Lambda function with DynamoDB

Introduction

A Lambda function lets you run code automatically when something happens. Using it with DynamoDB helps you save or get data without managing servers.

You want to save user info automatically when they sign up.

You need to update inventory counts when a sale happens.

You want to fetch data from DynamoDB when a website visitor requests it.

You want to process data changes in DynamoDB and react to them.

You want to run small tasks triggered by events without running a full server.

Syntax

DynamoDB

exports.handler = async (event) => {
  const AWS = require('aws-sdk');
  const dynamo = new AWS.DynamoDB.DocumentClient();

  const params = {
    TableName: 'YourTableName',
    Key: { id: event.id }
  };

  try {
    const data = await dynamo.get(params).promise();
    return data.Item;
  } catch (error) {
    return { error: error.message };
  }
};

This is a simple Lambda function in Node.js that reads an item from DynamoDB.

Replace 'YourTableName' and 'id' with your actual table and key names.

Examples

This example saves a new user to the 'Users' table using data from the event.

DynamoDB

exports.handler = async (event) => {
  const AWS = require('aws-sdk');
  const dynamo = new AWS.DynamoDB.DocumentClient();

  const params = {
    TableName: 'Users',
    Item: {
      id: event.id,
      name: event.name
    }
  };

  await dynamo.put(params).promise();
  return { message: 'User saved' };
};

This example fetches a user by id from the 'Users' table and returns it.

DynamoDB

exports.handler = async (event) => {
  const AWS = require('aws-sdk');
  const dynamo = new AWS.DynamoDB.DocumentClient();

  const params = {
    TableName: 'Users',
    Key: { id: event.id }
  };

  const data = await dynamo.get(params).promise();
  return data.Item || { message: 'User not found' };
};

Sample Program

This Lambda function saves a product to the 'Products' table and then retrieves it to confirm it was saved.

DynamoDB

exports.handler = async (event) => {
  const AWS = require('aws-sdk');
  const dynamo = new AWS.DynamoDB.DocumentClient();

  const params = {
    TableName: 'Products',
    Item: {
      id: '101',
      name: 'Coffee Mug',
      price: 12.99
    }
  };

  try {
    await dynamo.put(params).promise();
    const getParams = {
      TableName: 'Products',
      Key: { id: '101' }
    };
    const data = await dynamo.get(getParams).promise();
    return data.Item;
  } catch (error) {
    return { error: error.message };
  }
};

Important Notes

Make sure your Lambda has permission to access DynamoDB.

Use AWS SDK's DocumentClient for easier JSON handling.

Always handle errors to avoid crashes.

Summary

Lambda functions let you run code triggered by events without servers.

Use AWS SDK's DocumentClient to read and write DynamoDB data easily.

Remember to set correct permissions and handle errors in your code.

Practice

1. What is the main purpose of using a Lambda function with DynamoDB in AWS?

easy

A. To run code automatically when DynamoDB data changes

B. To manually update DynamoDB tables from the AWS console

C. To create new DynamoDB tables without coding

D. To backup DynamoDB data to S3 manually

Solution

1. Step 1: Understand Lambda triggers

   Lambda functions can be triggered by events, such as changes in DynamoDB tables.

2. Step 2: Identify the purpose of automation

   This allows automatic code execution when data changes, without manual intervention.

3. Final Answer:

   To run code automatically when DynamoDB data changes -> Option A

4. Quick Check:

   Lambda triggers = automatic code run [OK]

Hint: Lambda runs code on events like DynamoDB changes [OK]

Common Mistakes:

• Thinking Lambda manually updates tables
• Confusing Lambda with backup tools
• Assuming Lambda creates tables automatically

2. Which of the following is the correct way to initialize AWS DynamoDB DocumentClient in a Node.js Lambda function?

easy

A. const docClient = new AWS.DynamoDBClient();

B. const docClient = new AWS.DynamoDB.DocumentClient();

C. const docClient = AWS.DynamoDB.DocumentClient();

D. const docClient = AWS.DynamoDB.Document();

Solution

1. Step 1: Recall correct DocumentClient syntax

   The AWS SDK requires using 'new' keyword to create a DocumentClient instance.

2. Step 2: Match correct class name and usage

   The correct class is AWS.DynamoDB.DocumentClient, not DynamoDBClient or Document.

3. Final Answer:

   const docClient = new AWS.DynamoDB.DocumentClient(); -> Option B

4. Quick Check:

   Use 'new' with DocumentClient class [OK]

Hint: Use 'new AWS.DynamoDB.DocumentClient()' to initialize [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using wrong class name like DynamoDBClient
• Calling Document() instead of DocumentClient

3. Given this Lambda function snippet, what will be the output if the item with id '123' exists in the DynamoDB table?

const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient();

exports.handler = async (event) => {
  const params = {
    TableName: 'Users',
    Key: { id: '123' }
  };
  const data = await docClient.get(params).promise();
  return data.Item;
};

medium

A. An error because 'get' method is incorrect

B. A list of all items in the Users table

C. An empty object {}

D. The item object with id '123' from the Users table

Solution

1. Step 1: Understand docClient.get usage

   The get method retrieves a single item by key from the specified table.

2. Step 2: Analyze the return value

   If the item with id '123' exists, data.Item will contain that item object.

3. Final Answer:

   The item object with id '123' from the Users table -> Option D

4. Quick Check:

   docClient.get returns item if found [OK]

Hint: docClient.get returns single item by key [OK]

Common Mistakes:

• Expecting a list instead of single item
• Thinking get method is invalid
• Assuming empty object if item exists

4. Identify the error in this Lambda function code that writes an item to DynamoDB:

const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient();

exports.handler = async (event) => {
  const params = {
    TableName: 'Products',
    Item: {
      productId: '001',
      name: 'Book'
    }
  };
  await docClient.putItem(params).promise();
  return 'Success';
};

medium

A. Using 'putItem' instead of 'put' method on DocumentClient

B. Missing 'Key' property in params

C. TableName should be lowercase

D. Missing callback function in putItem

Solution

1. Step 1: Check DocumentClient method names

   DocumentClient uses 'put' method, not 'putItem'.

2. Step 2: Confirm correct method usage

   Using 'putItem' causes a runtime error because it does not exist on DocumentClient.

3. Final Answer:

   Using 'putItem' instead of 'put' method on DocumentClient -> Option A

4. Quick Check:

   Use 'put' to write items with DocumentClient [OK]

Hint: Use 'put', not 'putItem', with DocumentClient [OK]

Common Mistakes:

• Confusing low-level DynamoDB client with DocumentClient
• Assuming 'putItem' exists in DocumentClient
• Ignoring method naming conventions

5. You want a Lambda function to update a user's email in DynamoDB only if the user exists. Which approach correctly ensures this conditional update?

hard

A. Use docClient.delete before updating

B. Use docClient.put without any condition

C. Use docClient.update with ConditionExpression 'attribute_exists(id)'

D. Use docClient.get and then docClient.put unconditionally

Solution

1. Step 1: Understand conditional update in DynamoDB

   To update only if an item exists, use ConditionExpression with 'attribute_exists' on the key.

2. Step 2: Apply ConditionExpression in update method

   docClient.update supports ConditionExpression to prevent overwriting non-existing items.

3. Step 3: Evaluate other options

   Using put without condition overwrites or creates new items; delete before update is inefficient; get then put is not atomic.

4. Final Answer:

   Use docClient.update with ConditionExpression 'attribute_exists(id)' -> Option C

5. Quick Check:

   Conditional update = update + attribute_exists [OK]

Hint: Use ConditionExpression 'attribute_exists' for safe updates [OK]

Common Mistakes:

• Using put which overwrites without condition
• Deleting item before update unnecessarily
• Not using atomic conditional update