mysql insert race condition

Question

How do you stop race conditions in MySQL? the problem at hand is caused by a simple algorithm:

1. select a row from table
2. if it doesn\'t exist, insert it

Answer 1

I ran into the same problem and searched the Net for a moment :)

Finally I came up with solution similar to method to creating filesystem objects in shared (temporary) directories to securely open temporary files:

$exists = $success = false;
do{
 $exists = check();// select a row in the table 
 if (!$exists)
  $success = create_record();
  if ($success){
   $exists = true;
  }else if ($success != ERROR_DUP_ROW){
    log_error("failed to create row not 'coz DUP_ROW!");
    break;
  }else{
    //probably other process has already created the record,
    //so try check again if exists
  }
}while(!$exists)

Don't be afraid of busy-loop - normally it will execute once or twice.