Preventing Multiple Simultaneous Votes From Different Users

Question

I am not 100% sure on how to describe this, so here's a quick explanation of what I am doing: I have a website where people can vote UP or DOWN to a "champion." These champions start with 100 health. If you were to UP vote a specific champion, their health would now be 101. DOWN voting it would be 99.

This site has been up and running for 5 seasons now (there's over 1200 members that play). So there's a lot of voting going on at once. It all works fine now. For this next season, I am implementing "real-time" voting with jQuery/getJSON/ajax (so the page doesn't need to refresh every time you vote).

I have this working amazing right now (I am shocked actually :P) except for one minor detail... If a "champion" is down to 1 health, and more than one person click DOWN VOTE at the same time, the server fires it off multiple times making the champion have -1 or whatever amount of health. It also INSERTS (into the eliminated table of the database) each person who clicked DOWN at the same time as the one who eliminated that champion.

Really have no clue what to do here...but I hope I explained this well enough. If you guys need any code, such as the php file that inserts the information into the database or even the jQuery part, please let me know! Any help would be greatly appreciated!

Answer 1

I see you have already some code based from this post: link.

What you can do is:

• Change the UPDATE function to first SELECT the amount of health an user has. Then only update it when it actually is > 0. If it is < 1 then you could decide to set it also to 0, to prevent accidents with -1. If that happened by accident still, then you set it back. But that chanche is very small.
• When performing your INSERT into the eliminated table, check if that record is not already present in there. If not, then insert it. As additional check you could remove duplicates if it happened by accident. This prevents you from having multiple people eliminating the same person (which sounds impossible).