amazonka-dynamodb-0.0.8: Amazon DynamoDB SDK.

Safe HaskellNone
LanguageHaskell2010

Network.AWS.DynamoDB.BatchGetItem

Contents

Description

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 16 MB of data, which can contain 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 300 KB in size, the system returns 52 items (so as not to exceed the 16 MB limit). It also returns 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 data set.

If none of the items can be processed due to insufficient provisioned throughput on all of the tables in the request, then BatchGetItem will return a ProvisionedThroughputExceededException. If at least one of the items is successfully processed, then BatchGetItem completes successfully, while returning the keys of the unread items in UnprocessedKeys.

If DynamoDB returns any unprocessed items, you should retry the batch operation on those items. However, we strongly recommend that you use anexponential backoff algorithm. If you retry the batch operation immediately, the underlying read or write requests can still fail due to throttling on the individual tables. If you delay the batch operation using exponential backoff, the individual requests in the batch are much more likely to succeed.

For more information, go to Batch Operations and Error Handling in the Amazon DynamoDB Developer Guide.

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 retrieves items in parallel.

When designing your application, keep in mind that 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 UnitsCalculations in the Amazon DynamoDB Developer Guide.

http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_BatchGetItem.html

Synopsis

Request

Request constructor

Request lenses

bgiRequestItems :: Lens' BatchGetItem (HashMap Text KeysAndAttributes) Source

A map of one or more table names and, for each table, the corresponding primary keys for the items to retrieve. Each table name can be invoked only once.

Each element in the map consists of the following:

Keys - An array of primary key attribute values that define specific items in the table. For each primary key, you must provide all of the key attributes. For example, with a hash type primary key, you only need to specify the hash attribute. For a hash-and-range type primary key, you must specify both the hash attribute and the range attribute.

AttributesToGet - One or more attributes to be retrieved from the table. By default, all attributes are returned. If a specified attribute is not found, it does 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.

ConsistentRead - If true, a strongly consistent read is used; if false (the default), an eventually consistent read is used.

Response

Response constructor

Response lenses

bgirConsumedCapacity :: Lens' BatchGetItemResponse [ConsumedCapacity] Source

The write capacity units consumed by the operation.

Each element consists of:

TableName - The table that consumed the provisioned throughput.

CapacityUnits - The total number of capacity units consumed.

bgirResponses :: Lens' BatchGetItemResponse (HashMap Text [HashMap Text AttributeValue]) Source

A map of table name to a list of items. Each object in Responses consists of a table name, along with a map of attribute data consisting of the data type and attribute value.

bgirUnprocessedKeys :: Lens' BatchGetItemResponse (HashMap Text KeysAndAttributes) Source

A map of tables and their respective keys that were not processed with the current response. The UnprocessedKeys value is in the same form as RequestItems, so the value can be provided directly to a subsequent BatchGetItem operation. For more information, see RequestItems in the Request Parameters section.

Each element consists of:

Keys - An array of primary key attribute values that define specific items in the table.

AttributesToGet - One or more attributes to be retrieved from the table or index. By default, all attributes are returned. If a specified attribute is not found, it does not appear in the result.

ConsistentRead - The consistency of a read operation. If set to true, then a strongly consistent read is used; otherwise, an eventually consistent read is used.

If there are no unprocessed keys remaining, the response contains an empty UnprocessedKeys map.