The batch_get_item 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. batch_get_item
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,
batch_get_item 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
batch_get_item returns a
ProvisionedThroughputExceededException. If at least one of the items
is successfully processed, then
batch_get_item 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, batch_get_item 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,
batch_get_item 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.
dynamodb_batch_get_item(RequestItems, ReturnConsumedCapacity)A list with the following syntax:
list(
Responses = list(
list(
list(
list(
S = "string",
N = "string",
B = raw,
SS = list(
"string"
),
NS = list(
"string"
),
BS = list(
raw
),
M = list(
list()
),
L = list(
list()
),
NULL = TRUE|FALSE,
BOOL = TRUE|FALSE
)
)
)
),
UnprocessedKeys = list(
list(
Keys = list(
list(
list(
S = "string",
N = "string",
B = raw,
SS = list(
"string"
),
NS = list(
"string"
),
BS = list(
raw
),
M = list(
list()
),
L = list(
list()
),
NULL = TRUE|FALSE,
BOOL = TRUE|FALSE
)
)
),
AttributesToGet = list(
"string"
),
ConsistentRead = TRUE|FALSE,
ProjectionExpression = "string",
ExpressionAttributeNames = list(
"string"
)
)
),
ConsumedCapacity = list(
list(
TableName = "string",
CapacityUnits = 123.0,
ReadCapacityUnits = 123.0,
WriteCapacityUnits = 123.0,
Table = list(
ReadCapacityUnits = 123.0,
WriteCapacityUnits = 123.0,
CapacityUnits = 123.0
),
LocalSecondaryIndexes = list(
list(
ReadCapacityUnits = 123.0,
WriteCapacityUnits = 123.0,
CapacityUnits = 123.0
)
),
GlobalSecondaryIndexes = list(
list(
ReadCapacityUnits = 123.0,
WriteCapacityUnits = 123.0,
CapacityUnits = 123.0
)
)
)
)
)
[required] 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 batch_get_item
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
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. Use
ProjectionExpression instead. For more information, see
AttributesToGet
in the Amazon DynamoDB Developer Guide.
svc$batch_get_item(
RequestItems = list(
list(
Keys = list(
list(
list(
S = "string",
N = "string",
B = raw,
SS = list(
"string"
),
NS = list(
"string"
),
BS = list(
raw
),
M = list(
list()
),
L = list(
list()
),
NULL = TRUE|FALSE,
BOOL = TRUE|FALSE
)
)
),
AttributesToGet = list(
"string"
),
ConsistentRead = TRUE|FALSE,
ProjectionExpression = "string",
ExpressionAttributeNames = list(
"string"
)
)
),
ReturnConsumedCapacity = "INDEXES"|"TOTAL"|"NONE"
)
if (FALSE) {
# This example reads multiple items from the Music table using a batch of
# three GetItem requests. Only the AlbumTitle attribute is returned.
svc$batch_get_item(
RequestItems = list(
Music = list(
Keys = list(
list(
Artist = list(
S = "No One You Know"
),
SongTitle = list(
S = "Call Me Today"
)
),
list(
Artist = list(
S = "Acme Band"
),
SongTitle = list(
S = "Happy Day"
)
),
list(
Artist = list(
S = "No One You Know"
),
SongTitle = list(
S = "Scared of My Shadow"
)
)
),
ProjectionExpression = "AlbumTitle"
)
)
)
}
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