多次写入文件

Question

I have a web application and when the users clicks save, a write to a file should occur. The application is "a football scheduler", thus more than one players can click on the save button at the very same time.

Initially I thought that this would be something rare, but this is not the case, since when the match in announced players rush into, because the match will be full in a short period of time.

What happens if two players press save at the same time? Only the save of one player will take effect and the other one will be lost (and it is not rare that this happens with the last available position and then there is some conflict).

How can I cope with this phenomenon?

// removed code since it wasn't needed

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I would like to know if this is possible without using a database. So a no answer is also accepted.

Answer 1

I would suppose you would handle this the same way that a database would. Add a lock to the file and wait.

Something like:

while (file_exists('file.lock')) {
  usleep(10000);
}
touch('file.lock');
...
unlink('file.lock');

You may want to add a timeout. I would think this would be safe enough, but I suppose it is still possible for two users to run this at the EXACT same time. In that case, you may want to add a check that their content was successfully saved before continuing.

Update: As Marc B pointed out, file locking appears to be included in php, didn't know about this function until he pointed it out flock() .

$fp = fopen("file", "r+");

// try to acquire an exclusive lock, otherwise sleep 10ms before trying again
while (!flock($fp, LOCK_EX)) {  
    usleep(10000);
}

fwrite($fp, "Write something here\n");
fflush($fp);            // flush output before releasing the lock
flock($fp, LOCK_UN);    // release the lock

fclose($fp)

This would be similar to the function I wrote above. Of course don't pay attention to the fwrite, I just copied the example from php.net to show how flock() works.

Answer 2

It is not possible to write to the same file at the same time.

You could use a database, which can handle more than one access at the same time. In my opinion this is the best solution for handling and accessing a lot of data!

Or you could store the data in seperate files for seperate users, which are all created in a certain folder, and if there are more files available, save them one after another and delete them afterwards. Then keep on writing in the masterfile until no more files are available. This would be a workaround and I really would not recommend it, but anyways it should work if you really don't want to use a database.