How can I scrape, parse and crawl files in Ruby?

Question

I have a number of data files to process from a data warehouse that have the following format:

:header 1 ...
:header n
# remarks 1 ...
# remarks n
# column header 1
# column header 2
DATA ROWS
(Example: "#### ## ## #####   ########  ####### ###afp##      ##e###")

The data is separated by white spaces and has both numbers and other ASCII chars. Some of those pieces of data will be split up and made more meaningful.

All of the data will go into a database, initially an SQLite db for development, and then pushed up to another, more permanent, storage.

These files will be pulled in actually via HTTP from the remote server and I will have to crawl a bit to get some of it as they span folders and many files.

I was hopeful to get some input what the best tools and methods may be to accomplish this the "Ruby way", as well as to abstract out some of this. Otherwise, I'll tackle it probably similar to how I would in Perl or other such approaches I've taken before.

I was thinking along the lines of using OpenURI to open each url, then if input is HTML collect links to crawl, otherwise process the data. I would use String.scan to break apart the file appropriately each time into a multi-dimensional array parsing each component based on the established formatting by the data provider. Upon completion, push the data into the database. Move on to next input file/uri. Rinse and repeat.

I figure I must be missing some libs that those with more experience would use to clean/quicken this process up dramatically and make the script much more flexible for reuse on other data sets.

Additionally, I will be graphing and visualizing this data as well as generating reports, so perhaps that should too be considered.

Any input as to perhaps a better approach or libs to simply this?

Answer 1

Your question focuses on a lot on "low level" details -- parsing URL's and so on. One key aspect of the "Ruby Way" is "Don't reinvent the wheel." Leverage existing libraries. :)

My recommendation? First, leverage a crawler such as spider or anemone. Second, use Nokogiri for HTML/XML parsing. Third, store the results. I recommend this because you might do different analyses later and you don't want to throw away the hard work of your spidering.

Without knowing too much about your constraints, I would look at storing your results in MongoDB. After thinking this, I did a quick search and found a nice tutorial Scraping a blog with Anemone and MongoDB.

Answer 2

I've written probably a bajillion spiders and site analyzers and find that Ruby has some nice tools that should make this an easy process.

OpenURI makes it easy to retrieve pages.

URI.extract makes it easy to find links in pages. From the docs:

Description

Extracts URIs from a string. If block given, iterates through all matched URIs. Returns nil if block given or array with matches.

  require "uri"

  URI.extract("text here http://foo.example.org/bla and here mailto:test@example.com and here also.")
  # => ["http://foo.example.com/bla", "mailto:test@example.com"]

Simple, untested, logic to start might look like:

require "openuri"
require "uri"

urls_to_scan = %w[
  http://www.example.com/page1
  http://www.example.com/page2
]

loop do
  break if urls_to_scan.empty?
  url = urls_to_scan.shift
  html = open(url).read

  # you probably want to do something to make sure the URLs are not
  # pointing outside the site you're walking.
  #
  # Something like:
  # 
  #     URI.extract(html).select{ |u| u[%r{^http://www\.example\.com}i] }
  #
  new_urls = URI.extract(html)

  if (new_urls.any?)
    urls_to_scan += new_urls
  else
    ; # parse your file as data using the content in html
  end
end

Unless you own the site you're crawling, you want to be kind and gentle: Don't run as fast as possible because it's not your pipe. Pay attention to the site's robot.txt file or risk being banned.

There are true web-crawler gems for Ruby, but the basic task is so simple I never bother with them. If you want to check out other alternatives, visit some of the links to the right for other questions on SO that touch on this subject.

If you need more power or flexibility, the Nokogiri gem makes short work of parsing HTML, allowing you to use CSS accessors to search for tags of interest. There are some pretty powerful gems for making it easy to grab pages such as typhoeus.

Finally, while ActiveRecord, which is recommended in some comments, is nice, finding documentation for using it outside of Rails can be difficult or confusing. I recommend using Sequel. It is a great ORM, very flexible, and well documented.

Answer 3

Hi I would start by taking a very close look at the gem called Mechanize before firing up any basic open-uri stuff - cause it's build into mechanize. It's a brilliant, fast, and easy to use gem for automating web-crawling. Since your data-format is pretty strange (at least compared to json, xml or html) I don't think you will make any use of the build-in parser - but you could still take a look at it. it's called nokogiri and is extremely smart as well. But in the last end, after crawling and fetching the resources, you will probably have to go with some good old regular expression stuff.

Good luck!