Step Functions with DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this DynamoDB query in Step Functions?

Consider a DynamoDB table named Orders with a partition key OrderId and an attribute Status. A Step Function executes a Query operation with the following parameters:

{
  "TableName": "Orders",
  "KeyConditionExpression": "OrderId = :oid",
  "ExpressionAttributeValues": {
    ":oid": {"S": "12345"}
  }
}

If the table contains one item with OrderId = "12345" and Status = "Shipped", what will the query return?

A {"Items": [], "Count": 0}

B Runtime error because Status attribute is not indexed

C SyntaxError due to missing FilterExpression

D {"Items": [{"OrderId": {"S": "12345"}, "Status": {"S": "Shipped"}}], "Count": 1}

💡 Hint

The query uses the partition key to find matching items.

The query uses the partition key OrderId with value "12345". Since the table has one item with that key, the query returns that item with its attributes.

🧠 Conceptual

intermediate

Which Step Function state type is best to perform a DynamoDB update operation?

You want to update an item in a DynamoDB table as part of a Step Function workflow. Which state type should you use?

A Pass state to pass data without changes

B Task state with AWS SDK integration calling DynamoDB UpdateItem

C Choice state to decide which item to update

D Wait state to delay the update operation

💡 Hint

Updating data requires an action that interacts with DynamoDB.

A Task state can call AWS SDK actions like DynamoDB UpdateItem to modify data. Choice, Pass, and Wait states do not perform data updates.

📝 Syntax

advanced

Identify the syntax error in this Step Function DynamoDB query state

Given this Step Function state snippet that queries DynamoDB, which option correctly identifies the syntax error?

{
  "Type": "Task",
  "Resource": "arn:aws:states:::aws-sdk:dynamodb:query",
  "Parameters": {
    "TableName": "Users",
    "KeyConditionExpression": "UserId = :uid",
    "ExpressionAttributeValues": {
      ":uid": {"S": "abc123"}
    }
  },
  "End": true
}

A No syntax error; the state is valid

B KeyConditionExpression must use #uid instead of :uid

C ExpressionAttributeValues keys must be strings without colon prefix

D Missing comma after ExpressionAttributeValues object

💡 Hint

Check the AWS SDK DynamoDB query syntax for Step Functions.

The syntax shown is correct for a DynamoDB query in Step Functions. The colon prefix in ExpressionAttributeValues is required and commas are correctly placed.

❓ optimization

advanced

How to optimize a Step Function that scans a large DynamoDB table?

You have a Step Function that performs a Scan operation on a large DynamoDB table. The scan is slow and expensive. Which approach best optimizes this workflow?

A Use Query with a partition key filter instead of Scan

B Increase the ScanLimit parameter to scan more items at once

C Add a Wait state between scans to reduce throughput

D Use Scan with FilterExpression to reduce returned items

💡 Hint

Query operations are more efficient than Scan when possible.

Query uses indexes and partition keys to efficiently retrieve items. Scan reads the entire table and is slower and more costly. Using Query with a partition key filter is the best optimization.

🔧 Debug

expert

Why does this Step Function DynamoDB PutItem state fail with a ValidationException?

Consider this Step Function state that tries to put an item into DynamoDB:

{
  "Type": "Task",
  "Resource": "arn:aws:states:::aws-sdk:dynamodb:putItem",
  "Parameters": {
    "TableName": "Products",
    "Item": {
      "ProductId": {"S": "p123"},
      "Price": {"N": "twenty"}
    }
  },
  "End": true
}

It fails with a ValidationException. What is the cause?

A TableName parameter is case-sensitive and should be "products"

B Missing required attribute ProductName in the item

C The Price attribute value "twenty" is not a valid number format

D Resource ARN is incorrect for PutItem operation

💡 Hint

Check the data types expected by DynamoDB for number attributes.

DynamoDB expects number attributes to be strings representing numeric values. The string "twenty" is not a valid number format, causing the ValidationException.