phpSpider advanced guide: How to implement multi-threaded parallel crawling?

phpSpider advanced guide: How to implement multi-threaded parallel crawling?

Introduction:
In the development of web crawlers, improving crawling efficiency is an important issue. Traditional single-threaded crawling is slow and cannot take full advantage of the multi-core advantages of modern computers. Multi-threaded parallel crawling can significantly improve crawling efficiency. This article will introduce how to use PHP to write multi-threaded parallel crawlers, and attach corresponding code examples.

1. Advantages of multi-threaded parallel crawlers
1.1 Improve crawling speed: Multi-threaded parallel crawlers can process multiple requests at the same time, thereby reducing the response time of requests and improving crawling speed.
1.2 Make full use of computer resources: The computer's multi-core processor can process multiple threads at the same time, and multi-threaded parallel crawlers can make full use of these computing resources to improve crawling efficiency.

2. Methods to implement multi-threaded parallel crawling
2.1 Use thread pool: Create a thread pool that contains multiple threads, each thread is responsible for processing a request. Through the thread pool, multiple threads can be managed and scheduled to avoid frequent creation and destruction of threads and improve efficiency.
2.2 Utilize PHP’s multi-process extension: PHP provides multi-process extension, which can create multiple sub-processes to perform crawling tasks at the same time. Each sub-process is responsible for processing a request, passing data through inter-process communication, and implementing parallel crawling.

3. Use the thread pool to implement multi-threaded parallel crawling
The following is a code example of using the thread pool to implement multi-threaded parallel crawling:

// 引入线程池库
require 'Threadpool.php';

// 创建线程池，参数为最大线程数
$pool = new Threadpool(5);

// 添加任务到线程池
for ($i=0; $i<10; $i++) {
    $url = 'https://www.example.com/page' . $i;
    $pool->addTask(function() use ($url) {
        // 发送HTTP请求并解析响应
        $response = file_get_contents($url);
        // 处理响应数据
        processResponse($response);
    });
}

// 等待所有任务完成
$pool->waitForTasks();

// 停止线程池
$pool->shutdown();

// 处理响应数据的函数
function processResponse($response) {
    // 解析响应数据
    // ...
    // 处理解析结果
    // ...
}

In the above code, use the Threadpool class to create A thread pool and set the maximum number of threads to 5. Then add crawling tasks to the thread pool in a loop. Each task is a closure function responsible for sending HTTP requests and processing responses. Finally, the waitForTasks method is called to wait for all tasks to be completed, and the shutdown method is called to stop the running of the thread pool.

4. Use PHP's multi-process extension to implement multi-threaded parallel crawling
The following is a code example that uses PHP's multi-process extension to implement multi-threaded parallel crawling:

// 创建多个子进程
for ($i=0; $i<10; $i++) {
    $pid = pcntl_fork();
    if ($pid == -1) {
        // 创建子进程失败，报错并退出
        die('fork failed');
    } elseif ($pid == 0) {
        // 子进程代码，负责处理爬取任务
        $url = 'https://www.example.com/page' . $i;
        // 发送HTTP请求并解析响应
        $response = file_get_contents($url);
        // 处理响应数据
        processResponse($response);
        exit(); // 子进程处理完任务后退出
    }
}

// 等待所有子进程退出
while (pcntl_waitpid(0, $status) != -1) {
    $status = pcntl_wexitstatus($status);
    // 可以在这里记录子进程运行结果等信息
}

// 处理响应数据的函数
function processResponse($response) {
    // 解析响应数据
    // ...
    // 处理解析结果
    // ...
}

The above code , use the pcntl_fork function to create multiple child processes, and use the process ID returned by the function to determine whether the current process is a child process or a parent process. The child process is responsible for handling crawling tasks, while the parent process waits for all child processes to exit and processes the running results of the child processes.

Summary:
This article introduces the method of using PHP to implement multi-threaded parallel crawling, and gives corresponding code examples. By using a thread pool or PHP's multi-process extension, you can make full use of the computer's multi-core advantages and improve crawling efficiency. However, it should be noted that when writing a multi-threaded parallel crawler, issues such as thread safety and resource competition must be considered, as well as the number of threads should be reasonably controlled to avoid excessive access pressure on the target website.

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