How to use PHP bloom filter for URL deduplication and website crawling management

How to use PHP bloom filter for URL deduplication and website crawling management

Overview:
When crawling a website, an important task is to remove duplicate URLs to avoid Crawling the same pages repeatedly wastes resources and time. Bloom filter is an efficient data structure suitable for quickly determining whether an element exists in a large set. This article will introduce how to use PHP Bloom filter for URL deduplication and website crawling management.

1. Install the Bloom filter extension
   First, we need to install the PHP Bloom filter extension. You can install it using PECL with the following command:

   $ pecl install bloom_filter

   After the installation is complete, you need to add the extension to the php.ini file:

   extension=bloom_filter.so

2. Create Bloom Filter Object
   Before using the Bloom filter, we need to create a Bloom filter object. You can use the bloom_filter_new function to create a new Bloom filter:

   $false_positive_rate = 0.01; // 误判率
   $estimated_element_count = 100000; // 预计元素个数
   $filter = bloom_filter_new($false_positive_rate, $estimated_element_count);

3. Add URL to Bloom filter
   When crawling the website, Every time a new URL is obtained, we need to add it to the bloom filter. You can use the bloom_filter_add function to add:

   $url = "http://example.com";
   if (!bloom_filter_add($filter, $url)) {
    // URL已存在，不需要进行爬取
    return;
   }

   Note: When the bloom filter determines that the URL may exist, it is "may exist", so there is still a certain probability of misjudgment. We are Additional judgment needs to be made in the code.

4. Determine whether the URL already exists
   Before adding the URL, we need to determine whether the URL already exists in the Bloom filter to avoid repeated addition. You can use the bloom_filter_contains function to determine:

   $url = "http://example.com";
   if (bloom_filter_contains($filter, $url)) {
    // URL已存在，不需要再次添加
    return;
   }

5. Website crawling management example
   The following is a simple example showing how to use the PHP bloom filter. Website crawling management:

   $false_positive_rate = 0.01; // 误判率
   $estimated_element_count = 100000; // 预计元素个数
   $filter = bloom_filter_new($false_positive_rate, $estimated_element_count);
   
   function crawl_website($url) {
    // 如果URL已存在于布隆过滤器中，则不需要进行爬取
    if (bloom_filter_contains($filter, $url)) {
        return;
    }
    
    // 进行网站爬取操作
    
    // 将URL添加到布隆过滤器中
    bloom_filter_add($filter, $url);
   }

Conclusion:
Use PHP Bloom filter to quickly deduplicate and manage URLs in crawled websites. By adding Bloom filter judgment, you can avoid crawling the same URL repeatedly and improve crawling efficiency. In practical applications, the false positive rate and the expected number of elements can be adjusted according to actual needs to balance the memory footprint and the accuracy of the Bloom filter.

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