Lambda function not triggered by dynamodb update: Key Error - amazon-web-services

Question

I am attempting to load a simple transactions.txt table into a S3 bucket where a Lambda function reads the file and populates DynamoDB tables for Customers and Transactions. This all works fine. However, I also have a Lambda function that is supposed to read the Transactions table as they populate the table and sum up the transaction totals by customer and insert them into another DynamoDB table--TransactionTotal.
My TotalNotifier Lambda function throws a "KeyError" regarding a "New Image". I believe the code is fine, and I have tried changing the type of Streams from 'New and Old' to just 'New' for the Transactions table and still encounter same error.
from __future__ import print_function
import json, boto3
# Connect to SNS
sns = boto3.client('sns')
alertTopic = 'HighBalanceAlert'
snsTopicArn = [t['TopicArn'] for t in sns.list_topics()['Topics'] if t['TopicArn'].endswith(':' + alertTopic)][0]
# Connect to DynamoDB
dynamodb = boto3.resource('dynamodb')
transactionTotalTableName = 'TransactionTotal'
transactionsTotalTable = dynamodb.Table(transactionTotalTableName);
# This handler is executed every time the Lambda function is triggered
def lambda_handler(event, context):
# Show the incoming event in the debug log
print("Event received by Lambda function: " + json.dumps(event, indent=2))
# For each transaction added, calculate the new Transactions Total
for record in event['Records']:
customerId = record['dynamodb']['NewImage']['CustomerId']['S']
transactionAmount = int(record['dynamodb']['NewImage']['TransactionAmount']['N'])
# Update the customer's total in the TransactionTotal DynamoDB table
response = transactionsTotalTable.update_item(
Key={
'CustomerId': customerId
},
UpdateExpression="add accountBalance :val",
ExpressionAttributeValues={
':val': transactionAmount
},
ReturnValues="UPDATED_NEW"
)
Here is a sample error from the CloudWatch log:
'NewImage': KeyError
Traceback (most recent call last):
File "/var/task/lambda_function.py", line 30, in lambda_handler
customerId = record['dynamodb']['NewImage']['CustomerId']['S']
KeyError: 'NewImage'

Answer 1

To elaborate on Oluwafemi's comment, you're likely experiencing this error when receiving a REMOVE event. Regardless of whether your stream is new and old images, or just new, you won't receive a NEW_IMAGE on a REMOVE event, since there is no new image. Check out the example events on aws docs.
A check on the value of record['eventName'] should solve the issue.