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AWS Documentation » Amazon DynamoDB » API Reference » Actions » Amazon DynamoDB » BatchGetItem

BatchGetItem

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 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 Capacity Units Calculations in the Amazon DynamoDB Developer Guide.

Request Syntax

{
   "RequestItems": { 
      "string" : { 
         "AttributesToGet": [ "string" ],
         "ConsistentRead": boolean,
         "ExpressionAttributeNames": { 
            "string" : "string" 
         },
         "Keys": [ 
            { 
               "string" : { 
                  "B": blob,
                  "BOOL": boolean,
                  "BS": [ blob ],
                  "L": [ 
                     "AttributeValue"
                  ],
                  "M": { 
                     "string" : "AttributeValue"
                  },
                  "N": "string",
                  "NS": [ "string" ],
                  "NULL": boolean,
                  "S": "string",
                  "SS": [ "string" ]
               }
            }
         ],
         "ProjectionExpression": "string"
      }
   },
   "ReturnConsumedCapacity": "string"
}

Request Parameters

The request accepts the following data in JSON format.

Note

In the following list, the required parameters are described first.

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 - 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

    Note

    Tokens that begin with the : character are expression attribute values, which are placeholders for the actual value at runtime.

    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 - This is a legacy parameter. Use ProjectionExpression instead. For more information, see AttributesToGet in the Amazon DynamoDB Developer Guide.

Type: String to KeysAndAttributes object map

Key Length Constraints: Minimum length of 3. Maximum length of 255.

Key Pattern: [a-zA-Z0-9_.-]+

Required: Yes

ReturnConsumedCapacity

Determines the level of detail about provisioned throughput consumption that is returned in the response:

  • INDEXES - The response includes the aggregate ConsumedCapacity for the operation, together with ConsumedCapacity for each table and secondary index that was accessed.

    Note that some operations, such as GetItem and BatchGetItem, do not access any indexes at all. In these cases, specifying INDEXES will only return ConsumedCapacity information for table(s).

  • TOTAL - The response includes only the aggregate ConsumedCapacity for the operation.

  • NONE - No ConsumedCapacity details are included in the response.

Type: String

Valid Values: INDEXES | TOTAL | NONE

Required: No

Response Syntax

{
   "ConsumedCapacity": [ 
      { 
         "CapacityUnits": number,
         "GlobalSecondaryIndexes": { 
            "string" : { 
               "CapacityUnits": number,
               "ReadCapacityUnits": number,
               "WriteCapacityUnits": number
            }
         },
         "LocalSecondaryIndexes": { 
            "string" : { 
               "CapacityUnits": number,
               "ReadCapacityUnits": number,
               "WriteCapacityUnits": number
            }
         },
         "ReadCapacityUnits": number,
         "Table": { 
            "CapacityUnits": number,
            "ReadCapacityUnits": number,
            "WriteCapacityUnits": number
         },
         "TableName": "string",
         "WriteCapacityUnits": number
      }
   ],
   "Responses": { 
      "string" : [ 
         { 
            "string" : { 
               "B": blob,
               "BOOL": boolean,
               "BS": [ blob ],
               "L": [ 
                  "AttributeValue"
               ],
               "M": { 
                  "string" : "AttributeValue"
               },
               "N": "string",
               "NS": [ "string" ],
               "NULL": boolean,
               "S": "string",
               "SS": [ "string" ]
            }
         }
      ]
   },
   "UnprocessedKeys": { 
      "string" : { 
         "AttributesToGet": [ "string" ],
         "ConsistentRead": boolean,
         "ExpressionAttributeNames": { 
            "string" : "string" 
         },
         "Keys": [ 
            { 
               "string" : { 
                  "B": blob,
                  "BOOL": boolean,
                  "BS": [ blob ],
                  "L": [ 
                     "AttributeValue"
                  ],
                  "M": { 
                     "string" : "AttributeValue"
                  },
                  "N": "string",
                  "NS": [ "string" ],
                  "NULL": boolean,
                  "S": "string",
                  "SS": [ "string" ]
               }
            }
         ],
         "ProjectionExpression": "string"
      }
   }
}

Response Elements

If the action is successful, the service sends back an HTTP 200 response.

The following data is returned in JSON format by the service.

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.

Type: Array of ConsumedCapacity objects

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.

Type: String to array of string to AttributeValue object maps map

Key Length Constraints: Minimum length of 3. Maximum length of 255.

Key Pattern: [a-zA-Z0-9_.-]+

Key Length Constraints: Maximum length of 65535.

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 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.

Type: String to KeysAndAttributes object map

Key Length Constraints: Minimum length of 3. Maximum length of 255.

Key Pattern: [a-zA-Z0-9_.-]+

Errors

For information about the errors that are common to all actions, see Common Errors.

InternalServerError

An error occurred on the server side.

HTTP Status Code: 500

ProvisionedThroughputExceededException

Your request rate is too high. The AWS SDKs for DynamoDB automatically retry requests that receive this exception. Your request is eventually successful, unless your retry queue is too large to finish. Reduce the frequency of requests and use exponential backoff. For more information, go to Error Retries and Exponential Backoff in the Amazon DynamoDB Developer Guide.

HTTP Status Code: 400

RequestLimitExceeded

Throughput exceeds the current throughput limit for your account. Please contact AWS Support at AWS Support to request a limit increase.

HTTP Status Code: 400

ResourceNotFoundException

The operation tried to access a nonexistent table or index. The resource might not be specified correctly, or its status might not be ACTIVE.

HTTP Status Code: 400

Example

Retrieve Items From Multiple Tables

The following sample requests attributes from two different tables.

Sample Request


POST / HTTP/1.1
Host: dynamodb.<region>.<domain>;
Accept-Encoding: identity
Content-Length: <PayloadSizeBytes>
User-Agent: <UserAgentString>
Content-Type: application/x-amz-json-1.0
Authorization: AWS4-HMAC-SHA256 Credential=<Credential>, SignedHeaders=<Headers>, Signature=<Signature>
X-Amz-Date: <Date>
X-Amz-Target: DynamoDB_20120810.BatchGetItem

{
    "RequestItems": {
        "Forum": {
            "Keys": [
                {
                    "Name":{"S":"Amazon DynamoDB"}
                },
                {
                    "Name":{"S":"Amazon RDS"}
                },
                {
                    "Name":{"S":"Amazon Redshift"}
                }
            ],
            "ProjectionExpression":"Name, Threads, Messages, Views"
        },
        "Thread": {
            "Keys": [
                {
                    "ForumName":{"S":"Amazon DynamoDB"},
                    "Subject":{"S":"Concurrent reads"}
                }
            ],
            "ProjectionExpression":"Tags, Message"
        }
    },
    "ReturnConsumedCapacity": "TOTAL"
}

Sample Response


HTTP/1.1 200 OK
x-amzn-RequestId: <RequestId>
x-amz-crc32: <Checksum>
Content-Type: application/x-amz-json-1.0
Content-Length: <PayloadSizeBytes>
Date: <Date>
 {
    "Responses": {
        "Forum": [
            {
                "Name":{
                    "S":"Amazon DynamoDB"
                },
                "Threads":{
                    "N":"5"
                },
                "Messages":{
                    "N":"19"
                },
                "Views":{
                    "N":"35"
                }
            },
            {
                "Name":{
                    "S":"Amazon RDS"
                },
                "Threads":{
                    "N":"8"
                },
                "Messages":{
                    "N":"32"
                },
                "Views":{
                    "N":"38"
                }
            },
            {
                "Name":{
                    "S":"Amazon Redshift"
                },
                "Threads":{
                    "N":"12"
                },
                "Messages":{
                    "N":"55"
                },
                "Views":{
                    "N":"47"
                }
            }
        ]
        "Thread": [
            {
                "Tags":{
                    "SS":["Reads","MultipleUsers"]
                },
                "Message":{
                    "S":"How many users can read a single data item at a time? Are there any limits?"
                }
            }
        ]
    },
    "UnprocessedKeys": {
    },
    "ConsumedCapacity": [
        {
            "TableName": "Forum",
            "CapacityUnits": 3
        },
        {
            "TableName": "Thread",
            "CapacityUnits": 1
        }
    ]
}

See Also

For more information about using this API in one of the language-specific AWS SDKs, see the following: