运行并发PHP脚本

Question

I'm having the following problem with my VPS server.

I have a long-running PHP script that sends big files to the browser. It does something like this:

<?php
header("Content-type: application/octet-stream");
readfile("really-big-file.zip");
exit();
?>

This basically reads the file from the server's file system and sends it to the browser. I can't just use direct links(and let Apache serve the file) because there is business logic in the application that needs to be applied.

The problem is that while such download is running, the site doesn't respond to other requests.

Answer 1

The problem you are experiencing is related to the fact that you are using sessions. When a script has a running session, it locks the session file to prevent concurrent writes which may corrupt the session data. This means that multiple requests from the same client - using the same session ID - will not be executed concurrently, they will be queued and can only execute one at a time.

Multiple users will not experience this issue, as they will use different session IDs. This does not mean that you don't have a problem, because you may conceivably want to access the site whilst a file is downloading, or set multiple files downloading at once.

The solution is actually very simple: call session_write_close() before you start to output the file. This will close the session file, release the lock and allow further concurrent requests to execute.