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SafeStream: Atomic File Operations

The Nette\Utils\SafeStram for file manipulation guarantees atomicity and isolation of every file operation.

Why is it actually good? Let's start with a simple example, where we repeatedly write the same string to the file and then read it:

$s = str_repeat('Long String', 10000);

$counter = 1000;
while ($counter--) {
	file_put_contents('soubor', $s);       // write it
	$readed = file_get_contents('soubor'); // read it
	if ($s !== $readed) {                  // check it
		echo 'Strings are different!';

It may seem that the echo 'Strings are different!' command can't ever get executed. The opposite is true. Try to run this script in two browsers simultaneously. The error occurs almost immediately.

It's because the code is not safe when performed repeatedly at the same time (ie, in multiple threads). And that is nothing unusual on the Internet, where several people often connect to one website at the same time. Therefore, it's very important to ensure that your application can handle multiple threads at once – that it's thread-safe, because native PHP functions are not. Otherwise, you can expect data loss and strange errors occurring.

How to ensure, that functions like file_get_contents or fwrite behave atomically? The SafeStream protocol offers a secure solution, so we can atomically manipulate files through standard PHP functions. The protocol is available automatically when you load Nette Framework. After that, you just need prefix the filename with safe://:

$handle = fopen('safe://test.txt', 'x'); // prefix the filename with safe://

fwrite($handle, 'Nette Framework'); // for now, the data is written into a temporary file

fclose($handle); // and only now the file is renamed to test.txt

You can of course use all the familiar function, such as:

file_put_contents('safe://test.txt', $content);

$ini = parse_ini_file('safe://autoload.ini');

SafeStream guarantees:

  • Atomicity: The file is written either as a whole or not written at all.
  • Isolation: No one can start to read a file that is not yet fully written.

If you write to an existing file in the ‚a‘ mode (append), SafeStream creates it's copy and only after successfully writing it renames it to the original name. Write in this mode is therefore more resource-consuming than in other modes.