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如何在大型 php 应用程序中写入文件（多个问题）

发布于 2024-08-24 04:30:57 字数 176 浏览 6 评论 0原文

在大型 php 应用程序中写入文件的最佳方法是什么？假设每秒需要大量写入。最好的方法是如何解决这个问题。

我可以打开文件并附加数据吗？或者我应该打开、锁定、写入和解锁。

文件将发生什么，需要写入其他数据。此活动会丢失，还是会被保存。如果将其保存，将停止应用程序。

如果您去过，感谢您的阅读！

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夜清冷一曲。 2024-08-31 04:30:57

这里有一个简单的例子，强调了同时写入的危险：

<?php
for($i = 0; $i < 100; $i++) {
 $pid = pcntl_fork();
 //only spawn more children if we're not a child ourselves
 if(!$pid)
  break;
}

$fh = fopen('test.txt', 'a');

//The following is a simple attempt to get multiple threads to start at the same time.
$until = round(ceil(time() / 10.0) * 10);
echo "Sleeping until $until\n";
time_sleep_until($until);

$myPid = posix_getpid();
//create a line starting with pid, followed by 10,000 copies of
//a "random" char based on pid.
$line = $myPid . str_repeat(chr(ord('A')+$myPid%25), 10000) . "\n";
for($i = 0; $i < 1; $i++) {
    fwrite($fh, $line);
}

fclose($fh);

echo "done\n";

如果追加是安全的，您应该得到一个包含 100 行的文件，所有这些行大约有 10,000 个字符长，并且以整数开头。有时，当您运行此脚本时，这正是您将得到的结果。有时，一些附加会发生冲突，并且它会被破坏。

您可以使用 grep '^[^0-9]' test.txt 找到损坏的行，

这是因为文件附加仅在以下情况下是原子的：

您进行一次 fwrite() 调用
，并且 fwrite() 小于 PIPE_BUF （大约 1-4k）
，并且您写入到一个完全符合 POSIX 标准的文件系统

如果您在日志追加过程中对 fwrite 进行了多次调用，或者您写入的内容超过 4k，那么所有的赌注都会失败。

现在，至于这是否重要：您是否同意在重负载下日志中出现一些损坏的行？老实说，大多数时候这是完全可以接受的，并且您可以避免文件锁定的开销。

Here's a simple example that highlights the danger of simultaneous wites:

<?php
for($i = 0; $i < 100; $i++) {
 $pid = pcntl_fork();
 //only spawn more children if we're not a child ourselves
 if(!$pid)
  break;
}

$fh = fopen('test.txt', 'a');

//The following is a simple attempt to get multiple threads to start at the same time.
$until = round(ceil(time() / 10.0) * 10);
echo "Sleeping until $until\n";
time_sleep_until($until);

$myPid = posix_getpid();
//create a line starting with pid, followed by 10,000 copies of
//a "random" char based on pid.
$line = $myPid . str_repeat(chr(ord('A')+$myPid%25), 10000) . "\n";
for($i = 0; $i < 1; $i++) {
    fwrite($fh, $line);
}

fclose($fh);

echo "done\n";

If appends were safe, you should get a file with 100 lines, all of which roughly 10,000 chars long, and beginning with an integer. And sometimes, when you run this script, that's exactly what you'll get. Sometimes, a few appends will conflict, and it'll get mangled, however.

You can find corrupted lines with grep '^[^0-9]' test.txt

This is because file append is only atomic if:

You make a single fwrite() call
and that fwrite() is smaller than PIPE_BUF (somewhere around 1-4k)
and you write to a fully POSIX-compliant filesystem

If you make more than a single call to fwrite during your log append, or you write more than about 4k, all bets are off.

Now, as to whether or not this matters: are you okay with having a few corrupt lines in your log under heavy load? Honestly, most of the time this is perfectly acceptable, and you can avoid the overhead of file locking.

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