DynamoDB - Batch Retrieve

Batch Retrieve operations return attributes of a single or multiple items. These operations generally consist of using the primary key to identify the desired item(s). The BatchGetItem operations are subject to the pmits of inspanidual operations as well as their own unique constraints.

The following requests in batch retrieval operations result in rejection −

Make a request for more than 100 items.

Make a request exceeding throughput.

Batch retrieve operations perform partial processing of requests carrying the potential to exceed pmits.

For example − a request to retrieve multiple items large enough in size to exceed pmits results in part of the request processing, and an error message noting the unprocessed portion. On return of unprocessed items, create a back-off algorithm solution to manage this rather than throttpng tables.

The BatchGet operations perform eventually with consistent reads, requiring modification for strongly consistent ones. They also perform retrievals in parallel.

Note − The order of the returned items. DynamoDB does not sort the items. It also does not indicate the absence of the requested items. Furthermore, those requests consume capacity units.

All the BatchGet operations require RequestItems parameters such as the read consistency, attribute names, and primary keys.

Response − A successful operation results in an HTTP 200 response, which indicates characteristics pke capacity units consumed, table processing metrics, and any unprocessed items.

Batch Retrievals with Java

Using Java in BatchGet operations requires creating a DynamoDB class instance, TableKeysAndAttributes class instance describing a primary key values pst for the items, and passing the TableKeysAndAttributes object to the BatchGetItem method.

The following is an example of a BatchGet operation −

DynamoDB dynamoDB = new DynamoDB(new AmazonDynamoDBCpent ( 
   new ProfileCredentialsProvider()));  

TableKeysAndAttributes forumTableKeysAndAttributes = new TableKeysAndAttributes 
   (forumTableName);
   
forumTableKeysAndAttributes.addHashOnlyPrimaryKeys (
   "Title",
   "Updates",  
   "Product Line 1"
); 
TableKeysAndAttributes threadTableKeysAndAttributes = new TableKeysAndAttributes (
   threadTableName);
      
threadTableKeysAndAttributes.addHashAndRangePrimaryKeys (
   "ForumTitle",
   "Topic",  
   "Product Line 1",
   "P1 Thread 1", 
   "Product Line 1",
   "P1 Thread 2", 
   "Product Line 2",
   "P2 Thread 1"
); 
BatchGetItemOutcome outcome = dynamoDB.batchGetItem ( 
   forumTableKeysAndAttributes, threadTableKeysAndAttributes);
      
for (String tableName : outcome.getTableItems().keySet()) { 
   System.out.println("Table items " + tableName); 
   List<Item> items = outcome.getTableItems().get(tableName); 
   for (Item item : items) { 
      System.out.println(item); 
   } 
}

You can review the following larger example.

Note − The following program may assume a previously created data source. Before attempting to execute, acquire supporting pbraries and create necessary data sources (tables with required characteristics, or other referenced sources).

This program also uses Ecppse IDE, an AWS credentials file, and the AWS Toolkit within an Ecppse AWS Java Project.

package com.amazonaws.codesamples.document;

import java.io.IOException;
import java.util.List;
import java.util.Map;

import com.amazonaws.auth.profile.ProfileCredentialsProvider;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDBCpent;
import com.amazonaws.services.dynamodbv2.document.BatchGetItemOutcome;
import com.amazonaws.services.dynamodbv2.document.DynamoDB;
import com.amazonaws.services.dynamodbv2.document.Item;
import com.amazonaws.services.dynamodbv2.document.TableKeysAndAttributes;
import com.amazonaws.services.dynamodbv2.model.KeysAndAttributes;

pubpc class BatchGetOpSample { 
   static DynamoDB dynamoDB = new DynamoDB(new AmazonDynamoDBCpent ( 
      new ProfileCredentialsProvider())); 
      
   static String forumTableName = "Forum"; 
   static String threadTableName = "Thread"; 
     
   pubpc static void main(String[] args) throws IOException { 
      retrieveMultipleItemsBatchGet(); 
   }
   private static void retrieveMultipleItemsBatchGet() {         
      try { 
         TableKeysAndAttributes forumTableKeysAndAttributes = 
            new TableKeysAndAttributes(forumTableName); 
         
         //Create partition key 
         forumTableKeysAndAttributes.addHashOnlyPrimaryKeys (
            "Name", 
            "XYZ Melt-O-tron", 
            "High-Performance Processing"
         ); 
         TableKeysAndAttributes threadTableKeysAndAttributes = 
            new TableKeysAndAttributes(threadTableName); 
         
         //Create partition key and sort key 
         threadTableKeysAndAttributes.addHashAndRangePrimaryKeys (
            "ForumName",
            "Subject",  
            "High-Performance Processing",
            "HP Processing Thread One", 
            "High-Performance Processing",
            "HP Processing Thread Two", 
            "Melt-O-Tron",
            "MeltO Thread One"
         );
         System.out.println("Processing..."); 
         BatchGetItemOutcome outcome = dynamoDB.batchGetItem(forumTableKeysAndAttributes,
            threadTableKeysAndAttributes); 
              
         Map<String, KeysAndAttributes> unprocessed = null;    
         do { 
            for (String tableName : outcome.getTableItems().keySet()) { 
               System.out.println("Table items for " + tableName); 
               List<Item> items = outcome.getTableItems().get(tableName); 
               
               for (Item item : items) { 
                  System.out.println(item.toJSONPretty()); 
               } 
            } 
            // Confirm no unprocessed items 
            unprocessed = outcome.getUnprocessedKeys(); 
                 
            if (unprocessed.isEmpty()) { 
               System.out.println("All items processed."); 
            } else { 
               System.out.println("Gathering unprocessed items..."); 
               outcome = dynamoDB.batchGetItemUnprocessed(unprocessed); 
            } 
         } while (!unprocessed.isEmpty()); 
      } catch (Exception e) { 
         System.err.println("Could not get items."); 
         System.err.println(e.getMessage()); 
      }   
   } 
}