| Copyright | (c) 2013-2021 Brendan Hay |
|---|---|
| License | Mozilla Public License, v. 2.0. |
| Maintainer | Brendan Hay <brendan.g.hay+amazonka@gmail.com> |
| Stability | auto-generated |
| Portability | non-portable (GHC extensions) |
| Safe Haskell | None |
Amazonka.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 returns 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.
If you request more than 100 items, BatchGetItem returns 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 dataset.
If none of the items can be processed due to insufficient provisioned
throughput on all of the tables in the request, then BatchGetItem
returns 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 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 ProjectionExpression 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 Working with Tables in the Amazon DynamoDB Developer Guide.
Synopsis
- data BatchGetItem = BatchGetItem' {}
- newBatchGetItem :: BatchGetItem
- batchGetItem_returnConsumedCapacity :: Lens' BatchGetItem (Maybe ReturnConsumedCapacity)
- batchGetItem_requestItems :: Lens' BatchGetItem (HashMap Text KeysAndAttributes)
- data BatchGetItemResponse = BatchGetItemResponse' {}
- newBatchGetItemResponse :: Int -> BatchGetItemResponse
- batchGetItemResponse_unprocessedKeys :: Lens' BatchGetItemResponse (Maybe (HashMap Text KeysAndAttributes))
- batchGetItemResponse_responses :: Lens' BatchGetItemResponse (Maybe (HashMap Text [HashMap Text AttributeValue]))
- batchGetItemResponse_consumedCapacity :: Lens' BatchGetItemResponse (Maybe [ConsumedCapacity])
- batchGetItemResponse_httpStatus :: Lens' BatchGetItemResponse Int
Creating a Request
data BatchGetItem Source #
Represents the input of a BatchGetItem operation.
See: newBatchGetItem smart constructor.
Constructors
| BatchGetItem' | |
Fields
| |
Instances
newBatchGetItem :: BatchGetItem Source #
Create a value of BatchGetItem with all optional fields omitted.
Use generic-lens or optics to modify other optional fields.
The following record fields are available, with the corresponding lenses provided for backwards compatibility:
$sel:returnConsumedCapacity:BatchGetItem', batchGetItem_returnConsumedCapacity - Undocumented member.
$sel:requestItems:BatchGetItem', batchGetItem_requestItems - 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 theProjectionExpressionparameter. The following are some use cases for usingExpressionAttributeNames:- 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 about 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 are returned. If any of the requested attributes are not found, they do not appear in the result.
For more information, see Accessing Item Attributes in the Amazon DynamoDB Developer Guide.
AttributesToGet- This is a legacy parameter. UseProjectionExpressioninstead. For more information, see AttributesToGet in the Amazon DynamoDB Developer Guide.
Request Lenses
batchGetItem_returnConsumedCapacity :: Lens' BatchGetItem (Maybe ReturnConsumedCapacity) Source #
Undocumented member.
batchGetItem_requestItems :: Lens' BatchGetItem (HashMap Text KeysAndAttributes) Source #
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 theProjectionExpressionparameter. The following are some use cases for usingExpressionAttributeNames:- 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 about 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 are returned. If any of the requested attributes are not found, they do not appear in the result.
For more information, see Accessing Item Attributes in the Amazon DynamoDB Developer Guide.
AttributesToGet- This is a legacy parameter. UseProjectionExpressioninstead. For more information, see AttributesToGet in the Amazon DynamoDB Developer Guide.
Destructuring the Response
data BatchGetItemResponse Source #
Represents the output of a BatchGetItem operation.
See: newBatchGetItemResponse smart constructor.
Constructors
| BatchGetItemResponse' | |
Fields
| |
Instances
newBatchGetItemResponse Source #
Arguments
| :: Int | |
| -> BatchGetItemResponse |
Create a value of BatchGetItemResponse with all optional fields omitted.
Use generic-lens or optics to modify other optional fields.
The following record fields are available, with the corresponding lenses provided for backwards compatibility:
$sel:unprocessedKeys:BatchGetItemResponse', batchGetItemResponse_unprocessedKeys - 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.ProjectionExpression- 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.
$sel:responses:BatchGetItemResponse', batchGetItemResponse_responses - 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.
$sel:consumedCapacity:BatchGetItemResponse', batchGetItemResponse_consumedCapacity - The read capacity units consumed by the entire BatchGetItem operation.
Each element consists of:
TableName- The table that consumed the provisioned throughput.CapacityUnits- The total number of capacity units consumed.
$sel:httpStatus:BatchGetItemResponse', batchGetItemResponse_httpStatus - The response's http status code.
Response Lenses
batchGetItemResponse_unprocessedKeys :: Lens' BatchGetItemResponse (Maybe (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.ProjectionExpression- 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.
batchGetItemResponse_responses :: Lens' BatchGetItemResponse (Maybe (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.
batchGetItemResponse_consumedCapacity :: Lens' BatchGetItemResponse (Maybe [ConsumedCapacity]) Source #
The read capacity units consumed by the entire BatchGetItem operation.
Each element consists of:
TableName- The table that consumed the provisioned throughput.CapacityUnits- The total number of capacity units consumed.
batchGetItemResponse_httpStatus :: Lens' BatchGetItemResponse Int Source #
The response's http status code.