Lambda function with DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Lambda function when querying DynamoDB?

Consider a Lambda function written in Node.js that queries a DynamoDB table named Users with a partition key userId. The function queries for userId = '123'. What will the function return if the item exists?

DynamoDB

import AWS from 'aws-sdk';
const dynamoDb = new AWS.DynamoDB.DocumentClient();

export const handler = async (event) => {
  const params = {
    TableName: 'Users',
    Key: { userId: '123' }
  };
  try {
    const data = await dynamoDb.get(params).promise();
    return data.Item || null;
  } catch (error) {
    return { error: error.message };
  }
};

A {"userId":"123","name":"Alice","age":30}

B null

C {"error":"ResourceNotFoundException"}

D {"userId":"123"}

💡 Hint

If the item exists, the get method returns the full item under data.Item.

The dynamoDb.get method returns an object with an Item property containing the item if found. If the item exists, it returns the full item data. If not found, Item is undefined, so the function returns null.

📝 Syntax

intermediate

Which option correctly updates an item in DynamoDB using Lambda?

You want to update the age attribute of a user with userId = '456' in DynamoDB using a Lambda function. Which code snippet is syntactically correct?

DynamoDB

import AWS from 'aws-sdk';
const dynamoDb = new AWS.DynamoDB.DocumentClient();

export const handler = async () => {
  const params = {
    TableName: 'Users',
    Key: { userId: '456' },
    UpdateExpression: 'set age = :a',
    ExpressionAttributeValues: { ':a': 35 },
    ReturnValues: 'UPDATED_NEW'
  };
  return await dynamoDb.update(params).promise();
};

A UpdateExpression: 'set age = :a', ExpressionAttributeValues: { ':a': 35 }

B UpdateExpression: 'set age = a', ExpressionAttributeValues: { 'a': 35 }

C UpdateExpression: 'set age = :a', ExpressionAttributeValues: { a: 35 }

D UpdateExpression: 'set age = :a', ExpressionAttributeValues: { ':a' => 35 }

💡 Hint

Expression attribute values must use placeholders starting with a colon : and be keys in an object.

The correct syntax requires the placeholder :a in the update expression and the corresponding key in ExpressionAttributeValues must be a string with the colon. Option A follows this syntax correctly.

❓ optimization

advanced

How to optimize a Lambda function to batch write multiple items to DynamoDB?

You want to insert 25 user records into a DynamoDB table in a single Lambda invocation. Which approach is the most efficient and correct?

A Call <code>dynamoDb.put</code> 25 times in parallel without waiting for promises.

B Call <code>dynamoDb.put</code> 25 times sequentially inside a loop.

C Use <code>dynamoDb.update</code> with 25 update expressions in one call.

D Use <code>dynamoDb.batchWrite</code> with up to 25 <code>PutRequest</code> items in one call.

💡 Hint

DynamoDB supports batch writes with a limit of 25 items per batch.

Using batchWrite with up to 25 items is the most efficient way to insert multiple items in one request. Calling put multiple times is less efficient and may cause throttling.

🔧 Debug

advanced

Why does this Lambda function fail with a ProvisionedThroughputExceededException?

This Lambda function writes items to DynamoDB but sometimes fails with ProvisionedThroughputExceededException. What is the likely cause?

DynamoDB

import AWS from 'aws-sdk';
const dynamoDb = new AWS.DynamoDB.DocumentClient();

export const handler = async (event) => {
  const params = {
    TableName: 'Users',
    Item: event.item
  };
  try {
    await dynamoDb.put(params).promise();
    return { status: 'success' };
  } catch (error) {
    return { error: error.code };
  }
};

A The AWS SDK is not configured with correct credentials.

B The Item parameter is missing a required attribute.

C The function is writing too many items too quickly, exceeding the table's write capacity.

D The table name 'Users' is misspelled.

💡 Hint

ProvisionedThroughputExceededException means the request rate is too high for the provisioned capacity.

This error occurs when the number of write requests exceeds the provisioned write capacity units of the DynamoDB table. The function likely writes too fast or in bursts.

🧠 Conceptual

expert

What is the best practice for handling DynamoDB pagination in a Lambda function?

You query a DynamoDB table with a large dataset that returns paginated results. How should your Lambda function handle pagination to retrieve all items efficiently?

A Set <code>Limit</code> to a very high number to get all items in one query call.

B Use the <code>LastEvaluatedKey</code> from the query response to iteratively request the next page until no more keys remain.

C Ignore pagination and process only the first page of results.

D Use <code>Scan</code> instead of <code>Query</code> to avoid pagination.

💡 Hint

DynamoDB paginates query results and provides LastEvaluatedKey to continue fetching.

To retrieve all items, the Lambda function should check if LastEvaluatedKey exists in the response and use it as ExclusiveStartKey in the next query call, repeating until no key remains.