Lambda trigger on stream events in DynamoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Lambda trigger on DynamoDB stream events?

It is a way to automatically run AWS Lambda functions when changes happen in a DynamoDB table, like adding, updating, or deleting items.

beginner

How does DynamoDB Streams help Lambda functions?

DynamoDB Streams captures table activity and sends event records to Lambda, which then processes these changes in near real-time.

beginner

What types of events can trigger a Lambda function from a DynamoDB stream?

INSERT (new item added), MODIFY (item updated), and REMOVE (item deleted) events can trigger Lambda functions.

intermediate

Why use Lambda triggers on DynamoDB streams instead of polling the table?

Lambda triggers respond instantly to changes without constant checking, saving resources and providing faster reactions.

intermediate

What must you enable on a DynamoDB table to use Lambda triggers on stream events?

You must enable DynamoDB Streams on the table and configure the stream to capture the desired data (keys only, new image, old image, or both).

Which DynamoDB stream event type triggers when an item is deleted?

A UPDATE

B MODIFY

C INSERT

D REMOVE

REMOVE events occur when an item is deleted from the table.

What must be enabled on a DynamoDB table to use Lambda triggers on stream events?

A DynamoDB Streams

B Auto Scaling

C Global Tables

D Backup and Restore

DynamoDB Streams must be enabled to capture changes and trigger Lambda functions.

Which of the following is NOT a benefit of using Lambda triggers on DynamoDB streams?

A Manual polling of the table for changes

B Automatic scaling of Lambda functions

C Reduced resource usage compared to polling

D Real-time processing of data changes

Manual polling is not a benefit; Lambda triggers avoid the need for polling.

What kind of data can DynamoDB Streams capture for Lambda triggers?

A Only the primary key

B New item image, old item image, or both

C Only deleted items

D Only updated attributes

Streams can capture new images, old images, or both depending on configuration.

When a Lambda function is triggered by a DynamoDB stream, what does it receive?

A The entire DynamoDB table

B Only the updated attributes

C A batch of event records describing changes

D A snapshot of the database

Lambda receives a batch of event records describing the changes made.

Explain how a Lambda trigger works with DynamoDB stream events.

Think about what happens when data changes in the table.

List the steps to set up a Lambda trigger on DynamoDB stream events.

Consider both DynamoDB and Lambda configurations.

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