How to implement asynchronous multi-threaded crawler with PHP

In the implementation of web crawlers, asynchronous multi-threading can greatly improve the efficiency of crawling. As a mainstream programming language, PHP can also implement asynchronous multi-threaded crawlers through concurrent programming. This article will introduce the specific implementation method.

1. Overview of asynchronous multi-threaded crawlers

Asynchronous multi-threaded crawlers mainly rely on two technologies: asynchronous IO and multi-threaded processing. In traditional synchronous IO, the thread will wait for the IO operation to complete before proceeding to the next step. In asynchronous IO, threads can perform other operations while waiting for IO operations, thereby improving program running efficiency. Multi-threading can perform multiple tasks at the same time, speeding up task processing.

2. Asynchronous multi-threading implementation principle

Implementing asynchronous multi-threading in PHP mainly relies on two extensions: pthread and cURL. The pthread extension is a multi-thread extension based on the POSIX thread standard, which can enable multi-threading functionality in PHP. cURL is the network library used in PHP, which can realize the transmission of network data through cURL.

The main process of implementing an asynchronous multi-threaded crawler is as follows:

1. Create a main thread and multiple sub-threads, and the sub-threads can be created and destroyed as needed.
2. When the main thread starts, it takes out a pending task from the task queue and assigns the task to a child thread for processing.
3. When the child thread starts, initiate a network request through cURL to obtain the required data.
4. While waiting for the network response, the sub-thread can perform other tasks to speed up the crawler operation efficiency.
5. After the sub-thread request is completed, the crawled data will be sent to the main thread, and the main thread will store the results in the specified storage location.
6. If there are still tasks to be processed in the task queue, repeat the above steps.

3. Implementation steps

1. Install the pthread extension

In Linux, you can use the following command to install the pthread extension:

sudo pecl install pthreads

In Windows, you can obtain the pthread extension DLL file from the PHP official website for installation.

2. Create the main thread and sub-threads

The creation of the main thread and sub-threads can be achieved through the Thread class in PHP.

class SpiderThread extends Thread {

private $url;

public function __construct($url) {
    $this->url = $url;
}

public function run() {
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $this->url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    $result = curl_exec($ch);
    curl_close($ch);
    $this->synchronized(function($thread){
        $thread->notify();
    }, $this);
    return $result;
}

}

The main thread can be synchronized through the Mutex class extended by pthreads.

$mutex = new Mutex();
$threads = array();
foreach($urls as $url) {

$mutex->lock();
$threads[] = new SpiderThread($url);
end($threads)->start();
$mutex->unlock();
$mutex->synchronized(function($mutex){
    $mutex->wait();
}, $mutex);

}
foreach($threads as $thread) {

$result = $thread->join();
//处理爬取结果

}

In the above code, $urls is an array that stores links to be crawled. The main thread creates The child thread performs task processing, and the result returned by the child thread is stored in $result.

3. Implementing thread pool

In order to improve program running efficiency, we can use thread pool technology to manage the creation and destruction of child threads. A certain number of child threads are maintained in the thread pool. When the main thread submits a task to the thread pool, the thread pool will select one of the idle threads for task processing based on the real-time status of the thread.

The following is a simple thread pool implementation example:

class ThreadPool {

private $pool;
private $tasks;

public function __construct($size) {
    $this->pool = new SplQueue();
    for($i = 0; $i < $size; $i++) {
        $this->pool->enqueue(new SpiderThread());
    }
    $this->tasks = new SplQueue();
}

public function execute($task) {
    if($this->pool->isEmpty()) {
        $this->tasks->enqueue($task);
    } else {
        $thread = $this->pool->dequeue();
        $thread->execute($task);
    }
}

public function addThread($thread) {
    $this->pool->enqueue($thread);
}

public function addTask($task) {
    $this->tasks->enqueue($task);
    $this->checkTask();
}

public function checkTask() {
    if(!$this->tasks->isEmpty() && !$this->pool->isEmpty()) {
        $thread = $this->pool->dequeue();
        $task = $this->tasks->dequeue();
        $thread->execute($task);
    }
}

}

IV. Summary

This article introduces the basic method of implementing asynchronous multi-threaded crawlers in PHP. Multi-threading and network data transmission are realized through pthread and cURL, which can greatly improve the operating efficiency of the crawler. In practical applications, program running efficiency can be further improved by using thread pool technology.

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