Scheduling one time tasks with AWS

Question

I have to implement functionality that requires delayed sending of a message to a user once on a specific date, which can be anytime - from tomorrow till in a few months from now.

All our code is so far implemented as lambda functions.

I'm considering three options on how to implement this:

1. Create an entry in DynamoDB with hash key being date and range key being unique ID. Schedule lambda to run once a day and pick up all entries/tasks scheduled for this day, send a message for each of them.

2. Using SDK Create cloudwatch event rule with cron expression indicating single execution and make it invoke lambda function (target) with ID of user/message. The lambda would be invoked on a specific schedule with a specific user/message to be delivered.

3. Create a step function instance and configure it to sleep & invoke step with logic to send a message when the right moment comes.

Do you have perhaps any recommendation on what would be best practice to implement this kind of business requirement? Perhaps an entirely different approach?

Answer 1

It largely depends on scale. If you'll only have a few scheduled at any point in time then I'd use the CloudWatch events approach. It's very low overhead and doesn't involve running code and doing nothing.

If you expect a LOT of schedules then the DynamoDB approach is very possibly the best approach. Run the lambda on a fixed schedule, see what records have not yet been run, and are past/equal to current time. In this model you'll want to delete the records that you've already processed (or mark them in some way) so that you don't process them again. Don't rely on the schedule running at certain intervals and checking for records between the last time and the current time unless you are recording when the last time was (i.e. don't assume you ran a minute ago because you scheduled it to run every minute).

Step functions could work if the time isn't too far out. You can include a delay in the step that causes it to just sit and wait. The delays in step functions are just that, delays, not scheduled times, so you'd have to figure out that delay yourself, and hope it fires close enough to the time you expect it. This one isn't a bad option for mid to low volume.

Edit: Step functions include a wait_until option on wait states now. This is a really good option for what you are describing.