Lambda trigger on stream events in DynamoDB - Mini Project: Build & Apply

Lambda Trigger on DynamoDB Stream Events

📖 Scenario: You are building a simple inventory system using DynamoDB. You want to automatically process changes to the inventory items whenever they are added or updated. To do this, you will set up a Lambda function that triggers on DynamoDB stream events.

🎯 Goal: Create a DynamoDB table with a stream enabled, configure a Lambda function to trigger on the stream events, and write the Lambda function code to process the incoming stream records.

📋 What You'll Learn

Create a DynamoDB table named Inventory with a primary key ItemId (string).

Enable a stream on the Inventory table with NEW_IMAGE view type.

Create a Lambda function named ProcessInventoryStream that triggers on the DynamoDB stream events.

Write Lambda function code to iterate over the stream records and log the ItemId and Quantity of each new image.

💡 Why This Matters

🌍 Real World

Automatically reacting to database changes is common in real-time applications like inventory management, notifications, and analytics.

💼 Career

Understanding how to connect DynamoDB streams with Lambda functions is a key skill for AWS developers and cloud engineers working on serverless architectures.

1

Create DynamoDB Table with Stream Enabled

Create a DynamoDB table named Inventory with a primary key called ItemId of type string. Enable the stream on this table with the view type set to NEW_IMAGE.

DynamoDB

# Your code here to create DynamoDB table Inventory with ItemId as string primary key and stream enabled with NEW_IMAGE

Hint

Use the AWS CLI aws dynamodb create-table command with the --stream-specification option.

2

Configure Lambda Trigger on DynamoDB Stream

Create a Lambda function named ProcessInventoryStream and configure it to trigger on the DynamoDB stream of the Inventory table. Use the stream ARN from the table description as the event source.

DynamoDB

aws dynamodb create-table \
    --table-name Inventory \
    --attribute-definitions AttributeName=ItemId,AttributeType=S \
    --key-schema AttributeName=ItemId,KeyType=HASH \
    --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5 \
    --stream-specification StreamEnabled=true,StreamViewType=NEW_IMAGE

# Your code here to create Lambda function ProcessInventoryStream and add DynamoDB stream trigger

Hint

Use aws lambda create-function to create the Lambda and aws lambda create-event-source-mapping to connect the stream.

3

Write Lambda Function Code to Process Stream Records

Write the Lambda function code in Python that iterates over the Records in the event. For each record, extract the ItemId and Quantity from the NewImage and log them using print.

DynamoDB

def lambda_handler(event, context):
    # Your code here to process stream records and print ItemId and Quantity

Hint

Use event['Records'] to loop through records and access NewImage data.

4

Deploy Lambda Function Code and Finalize Setup

Package the Lambda function code into a zip file named function.zip and update the Lambda function ProcessInventoryStream with this code. Ensure the Lambda function has permission to read from the DynamoDB stream.

DynamoDB

# Your code here to update Lambda function code with function.zip and add necessary permissions

Hint

Use aws lambda update-function-code to deploy and aws lambda add-permission to grant stream invoke rights.

Practice

1. What is the main purpose of a Lambda trigger on DynamoDB stream events?

easy

A. To backup the DynamoDB table data to S3

B. To manually update the DynamoDB table from the Lambda console

C. To disable the DynamoDB stream temporarily

D. To automatically run code when data in the DynamoDB table changes

Solution

1. Step 1: Understand DynamoDB streams

   DynamoDB streams capture data changes like inserts, updates, and deletes.

2. Step 2: Role of Lambda triggers

   Lambda triggers run code automatically when these stream events occur.

3. Final Answer:

   To automatically run code when data in the DynamoDB table changes -> Option D

4. Quick Check:

   Lambda triggers = auto-run code on data change [OK]

Hint: Lambda triggers run code automatically on data changes [OK]

Common Mistakes:

• Thinking Lambda triggers require manual execution
• Confusing stream enabling with backup
• Assuming Lambda disables streams

2. Which of the following is the correct way to specify a Lambda trigger for a DynamoDB stream in AWS Console?

easy

A. Enable stream on the table, then add Lambda as a trigger on the stream

B. Create Lambda function first, then enable stream on the table

C. Enable stream on the table and Lambda separately without linking

D. Add Lambda trigger before enabling the stream on the table

Solution

1. Step 1: Enable stream on DynamoDB table

   Streams must be enabled first to capture data changes.

2. Step 2: Add Lambda as a trigger on the enabled stream

   Lambda triggers are linked to the stream to react to events.

3. Final Answer:

   Enable stream on the table, then add Lambda as a trigger on the stream -> Option A

4. Quick Check:

   Enable stream first, then add Lambda trigger [OK]

Hint: Enable stream first, then attach Lambda trigger [OK]

Common Mistakes:

• Trying to add Lambda trigger before enabling stream
• Not linking Lambda to the stream
• Enabling stream and Lambda separately without connection

3. Given this Lambda function snippet triggered by a DynamoDB stream event:

def lambda_handler(event, context):
    for record in event['Records']:
        if record['eventName'] == 'INSERT':
            print(record['dynamodb']['NewImage']['id']['S'])

What will this code output when a new item with id '123' is inserted?

medium

A. It prints '123' to the logs

B. It prints the old image of the item

C. It raises a KeyError because 'NewImage' is missing

D. It prints nothing because eventName is 'MODIFY'

Solution

1. Step 1: Check eventName condition

   The code checks if eventName is 'INSERT', which matches new item insertion.

2. Step 2: Access NewImage id value

   It accesses 'NewImage' dictionary and prints the 'id' string value '123'.

3. Final Answer:

   It prints '123' to the logs -> Option A

4. Quick Check:

   INSERT event prints new id '123' [OK]

Hint: INSERT event uses NewImage to get new item data [OK]

Common Mistakes:

• Confusing NewImage with OldImage
• Assuming eventName is MODIFY instead of INSERT
• Not accessing nested dictionary keys correctly

4. You wrote this Lambda code to process DynamoDB stream events but it throws a runtime error:

def lambda_handler(event, context):
    for record in event['Records']:
        print(record['dynamodb']['NewImage']['id']['S'])

What is the likely cause of the error?

medium

A. The 'id' attribute is always an integer, not a string

B. The event may contain DELETE events without NewImage data

C. The event['Records'] key is missing

D. Lambda functions cannot access DynamoDB streams

Solution

1. Step 1: Understand stream event types

   DELETE events do not have 'NewImage', only 'OldImage'.

2. Step 2: Accessing NewImage without check causes error

   Trying to access 'NewImage' on DELETE events causes a KeyError.

3. Final Answer:

   The event may contain DELETE events without NewImage data -> Option B

4. Quick Check:

   DELETE events lack NewImage causing runtime error [OK]

Hint: Check event type before accessing NewImage [OK]

Common Mistakes:

• Assuming all events have NewImage
• Ignoring DELETE event structure
• Believing Lambda can't access streams

5. You want your Lambda triggered by DynamoDB streams to process only UPDATE events where the attribute 'status' changed to 'completed'. Which approach is best?

hard

A. Filter events in DynamoDB stream settings to only send UPDATE events with 'completed' status

B. Use a separate Lambda for each event type and status value

C. In Lambda, check if eventName is 'MODIFY' and compare OldImage and NewImage 'status' values

D. Enable stream with only KEYS_ONLY to reduce data size

Solution

1. Step 1: Understand stream filtering limits

   DynamoDB streams cannot filter by attribute values, only event types.

2. Step 2: Use Lambda code to check eventName and compare status

   Lambda can check if eventName is 'MODIFY' and compare OldImage and NewImage 'status' to detect change to 'completed'.

3. Final Answer:

   In Lambda, check if eventName is 'MODIFY' and compare OldImage and NewImage 'status' values -> Option C

4. Quick Check:

   Filter in Lambda by event and attribute change [OK]

Hint: Filter attribute changes inside Lambda, not in stream settings [OK]

Common Mistakes:

• Trying to filter by attribute in stream settings
• Using KEYS_ONLY which lacks attribute data
• Creating multiple Lambdas unnecessarily