How can i stop two people booking the same appointment time

As the title states, how can i stop two users from booking the same appointment time.

Example, two users 开发者_运维知识库logging, on there screen they can both see that a 1pm appointment is available. They both try to book themselves into that appointment time.

How can i stop this from happening and ensure only one user can book it, then refresh the screen to show the next available booking time to the other user.

Thanks.

---

That's a fairly classic use case. You can simply display the appointment plan at a specific time. That can remain static or you can set up a periodic process (for example, every five seconds) to update the plan with new information.

Then, when the user/operator decides to book a free timeslot, it tries to do an (atomic) update that will fail if someone else has slipped in (using primary key or some other unique constraint). The atomicity of the update operation guarantees that only one person can book the timeslot. If the update works, voila, you have your time booked.

If it fails, notify the user of that fact and then load up the new appointment plan.

Rinse and repeat until the user has their booking or they wander off, disgruntled.

---

I do not think that this is optimal. Why don't you opt for a first clicked/first served pattern ? What may happen if you have 10 users or more viewing the same page ? A user viewing a page does not mean that he will click on a time slot. So IMHO wait for a user to REALLY click and then notify eventually other users that a time slot they are viewing has been booked (As it happens with StackOverFlow when somebody answers a question while you are answering)