a partial result is returned, the operation returns a value for UnprocessedKeys . You can use this value to retry the operation starting with the next item to get.

For example, if you ask to retrieve 100 items, but each individual item is 50 KB in size, the system returns 20 items (1 MB) and an appropriate UnprocessedKeys value so you can get the next page of results. If desired, your application can include its own logic to assemble the pages of results into one dataset.

If no items can be processed because of insufficient provisioned throughput on each of the tables involved in the request, BatchGetItem throws ProvisionedThroughputExceededException .

By default, BatchGetItem performs eventually consistent reads on every table in the request. If you want strongly consistent reads instead, you can set ConsistentRead to true for any or all tables.

In order to minimize response latency, BatchGetItem fetches items in parallel.

When designing your application, keep in mind that Amazon DynamoDB does not return attributes in any particular order. To help parse the response by item, include the primary key values for the items in your request in the AttributesToGet parameter.

If a requested item does not exist, it is not returned in the result. Requests for nonexistent items consume the minimum read capacity units according to the type of read. For more information, see Capacity Units Calculations in the Amazon DynamoDB Developer Guide.

Syntax

public abstract BatchGetItemResponse BatchGetItem(
         BatchGetItemRequest request
)

Examples

The following examples show how to batch get item from two tables.

This example will construct the collection of keys for the items to retrieve from the first table.

BatchGet sample - First table

// Define attributes to get and keys to retrieve
List<string> attributesToGet = new List<string> { "Author", "Title", "Year" };
List<Dictionary<string, AttributeValue>> sampleTableKeys = new List<Dictionary<string, AttributeValue>>
{
    new Dictionary<string, AttributeValue>
    {
        { "Author", new AttributeValue { S = "Mark Twain" } },
        { "Title", new AttributeValue { S = "The Adventures of Tom Sawyer" } }
    },
    new Dictionary<string, AttributeValue>
    {
        { "Author", new AttributeValue { S = "Mark Twain" } },
        { "Title", new AttributeValue { S = "Adventures of Huckleberry Finn" } }
    }
};

// Construct get-request for first table
KeysAndAttributes sampleTableItems = new KeysAndAttributes
{
    AttributesToGet = attributesToGet,
    Keys = sampleTableKeys
};

                

This example will construct the collection of keys for the items to retrieve from the second table.

BatchGet sample - Second table

// Define keys to retrieve
List<Dictionary<string, AttributeValue>> authorsTableKeys = new List<Dictionary<string, AttributeValue>>
{
    new Dictionary<string, AttributeValue>
    {
        { "Author", new AttributeValue { S = "Mark Twain" } },
    },
    new Dictionary<string, AttributeValue>
    {
        { "Author", new AttributeValue { S = "Booker Taliaferro Washington" } },
    }
};

// Construct get-request for second table
//  Skip setting AttributesToGet property to retrieve all attributes
KeysAndAttributes authorsTableItems = new KeysAndAttributes
{
    Keys = authorsTableKeys,
};

                

This example will construct the BatchGet request from the two earlier-created collections, will issue the call and in case some items are not processed, will attempt to retrieve the remaining items.

BatchGet sample - Service calls

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

// Construct table-keys mapping
Dictionary<string, KeysAndAttributes> requestItems = new Dictionary<string, KeysAndAttributes>();
requestItems["SampleTable"] = sampleTableItems;
requestItems["AuthorsTable"] = authorsTableItems;

// Construct request
BatchGetItemRequest request = new BatchGetItemRequest
{
    RequestItems = requestItems
};

BatchGetItemResult result;
do
{
    // Issue request and retrieve items
    result = client.BatchGetItem(request).BatchGetItemResult;

    // Iterate through responses
    Dictionary<string, List<Dictionary<string, AttributeValue>>> responses = result.Responses;
    foreach (string tableName in responses.Keys)
    {
        // Get items for each table
        List<Dictionary<string, AttributeValue>> tableItems = responses[tableName];

        // View items
        foreach (Dictionary<string, AttributeValue> item in tableItems)
        {
            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>()));
            }
        }
    }

    // Some items may not have been retrieved!
    //  Set RequestItems to the result's UnprocessedKeys and reissue request
    request.RequestItems = result.UnprocessedKeys;

} while (result.UnprocessedKeys.Count > 0);

                

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