Lambda event filter dynamodb TTL deletions

Question

I'm trying to trigger a Lambda from a DynamodDB stream, but only if the change is not a deletion due to TTL. I know that TTL deletes have 'userIdentity': {'principalId': 'dynamodb.amazonaws.com', 'type': 'Service'} in their events, while other events do not have useridentity at all, so I tried this filter: {"userIdentity": [ { "exists": false } ]}. This passes validation, but all events trigger the Lambda, including TTL deletes.

Any idea how I can filter the TTLs out? The reason I want to do that is I have a phase where no changes to the database should be made, and I'd like to verify that the Lambda is not triggered. Therefore I want to block TTL deletions before the Lambda invocation.

Answer

To filter out TTL deletions, you can't directly set up a filter on the event source mapping in Lambda. Instead, you can add a conditional check within your Lambda function to filter out TTL deletions based on the userIdentity field in the event record.

Here's an example of how you can modify your Lambda function to filter out TTL deletions:

```
import json

def lambda_handler(event, context):
    for record in event['Records']:
        if ('userIdentity' not in record['dynamodb']['NewImage']) or (record['dynamodb']['NewImage']['userIdentity']['principalId'] != 'dynamodb.amazonaws.com'):
            # Process the non-TTL deletion event
            print("Non-TTL event:", json.dumps(record, indent=2))

```

This code snippet checks each record in the event to see if the userIdentity field is present and if its principalId is equal to 'dynamodb.amazonaws.com'. If the condition is false, it processes the non-TTL event. By doing so, your Lambda function only processes events that are not TTL deletions.

Lambda event filter dynamodb TTL deletions

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