使用 Apache Spark 进行分布式 Web 爬网 - 有可能吗? [英] Distributed Web crawling using Apache Spark - Is it Possible?

问题描述

当我参加一次关于网络挖掘的采访时，我问了一个有趣的问题.问题是，是否可以使用 Apache Spark 抓取网站?

An interesting question asked of me when I attended one interview regarding web mining. The question was, is it possible to crawl the Websites using Apache Spark?

我猜是可以的，因为它支持Spark的分布式处理能力.面试后我搜索了这个，但找不到任何有趣的答案.Spark 可以做到吗?

I guessed that it was possible, because it supports distributed processing capacity of Spark. After the interview I searched for this, but couldn't find any interesting answer. Is that possible with Spark?

推荐答案

这种方式怎么样:

您的应用程序将获取一组网站 URL 作为您的抓取工具的输入，如果您只是实现一个普通应用程序，您可以按如下方式执行:

Your application would get a set of websites URLs as input for your crawler, if you are implementing just a normal app, you might do it as follows:

1. 将所有要爬取的网页拆分成一个单独的站点列表，每个站点都足够小，可以很好地放在一个线程中:例如:你要爬取www.example.com/news从20150301到20150401，拆分结果可以是:[www.example.com/news/20150301, www.example.com/news/20150302, ..., www.example.com/news/20150401]
2. 将每个基本 url(www.example.com/news/20150401) 分配给单个线程，它在真正发生数据获取的线程中
3. 将每个线程的结果保存到文件系统中.

1. split all the web pages to be crawled into a list of separate site, each site is small enough to fit in a single thread well: for example: you have to crawl www.example.com/news from 20150301 to 20150401, split results can be: [www.example.com/news/20150301, www.example.com/news/20150302, ..., www.example.com/news/20150401]
2. assign each base url(www.example.com/news/20150401) to a single thread, it is in the threads where the really data fetch happens
3. save the result of each thread into FileSystem.

当应用程序变成一个spark的时候，同样的过程发生，但是封装在Spark的概念中:我们可以自定义一个CrawlRDD做同样的工作:

When the application become a spark one, same procedure happens but encapsulate in Spark notion: we can customize a CrawlRDD do the same staff:

1. 拆分站点:def getPartitions: Array[Partition] 是执行拆分任务的好地方.
2. 抓取每个拆分的线程:def compute(part: Partition, context: TaskContext): Iterator[X] 将传播到应用程序的所有执行程序，并行运行.
3. 将 rdd 保存到 HDFS.

1. Split sites: def getPartitions: Array[Partition] is a good place to do the split task.
2. Threads to crawl each split: def compute(part: Partition, context: TaskContext): Iterator[X] will be spread to all the executors of your application, run in parallel.
3. save the rdd into HDFS.

最终的程序如下:

class CrawlPartition(rddId: Int, idx: Int, val baseURL: String) extends Partition {}

class CrawlRDD(baseURL: String, sc: SparkContext) extends RDD[X](sc, Nil) {

  override protected def getPartitions: Array[CrawlPartition] = {
    val partitions = new ArrayBuffer[CrawlPartition]
    //split baseURL to subsets and populate the partitions
    partitions.toArray
  }

  override def compute(part: Partition, context: TaskContext): Iterator[X] = {
    val p = part.asInstanceOf[CrawlPartition]
    val baseUrl = p.baseURL

    new Iterator[X] {
       var nextURL = _
       override def hasNext: Boolean = {
         //logic to find next url if has one, fill in nextURL and return true
         // else false
       }          

       override def next(): X = {
         //logic to crawl the web page nextURL and return the content in X
       }
    } 
  }
}

object Crawl {
  def main(args: Array[String]) {
    val sparkConf = new SparkConf().setAppName("Crawler")
    val sc = new SparkContext(sparkConf)
    val crdd = new CrawlRDD("baseURL", sc)
    crdd.saveAsTextFile("hdfs://path_here")
    sc.stop()
  }
}

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