阿帕奇是否为他们提供服务前，读锁文件？ [英] Does Apache read-lock files before serving them?

问题描述

我有一个移动应用，读取存储在Apache服务器上的JSON文件。该JSON文件的内容被再生（使用PHP脚本）如果事情是通过GUI改变。

I have an mobile app that reads a JSON file that is stored on an Apache server. The contents of that JSON file are regenerated (using a PHP script) if something is changed via a GUI.

我很担心，试图覆盖在它被阿帕奇服务的中间JSON文件可能会导致问题。

I am worried that trying to overwrite the JSON file in the middle of it being served by Apache might cause problems.

请问Apache获得任职文件之前读锁？如果不是这样，如果我尝试它在它提供服务的同时？

Does Apache obtain a read lock before serving files? If not, what will happen if I try to write it at the same time it is being served?

推荐答案

没有。在POSIX兼容的系统，所有的锁都咨询反正，所以即使阿帕奇会得到一个读锁，其他进程可以只写文件。

No. On POSIX-compatible systems, all locks are advisory anyways, so even if apache would get a read lock, the other process could just write the file.

您可以确定与 strace的：

[pid  7246] open("/var/www/file.json", O_RDONLY|O_CLOEXEC) = 11
[pid  7246] fcntl(11, F_GETFD)          = 0x1 (flags FD_CLOEXEC)
[pid  7246] mmap(NULL, 20, PROT_READ, MAP_SHARED, 11, 0) = 0x7f53f93da000
[pid  7246] munmap(0x7f53f93da000, 20)  = 0
[pid  7246] writev(10, [{"HTTP/1.1 200 OK\r\nDate: Thu, 26 J"}, ...) = 365
[pid  7246] close(11)                   = 0

因此​​，可能发生的是你的JSON文件仅部分写入。为了避免这个问题，写你的JSON文件在同一文件系统的临时文件，并使用原子改名来覆盖该文件。

这样的话，如果打开成功，Apache将继续为旧文件。如果说之前的改名完成了打开，Apache会得到新的，完成的文件。

That way, if the open has succeeded, apache will continue serving the old file. If the rename finishes before the open, apache will get the new, completed file.

如果你担心的一致性（在断电左右的情况下），你可能还需要调用 FSYNC 在关闭前写的JSON文件的应用程序。

If you worry about consistency (in the case of a power failure or so), you may also want to call fsync in the application that writes the JSON file before closing it.

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