Handling DynamoDB Triggers and Streams in Cucumber Tests

Amazon DynamoDB is a powerful NoSQL database service that provides fast and predictable performance with seamless scalability. One of its key features is the ability to trigger AWS Lambda functions through DynamoDB Streams. This capability enables real-time processing of data changes, making it ideal for building reactive applications. However, testing these triggers and streams can be challenging. In this blog post, we will explore how to handle DynamoDB triggers and streams in Cucumber tests, providing in-depth explanations and relevant code examples.

Setting Up DynamoDB Streams and Lambda Triggers

Before diving into testing, let’s briefly set up a DynamoDB table, enable streams, and configure a Lambda function to process these streams.

1. Create a DynamoDB Table:

import boto3

dynamodb = boto3.client('dynamodb', region_name='us-west-2')

table_name = 'ExampleTable'
dynamodb.create_table(
    TableName=table_name,
    KeySchema=[
        {'AttributeName': 'ID', 'KeyType': 'HASH'}  # Partition key
    ],
    AttributeDefinitions=[
        {'AttributeName': 'ID', 'AttributeType': 'S'}
    ],
    ProvisionedThroughput={
        'ReadCapacityUnits': 5,
        'WriteCapacityUnits': 5
    },
    StreamSpecification={
        'StreamEnabled': True,
        'StreamViewType': 'NEW_AND_OLD_IMAGES'
    }
)

2. Create a Lambda Function:

import zipfile

lambda_client = boto3.client('lambda', region_name='us-west-2')

# Create a zip file with Lambda function code
with zipfile.ZipFile('/tmp/lambda_function.zip', 'w') as zipf:
    zipf.write('lambda_function.py')

lambda_client.create_function(
    FunctionName='ExampleLambda',
    Runtime='python3.8',
    Role='<YourIAMRoleARN>',
    Handler='lambda_function.lambda_handler',
    Code={'ZipFile': open('/tmp/lambda_function.zip', 'rb').read()},
    Environment={'Variables': {'TABLE_NAME': table_name}}
)

3. Attach Lambda to DynamoDB Stream:

streams = dynamodb.describe_table(TableName=table_name)['Table']['LatestStreamArn']

lambda_client.create_event_source_mapping(
    EventSourceArn=streams,
    FunctionName='ExampleLambda',
    Enabled=True,
    BatchSize=100,
    StartingPosition='LATEST'
)

Writing Cucumber Tests for DynamoDB Triggers

Cucumber is a popular tool for Behavior-Driven Development (BDD). It allows you to write tests in a human-readable format. Here’s how you can use Cucumber to test DynamoDB triggers and streams.

1. Define Feature File:

Feature: DynamoDB Streams and Lambda Triggers

  Scenario: Inserting a new item into DynamoDB
    Given a DynamoDB table exists
    When I insert a new item into the table
    Then the Lambda function should be triggered
    And the item should be processed correctly

2. Implement Step Definitions:

import boto3
import time
from cucumber import given, when, then
from hamcrest import assert_that, has_item

dynamodb = boto3.resource('dynamodb', region_name='us-west-2')
lambda_client = boto3.client('lambda', region_name='us-west-2')

@given('a DynamoDB table exists')
def step_impl(context):
    context.table = dynamodb.Table('ExampleTable')

@when('I insert a new item into the table')
def step_impl(context):
    context.item = {'ID': '123', 'Name': 'Test Item'}
    context.table.put_item(Item=context.item)
    time.sleep(2)  # Wait for Lambda to process the item

@then('the Lambda function should be triggered')
def step_impl(context):
    logs = lambda_client.get_log_events(
        logGroupName='/aws/lambda/ExampleLambda',
        logStreamName='log-stream-name'
    )
    assert_that([event['message'] for event in logs['events']], has_item('Processing item: 123'))

@then('the item should be processed correctly')
def step_impl(context):
    processed_table = dynamodb.Table('ProcessedTable')
    response = processed_table.get_item(Key={'ID': '123'})
    assert 'Item' in response
    assert response['Item']['Name'] == 'Test Item Processed'

Detailed Explanation

1. Creating the DynamoDB Table: We create a DynamoDB table with a partition key (ID) and enable streams. The stream view type is set to NEW_AND_OLD_IMAGES to capture both new and old images of the item.
2. Creating the Lambda Function: We create a Lambda function with the necessary IAM role. The Lambda function code is packaged into a zip file and uploaded.
3. Attaching Lambda to DynamoDB Stream: We attach the Lambda function to the DynamoDB stream so that the Lambda function is triggered whenever there is a data change in the DynamoDB table.
4. Defining the Feature File: The feature file defines the scenarios we want to test in a human-readable format.
5. Implementing Step Definitions:
   • @given step ensures that the DynamoDB table exists.
   • @when step inserts a new item into the DynamoDB table.
   • @then step checks if the Lambda function was triggered by verifying the logs.
   • Another @then step checks if the item was processed correctly by querying the processed table.

Running the Tests

To run the tests, use the following command:

cucumber features/dynamodb_streams.feature

Conclusion

Testing DynamoDB triggers and streams with Cucumber can be challenging but is crucial for ensuring the reliability of your serverless applications. By following the steps outlined in this blog post, you can create comprehensive tests that cover the end-to-end flow of data changes in DynamoDB and their subsequent processing by Lambda functions.

If you found this article helpful and are interested in integrating Cucumber Automation Framework with AWS DynamoDB, I suggest you check out some of the other articles I’ve written for this series:

• Creating Reusable Cucumber Step Definitions for DynamoDB Operations
• Monitoring and Logging DynamoDB Interactions in Cucumber Tests
• Data-Driven Testing with Cucumber and DynamoDB
• Scalable Test Data Management with AWS DynamoDB and Cucumber

If you’re looking for a deeper dive into some of the concepts and specifics discussed in my article, feel free to reach out to me directly or as always you can checkout the official AWS DynamoDB Developer Documentation for more information.