Lambda with DynamoDB Streams in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Lambda Trigger Behavior on DynamoDB Stream Batch

You have a Lambda function triggered by a DynamoDB stream with batch size set to 5. The stream receives 7 new records at once. How many times will the Lambda function be invoked?

A The Lambda function will be invoked 2 times: once with 5 records and once with 2 records.

B The Lambda function will be invoked 1 time with 7 records in the batch.

C The Lambda function will be invoked 7 times, once per record.

D The Lambda function will not be invoked automatically.

💡 Hint

Think about how batch size controls the maximum number of records per invocation.

The batch size limits the number of records per Lambda invocation. With 7 records and batch size 5, Lambda is invoked twice: first with 5 records, then with 2.

❓ Architecture

intermediate

Ensuring Exactly-Once Processing with Lambda and DynamoDB Streams

You want to process DynamoDB stream records with a Lambda function ensuring each record is processed exactly once, even if retries happen. Which architectural choice helps achieve this?

A Use Lambda with DynamoDB Streams and implement idempotent processing in the Lambda function.

B Use Lambda with DynamoDB Streams and set batch size to 1 to avoid retries.

C Use Lambda with DynamoDB Streams and rely on Lambda's automatic retry without additional logic.

D Use Lambda with DynamoDB Streams and disable the stream to prevent duplicate processing.

💡 Hint

Consider how retries can cause duplicate processing and how to handle it.

Lambda retries can cause the same record to be processed multiple times. Implementing idempotent logic in the Lambda function ensures processing is safe and exactly-once.

❓ Configuration

advanced

Configuring Lambda Event Source Mapping for DynamoDB Streams

Given this AWS CLI command to create an event source mapping for a Lambda function triggered by a DynamoDB stream, what is the effect of the --starting-position TRIM_HORIZON parameter?

aws lambda create-event-source-mapping \
  --function-name MyLambdaFunction \
  --event-source-arn arn:aws:dynamodb:us-east-1:123456789012:table/MyTable/stream/2023-01-01T00:00:00.000 \
  --starting-position TRIM_HORIZON \
  --batch-size 10

A The Lambda function will start processing only new records added after the mapping is created.

B The Lambda function will ignore the stream and run on a schedule.

C The Lambda function will process records in reverse order, starting from the newest.

D The Lambda function will start processing from the oldest available record in the stream.

💡 Hint

Think about what 'TRIM_HORIZON' means in stream processing.

TRIM_HORIZON means the Lambda starts reading from the oldest record available in the stream, processing all existing records.

❓ security

advanced

IAM Permissions for Lambda to Read DynamoDB Streams

Which IAM permission is required for a Lambda function to read records from a DynamoDB stream?

A dynamodb:DeleteItem

B dynamodb:PutItem

C dynamodb:GetRecords

D lambda:InvokeFunction

💡 Hint

Consider what action allows reading stream records.

The permission dynamodb:GetRecords allows Lambda to read records from the DynamoDB stream.

✅ Best Practice

expert

Handling Partial Failures in Lambda Processing of DynamoDB Streams

Your Lambda function processes batches of DynamoDB stream records. If processing one record in the batch fails, what is the best practice to avoid losing other successfully processed records?

A Set batch size to 1 to avoid batch failures.

B Process records individually inside Lambda and handle failures per record, using partial batch response feature.

C Let the entire batch fail so Lambda retries all records, including successful ones.

D Disable retries in Lambda to prevent duplicate processing.

💡 Hint

Think about how to handle failures without reprocessing successful records.

Using the partial batch response feature lets Lambda acknowledge successful records and retry only failed ones, avoiding duplicate processing of successful records.

Practice

1. What is the main purpose of connecting AWS Lambda to DynamoDB Streams?

easy

A. To manually trigger Lambda functions from the AWS console

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

C. To backup DynamoDB data to S3 automatically

D. To increase the read capacity of the DynamoDB table

Solution

1. Step 1: Understand DynamoDB Streams role

   DynamoDB Streams capture data changes in the table as events.

2. Step 2: Understand Lambda integration

   Lambda can be triggered automatically by these stream events to run code.

3. Final Answer:

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

4. Quick Check:

   Lambda triggers on stream events = A [OK]

Hint: Streams trigger Lambda on data changes automatically [OK]

Common Mistakes:

• Thinking Lambda triggers manually from console
• Confusing streams with backups
• Assuming streams increase table capacity

2. Which of the following is the correct way to enable DynamoDB Streams on a table?

easy

A. Create a Lambda function and attach it to the table

B. Enable versioning on the DynamoDB table

C. Set the stream specification to NEW_AND_OLD_IMAGES in the table settings

D. Increase the write capacity units of the table

Solution

1. Step 1: Identify how to enable streams

   DynamoDB Streams are enabled by setting the stream specification on the table.

2. Step 2: Choose correct stream view type

   NEW_AND_OLD_IMAGES captures both before and after images of changes.

3. Final Answer:

   Set the stream specification to NEW_AND_OLD_IMAGES in the table settings -> Option C

4. Quick Check:

   Stream enabled via table settings = B [OK]

Hint: Enable streams in table settings, not in Lambda [OK]

Common Mistakes:

• Trying to enable streams by creating Lambda
• Confusing versioning with streams
• Changing capacity units instead of streams

3. Given this Lambda event handler code snippet triggered by DynamoDB Streams:

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

What will this code print when a new item with id '123' is added to the DynamoDB table?

medium

A. '123' printed once for the new item

B. 'id' key printed instead of its value

C. An error because 'NewImage' does not exist

D. Nothing, because eventName is not 'INSERT'

Solution

1. Step 1: Understand event structure for INSERT

   For an INSERT event, 'NewImage' contains the new item data including 'id'.

2. Step 2: Analyze code behavior

   The code prints the string value of 'id' from 'NewImage' when eventName is 'INSERT'.

3. Final Answer:

   '123' printed once for the new item -> Option A

4. Quick Check:

   INSERT event prints new id value = D [OK]

Hint: INSERT events have NewImage with new item data [OK]

Common Mistakes:

• Assuming eventName is different
• Expecting OldImage on INSERT
• Confusing key name with value

4. You deployed a Lambda function triggered by DynamoDB Streams but it never runs. Which of the following is the most likely cause?

medium

A. The DynamoDB table does not have streams enabled

B. The Lambda function code has a syntax error

C. The DynamoDB table has no read capacity units

D. The Lambda function has no execution role

Solution

1. Step 1: Check stream enablement

   Lambda triggers on DynamoDB Streams events, so streams must be enabled on the table.

2. Step 2: Consider other causes

   While code errors or roles cause failures, the function would still be invoked if streams exist.

3. Final Answer:

   The DynamoDB table does not have streams enabled -> Option A

4. Quick Check:

   Streams must be enabled for Lambda trigger = C [OK]

Hint: Check if streams are enabled on the table first [OK]

Common Mistakes:

• Assuming code errors prevent invocation
• Ignoring missing streams
• Confusing read capacity with triggers

5. You want your Lambda function to process only updated attributes from DynamoDB Streams and ignore inserts and deletes. Which stream view type and event filter should you use?

hard

A. Enable NEW_AND_OLD_IMAGES stream view and filter events where eventName is 'INSERT'

B. Enable KEYS_ONLY stream view and filter events where eventName is 'INSERT'

C. Enable OLD_IMAGE stream view and filter events where eventName is 'REMOVE'

D. Enable NEW_IMAGE stream view and filter events where eventName is 'MODIFY'

Solution

1. Step 1: Choose stream view for updated data

   NEW_IMAGE provides the new state after update, suitable for processing changes.

2. Step 2: Filter events for updates only

   Filtering on eventName 'MODIFY' ensures only update events trigger Lambda.

3. Final Answer:

   Enable NEW_IMAGE stream view and filter events where eventName is 'MODIFY' -> Option D

4. Quick Check:

   Updates = NEW_IMAGE + 'MODIFY' filter [OK]

Hint: Use NEW_IMAGE and 'MODIFY' event filter for updates only [OK]

Common Mistakes:

• Using KEYS_ONLY which lacks attribute data
• Filtering on INSERT or REMOVE instead of MODIFY
• Choosing OLD_IMAGE which shows old data only