Using parallelisation to scrape web pages with R

Question

I am trying to scrape a large amount of web pages to later analyse them. Since the number of URLs is huge, I had decided to use the parallel package along with XML .

Specifically, I am using the htmlParse() function from XML , which works fine when used with sapply , but generates empty objects of class HTMLInternalDocument when used with parSapply .

url1<- "http://forums.philosophyforums.com/threads/senses-of-truth-63636.html"
url2<- "http://forums.philosophyforums.com/threads/the-limits-of-my-language-impossibly-mean-the-limits-of-my-world-62183.html"
url3<- "http://forums.philosophyforums.com/threads/how-language-models-reality-63487.html"

myFunction<- function(x){
cl<- makeCluster(getOption("cl.cores",detectCores()))
ok<- parSapply(cl=cl,X=x,FUN=htmlParse)
return(ok)
}

urls<- c(url1,url2,url3)

#Works
output1<- sapply(urls,function(x)htmlParse(x))
str(output1[[1]])
> Classes 'HTMLInternalDocument', 'HTMLInternalDocument', 'XMLInternalDocument', 'XMLAbstractDocument', 'oldClass' <externalptr>
output1[[1]]


#Doesn't work
myFunction<- function(x){
cl<- makeCluster(getOption("cl.cores",detectCores()))
ok<- parSapply(cl=cl,X=x,FUN=htmlParse)
stopCluster(cl)
return(ok)
}

output2<- myFunction(urls)
str(output2[[1]])
> Classes 'HTMLInternalDocument', 'HTMLInternalDocument', 'XMLInternalDocument', 'XMLAbstractDocument', 'oldClass' <externalptr>
output2[[1]]
#empty

Thanks.

Answer 1

You can use getURIAsynchronous from Rcurl package that allows the caller to specify multiple URIs to download at the same time.

library(RCurl)
library(XML)
get.asynch <- function(urls){
  txt <- getURIAsynchronous(urls)
  ## this part can be easily parallelized 
  ## I am juste using lapply here as first attempt
  res <- lapply(txt,function(x){
    doc <- htmlParse(x,asText=TRUE)
    xpathSApply(doc,"/html/body/h2[2]",xmlValue)
  })
}

get.synch <- function(urls){
  lapply(urls,function(x){
    doc <- htmlParse(x)
    res2 <- xpathSApply(doc,"/html/body/h2[2]",xmlValue)
    res2
  })}

Here some benchmarking for 100 urls you divide the parsing time by a factor of 2.

library(microbenchmark)
uris = c("http://www.omegahat.org/RCurl/index.html")
urls <- replicate(100,uris)
microbenchmark(get.asynch(urls),get.synch(urls),times=1)

Unit: seconds
             expr      min       lq   median       uq      max neval
 get.asynch(urls) 22.53783 22.53783 22.53783 22.53783 22.53783     1
  get.synch(urls) 39.50615 39.50615 39.50615 39.50615 39.50615     1