php 45 lines · 7 steps

A race-free file counter in PHP

Using an exclusive file lock to safely read-modify-write a persistent integer counter.

Explained by highlit

Intermediate file-locking concurrency atomicity resource-management

1<?php
2 
3namespace App\Storage;
4 
5use RuntimeException;
6 
7final class CounterFile
8{
9    public function __construct(private readonly string $path)
10    {
11    }
12 
13    public function increment(int $by = 1): int
14    {
15        $handle = fopen($this->path, 'c+');
16 
17        if ($handle === false) {
18            throw new RuntimeException("Unable to open {$this->path}");
19        }
20 
21        try {
22            if (!flock($handle, LOCK_EX)) {
23                throw new RuntimeException("Could not acquire lock on {$this->path}");
24            }
25 
26            $contents = stream_get_contents($handle);
27            $current = (int) trim($contents);
28            $next = $current + $by;
29 
30            rewind($handle);
31            ftruncate($handle, 0);
32 
33            if (fwrite($handle, (string) $next) === false) {
34                throw new RuntimeException("Failed to write to {$this->path}");
35            }
36 
37            fflush($handle);
38 
39            return $next;
40        } finally {
41            flock($handle, LOCK_UN);
42            fclose($handle);
43        }
44    }
45}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1An exclusive lock turns a read-modify-write sequence into an atomic operation across concurrent processes.
2A finally block guarantees the lock is released and the handle closed even when a write fails midway.
3Truncating and rewinding before writing prevents stale trailing bytes from a shorter new value.

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