BatchGetItem in DynamoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the BatchGetItem operation in DynamoDB?

BatchGetItem lets you retrieve multiple items from one or more tables in a single request. It helps save time by fetching many items at once instead of one by one.

beginner

What is the maximum number of items you can request in a single BatchGetItem call?

You can request up to 100 items or 16 MB of data in one BatchGetItem call, whichever limit is reached first.

intermediate

How does DynamoDB handle unprocessed keys in a BatchGetItem response?

If DynamoDB can't process some keys due to throttling or other reasons, it returns them in the UnprocessedKeys field. You should retry fetching these keys later.

beginner

Can BatchGetItem retrieve items from multiple tables in one request?

Yes, BatchGetItem supports fetching items from multiple tables by specifying keys for each table in the request.

intermediate

Does BatchGetItem support conditional reads or filters on attributes?

No, BatchGetItem retrieves items by primary key only and does not support conditions or filters. You get the full items for the keys requested.

What is the main advantage of using BatchGetItem in DynamoDB?

A Update multiple items at once

B Create new tables

C Delete multiple tables

D Retrieve multiple items in one request

BatchGetItem is designed to fetch multiple items from one or more tables in a single request.

What happens if some keys are not processed in a BatchGetItem response?

A They are returned in UnprocessedKeys for retry

B DynamoDB deletes those keys

C The entire request fails

D They are ignored permanently

UnprocessedKeys contains keys that DynamoDB could not process, so you can retry fetching them.

What is the maximum number of items you can request in one BatchGetItem call?

A 100

B 50

C 10

D 1000

BatchGetItem supports up to 100 items or 16 MB of data per request.

Can BatchGetItem apply filters to return only certain attributes?

A Yes, it supports attribute filters

B Yes, but only on one table

C No, it returns full items by key

D Only on indexes

BatchGetItem returns full items for the requested keys without filtering attributes.

Is it possible to fetch items from multiple tables in a single BatchGetItem request?

A No, only one table per request

B Yes, multiple tables supported

C Only if tables share the same key schema

D Only if tables are in the same region

BatchGetItem allows specifying keys for multiple tables in one request.

Explain how BatchGetItem works and when you would use it.

Think about fetching many items quickly without multiple calls.

Describe the limits and restrictions of BatchGetItem in DynamoDB.

Consider what BatchGetItem cannot do compared to other read operations.

Practice

1. What is the main purpose of the BatchGetItem operation in DynamoDB?

easy

A. To update multiple items in a table at once.

B. To retrieve multiple items from one or more tables using their primary keys in a single request.

C. To delete multiple items from a table in one call.

D. To scan the entire table and return all items.

Solution

1. Step 1: Understand BatchGetItem functionality

   BatchGetItem is designed to fetch multiple items quickly by specifying their primary keys.

2. Step 2: Compare with other operations

   Unlike update or delete, BatchGetItem only retrieves data, not modifies it.

3. Final Answer:

   To retrieve multiple items from one or more tables using their primary keys in a single request. -> Option B

4. Quick Check:

   BatchGetItem = fetch multiple items [OK]

Hint: BatchGetItem fetches multiple items by keys in one call [OK]

Common Mistakes:

• Confusing BatchGetItem with update or delete operations
• Thinking it scans the whole table
• Assuming it modifies data

2. Which of the following is the correct syntax snippet to specify keys in a BatchGetItem request in DynamoDB?

easy

A. "RequestItems": { "TableName": { "FilterExpression": "Id = :id" } }

B. "RequestItems": { "TableName": { "AttributesToGet": ["Id"] } }

C. "RequestItems": { "TableName": { "Keys": [{ "Id": { "S": "123" } }] } }

D. "RequestItems": { "TableName": { "UpdateExpression": "SET Name = :name" } }

Solution

1. Step 1: Identify BatchGetItem key specification

   BatchGetItem requires a "Keys" array with primary key objects for each item to fetch.

2. Step 2: Check each option

   "RequestItems": { "TableName": { "Keys": [{ "Id": { "S": "123" } }] } } correctly uses "Keys" with primary key format. Snippets using "AttributesToGet", "FilterExpression", or "UpdateExpression" are not valid for BatchGetItem keys.

3. Final Answer:

   "RequestItems": { "TableName": { "Keys": [{ "Id": { "S": "123" } }] } } -> Option C

4. Quick Check:

   Keys array = correct syntax [OK]

Hint: BatchGetItem keys go inside "Keys" array per table [OK]

Common Mistakes:

• Using FilterExpression instead of Keys
• Confusing AttributesToGet with Keys
• Trying to use UpdateExpression in BatchGetItem

3. Given the following BatchGetItem request, what will be the output if the table contains items with Ids 101 and 102 only?

{
  "RequestItems": {
    "Products": {
      "Keys": [
        { "Id": { "N": "101" } },
        { "Id": { "N": "103" } }
      ]
    }
  }
}

medium

A. Returns only item with Id 101

B. Returns all items in the Products table

C. Returns no items because 103 does not exist

D. Returns items with Id 101 and 103

Solution

1. Step 1: Understand BatchGetItem behavior with missing keys

   BatchGetItem returns only items that exist for the specified keys. Missing keys are ignored.

2. Step 2: Analyze given keys

   Id 101 exists, so it will be returned. Id 103 does not exist, so it will be skipped.

3. Final Answer:

   Returns only item with Id 101 -> Option A

4. Quick Check:

   Only existing keys returned [OK]

Hint: BatchGetItem skips missing keys, returns existing only [OK]

Common Mistakes:

• Expecting error if key not found
• Assuming all keys must exist
• Thinking it returns all table items

4. You wrote this BatchGetItem request but it returns a syntax error:

{
  "RequestItems": {
    "Orders": {
      "Keys": {
        "OrderId": { "S": "A123" }
      }
    }
  }
}

What is the error and how to fix it?

medium

A. RequestItems key name is misspelled.

B. OrderId should be a number, not a string.

C. BatchGetItem does not support multiple tables.

D. Keys must be an array of key objects, not a single object.

Solution

1. Step 1: Identify the Keys format error

   Keys must be an array of objects, but here it is a single object, causing syntax error.

2. Step 2: Correct the Keys format

   Wrap the key object inside an array: "Keys": [{ "OrderId": { "S": "A123" } }]

3. Final Answer:

   Keys must be an array of key objects, not a single object. -> Option D

4. Quick Check:

   Keys = array of key objects [OK]

Hint: Keys must always be an array, even for one item [OK]

Common Mistakes:

• Using object instead of array for Keys
• Wrong data type for key attribute
• Misspelling RequestItems

5. You want to fetch items from two tables, "Users" and "Orders", using BatchGetItem. You have 3 user IDs and 2 order IDs. Which of the following request structures correctly fetches all these items in one call?

hard

A. { "RequestItems": { "Users": { "Keys": [ { "UserId": { "S": "U1" } }, { "UserId": { "S": "U2" } }, { "UserId": { "S": "U3" } } ] }, "Orders": { "Keys": [ { "OrderId": { "S": "O1" } }, { "OrderId": { "S": "O2" } } ] } } }

B. { "RequestItems": { "Users": { "Keys": { "UserId": ["U1", "U2", "U3"] } }, "Orders": { "Keys": { "OrderId": ["O1", "O2"] } } } }

C. { "RequestItems": { "Users": { "FilterExpression": "UserId IN (:u1, :u2, :u3)" }, "Orders": { "FilterExpression": "OrderId IN (:o1, :o2)" } } }

D. { "RequestItems": { "Users": { "AttributesToGet": ["UserId"] }, "Orders": { "AttributesToGet": ["OrderId"] } } }

Solution

1. Step 1: Understand BatchGetItem multi-table request format

   BatchGetItem requires "RequestItems" with each table name mapping to a "Keys" array of key objects.

2. Step 2: Evaluate each option

   { "RequestItems": { "Users": { "Keys": [ { "UserId": { "S": "U1" } }, { "UserId": { "S": "U2" } }, { "UserId": { "S": "U3" } } ] }, "Orders": { "Keys": [ { "OrderId": { "S": "O1" } }, { "OrderId": { "S": "O2" } } ] } } } correctly uses arrays of key objects for both tables. Formats using arrays inside objects, FilterExpression, or AttributesToGet are invalid for BatchGetItem keys.

3. Final Answer:

   { "RequestItems": { "Users": { "Keys": [ { "UserId": { "S": "U1" } }, { "UserId": { "S": "U2" } }, { "UserId": { "S": "U3" } } ] }, "Orders": { "Keys": [ { "OrderId": { "S": "O1" } }, { "OrderId": { "S": "O2" } } ] } } } -> Option A

4. Quick Check:

   Keys array per table = correct multi-table BatchGetItem [OK]

Hint: Use Keys array per table for multi-table BatchGetItem [OK]

Common Mistakes:

• Using FilterExpression instead of Keys
• Passing keys as arrays inside objects incorrectly
• Using AttributesToGet instead of Keys