Re-asking this Database Lock issue question

One way to overcome that is to setup a unique field requirement for the field. For example the record ID for the slot (if a dropdown) or a concatenated field (date+ time slot) will prevent users from saving a record if two users concurrently opened the form.

This sounds like a possible solution but I can't figure out in my head how to set up that unique requirement you mentioned.

Perhaps I should elaborate as I did before. 

We have two tables. One table is called Event Dates. The other is Event Registration Request table (the latter is a child of the former). Each event date has a total number of slots that can be requested(one event per day, with a limited number of slots available represented by a numeric field. Slots may be added if money is available, which means I can edit the Event Date record and change the number of available slots) . One user can request more than 1 slot on a certain date provided that the slots are still available. We created a formula field that calculates slots still available for each day. We have a rule set to abort the save and display a message indicating that the "Number of requested slots is greater than the number of slots available for this date". The form works fine if I go in the form and try to request 2 slots when only 1 is available. But if I and a colleague go in at the same time and request 1 slot each, the form can't stop us.

1. Send an email when the course is I subscribed and tell the user they were too slow to save.

2. use an Automation to set the Acceptance Status to �Cancelled due to course over subscribed�.