The issue

We want to create a system that can receive an HTTP request, schedule it to be sent back later via webhook, and ensure it is delivered at the exact time we specify, with a maximum margin of error of about one second.

Cron approach

So we start by saving the request in a database and we use a column like scheduled_at to store the time we want to send the request.

Now we need to create a cron job that will check the database for requests that are scheduled to be sent in the future and send them.

The downside

Most likely the cron job will be running every minute, and that cause a that the schedule may be off by a some amount of time that the cron job wasn't running.

The solution

The solution in this case is to use a queue like Amazon SQS. The only problem we have is that the MAXIMUM delay is 15 minutes. And probably our system can schedule request up to 30 days in the future.

So how can we solve this?...

Well, we keep the cron that we already have, but now instead of checking the schedule_at at the time the request is scheduled, we check the schedule_at at the time the request is scheduled minus 15 minutes or it could be less that that.

If that condition is true, we send the request to the queue. And now we use a worker to send the request to the webhook. And the worker will receive the request and send it at the exact time we specified.

So the final architecture could look like this: