Certificated Actions - One Click Only

01/14/2009 Updated CanProcessAction - left out where it creates an ID!

Here's something that comes up periodically, "how do you prevent the dreaded double-click?" From something as common as a banking or purchasing transaction getting run twice or more because of overzealous clicking, to batch processing for the night - to pushing items through workflow. How do you prevent it?

A typical strategy is to disable the button in javascript. This works, somewhat, but doesn't prevent someone from hitting refresh once they get to the next page.

My strategy, certificated submits:

A Table (in MySQL here):

 
CREATE TABLE CLICK_CERTIFICATES (
  certificate_id varchar(40) not null,
  primary key (certificate_id)
)

A way to create a GUID or unqiue id:

 
function CreateGUID(){
 srand((double)microtime()*1000000);
 $r = rand ;
 $u = uniqid($r);
 $m = md5 ($u);
 return($m);
}

A function that tells us if we can process the action:

  
function CanProcessAction($certificate_id){
  $certificate_id = addslashes($certificate_id);
if(trim($certificate_id)!=""){
  $fetch = mysql_query("select count(*) from CLICK_CERTIFICATES where certificate_id='$certificate_id'");
  $row = mysql_fetch_row($fetch);
  $exists =  ($row[0]!=0);
} else { $exists = false; $certificate_id=CreateGUID();}
  if(!exists){
    mysql_unbuffered_query("insert into CLICK_CERTIFICATES (certificate_id) values ('$certificate_id')");
  }
  return !$exists;
}

Now, just use a hidden variable on your form that creates the value of the certificate:
```
 
<input type="hidden" name="certificate_id" value="<? echo CreateGUID(); ?>">
```

And then just wrap your action on the receiving page like so:

  
$certificate_id = $_POST["certificate_id"];
if (CanProcessAction($certificate_id)){
   //do something only this once, even if they clicked twice, or refreshed the page.
}

You can also put other things in the table to "re-allow" an attempt after a certain amount of time, or log the extra clicks so you can nag your users. By doing the time control, you can prevent users from re-running resource intensive reports for a given interval, and caching the results.

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