| Copyright | (c) 2013-2016 Brendan Hay |
|---|---|
| License | Mozilla Public License, v. 2.0. |
| Maintainer | Brendan Hay <brendan.g.hay@gmail.com> |
| Stability | auto-generated |
| Portability | non-portable (GHC extensions) |
| Safe Haskell | None |
| Language | Haskell2010 |
Network.AWS.DynamoDB.BatchGetItem
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.
Important: If you request more than 100 items BatchGetItem will return a ValidationException with the message "Too many items requested for the BatchGetItem call".
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 .
Important: If DynamoDB returns any unprocessed items, you should retry the batch operation on those items. However, we strongly recommend that you use an exponential 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, see 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 items 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 .
- batchGetItem :: BatchGetItem
- data BatchGetItem
- bgiReturnConsumedCapacity :: Lens' BatchGetItem (Maybe ReturnConsumedCapacity)
- bgiRequestItems :: Lens' BatchGetItem (HashMap Text KeysAndAttributes)
- batchGetItemResponse :: Int -> BatchGetItemResponse
- data BatchGetItemResponse
- bgirsUnprocessedKeys :: Lens' BatchGetItemResponse (HashMap Text KeysAndAttributes)
- bgirsResponses :: Lens' BatchGetItemResponse (HashMap Text [HashMap Text AttributeValue])
- bgirsConsumedCapacity :: Lens' BatchGetItemResponse [ConsumedCapacity]
- bgirsResponseStatus :: Lens' BatchGetItemResponse Int
Creating a Request
batchGetItem :: BatchGetItem #
Creates a value of BatchGetItem with the minimum fields required to make a request.
Use one of the following lenses to modify other fields as desired:
bgiReturnConsumedCapacity- Undocumented member.bgiRequestItems- A map of one or more table names and, for each table, a map that describes one or more items to retrieve from that table. Each table name can be used only once per BatchGetItem request. Each element in the map of items to retrieve consists of the following: * ConsistentRead - Iftrue, a strongly consistent read is used; iffalse(the default), an eventually consistent read is used. * ExpressionAttributeNames - One or more substitution tokens for attribute names in the ProjectionExpression parameter. 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: *PercentileThe 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 = :valFor more information on expression attribute names, see Accessing Item Attributes in the Amazon DynamoDB Developer Guide . * 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 simple primary key, you only need to provide the partition key value. For a composite key, you must provide both the partition key value and the sort key value. * ProjectionExpression - A string that identifies one or more attributes to retrieve from the table. 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 . * AttributesToGet - Important: 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.
data BatchGetItem #
Represents the input of a BatchGetItem operation.
See: batchGetItem smart constructor.
Instances
Request Lenses
bgiReturnConsumedCapacity :: Lens' BatchGetItem (Maybe ReturnConsumedCapacity) #
Undocumented member.
bgiRequestItems :: Lens' BatchGetItem (HashMap Text KeysAndAttributes) #
A map of one or more table names and, for each table, a map that describes one or more items to retrieve from that table. Each table name can be used only once per BatchGetItem request. Each element in the map of items to retrieve consists of the following: * ConsistentRead - If true , a strongly consistent read is used; if false (the default), an eventually consistent read is used. * ExpressionAttributeNames - One or more substitution tokens for attribute names in the ProjectionExpression parameter. 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 For more information on expression attribute names, see Accessing Item Attributes in the Amazon DynamoDB Developer Guide . * 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 simple primary key, you only need to provide the partition key value. For a composite key, you must provide both the partition key value and the sort key value. * ProjectionExpression - A string that identifies one or more attributes to retrieve from the table. 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 . * AttributesToGet - Important: 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.
Destructuring the Response
Arguments
| :: Int | |
| -> BatchGetItemResponse |
Creates a value of BatchGetItemResponse with the minimum fields required to make a request.
Use one of the following lenses to modify other fields as desired:
bgirsUnprocessedKeys- 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 requested attribute is not found, it does not appear in the result. * ConsistentRead - The consistency of a read operation. If set totrue, 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.bgirsResponses- 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.bgirsConsumedCapacity- The read 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.bgirsResponseStatus- -- | The response status code.
data BatchGetItemResponse #
Represents the output of a BatchGetItem operation.
See: batchGetItemResponse smart constructor.
Response Lenses
bgirsUnprocessedKeys :: Lens' BatchGetItemResponse (HashMap Text KeysAndAttributes) #
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 requested 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.
bgirsResponses :: Lens' BatchGetItemResponse (HashMap Text [HashMap Text AttributeValue]) #
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.
bgirsConsumedCapacity :: Lens' BatchGetItemResponse [ConsumedCapacity] #
The read 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.
bgirsResponseStatus :: Lens' BatchGetItemResponse Int #
- - | The response status code.