Scheduling a reminder email in AWS step function (through events/SES) based on Dynamo DB attributes

Question

I have a step function with 3 lambdas, the last lambda is basically writing an entry in the dynamo DB with a timestamp, status = "unpaid" (this is updated to "paid" for some automatically based on another workflow), email and closes the execution. Now I want to schedule a reminder on any entry in the DynamoDB which is unpaid & over 7 days, a second reminder if any entry is unpaid over 14 days, a third last reminder on 19th day - sent via email. So the question is:

1. Is there any way to do this scheduling per Step function execution (that can then monitor that particular entry in ddb for 7, 14, 19 days and send reminders accordingly until the status is "unpaid"). If yes, would it be too much overhead since there could be millions of transactions.

2. The second way which I was thinking was to build another scheduler lambda sequence: the first lambda basically parsing through the whole ddb searching for entries valid for reminder (either 7, 14, 19). The second lambda getting the list from the first lambda and prepares the reminder based on whether its first, second or third (in loop) & the third Lambda one sending the reminder through SES.

Is there a better or easier way to do this? I know we can trigger step functions or lambdas through cloud events or we also have crons that we can use but they were not suiting the use case much. Any help here is appreciated?

Answer 1

DynamoDB does not have functionality for a delayed notification based on logic, you would need to design this flow yourself. Luckily AWS has all the tools you need to perform this.

I believe the best option would probably be to create a CloudWatch Events / EventBridge when the item is written to DynamoDB (either via your application or as a trigger via a Lambda using DynamoDB Streams ).

This event would be scheduled for 7 days time, in the 7 days any checks could be performed to validate if it has been paid or not. If it has not been paid you schedule the next event and send out the notification. If it had been paid you would simply exit the Lambda function. This would then continue for the next 2 time periods.

You could then further enhance this by using DynamoDB streams so that in the event of the DynamoDB table being updated a Lambda is triggered to detect whether status has changed from unpaid. If this occurs simply remove the event trigger to prevent it even having to process.