Crawler

A high performance web crawler / scraper in Elixir, with worker pooling and rate limiting via OPQ.

Features

• Crawl assets (javascript, css and images).
• Save to disk.
• Hook for scraping content.
• Restrict crawlable domains, paths or content types.
• Limit concurrent crawlers.
• Limit rate of crawling.
• Set the maximum crawl depth.
• Set timeouts.
• Set retries strategy.
• Set crawler's user agent.
• Manually pause/resume/stop the crawler.

See Hex documentation.

Architecture

Below is a very high level architecture diagram demonstrating how Crawler works.

Usage

Crawler.crawl("http://elixir-lang.org", max_depths: 2)

There are several ways to access the crawled page data:

1. Use Crawler.Store
2. Tap into the registry(?) Crawler.Store.DB
3. Use your own scraper
4. If the :save_to option is set, pages will be saved to disk in addition to the above mentioned places
5. Provide your own custom parser and manage how data is stored and accessed yourself

Configurations

Option	Type	Default Value	Description
:assets	list	[]	Whether to fetch any asset files, available options: "css", "js", "images".
:save_to	string	nil	When provided, the path for saving crawled pages.
:workers	integer	10	Maximum number of concurrent workers for crawling.
:interval	integer	0	Rate limit control - number of milliseconds before crawling more pages, defaults to 0 which is effectively no rate limit.
:max_depths	integer	3	Maximum nested depth of pages to crawl.
:max_pages	integer	:infinity	Maximum amount of pages to crawl.
:timeout	integer	5000	Timeout value for fetching a page, in ms. Can also be set to :infinity, useful when combined with Crawler.pause/1.
:retries	integer	2	Number of times to retry a fetch.
:store	module	nil	Module for storing the crawled page data and crawling metadata. You can set it to Crawler.Store or use your own module, see Crawler.Store.add_page_data/3 for implementation details.
:force	boolean	false	Force crawling URLs even if they have already been crawled, useful if you want to refresh the crawled data.
:scope	term	nil	Similar to :force, but you can pass a custom :scope to determine how Crawler should perform on links already seen.
:user_agent	string	Crawler/x.x.x (...)	User-Agent value sent by the fetch requests.
:url_filter	module	Crawler.Fetcher.UrlFilter	Custom URL filter, useful for restricting crawlable domains, paths or content types.
:retrier	module	Crawler.Fetcher.Retrier	Custom fetch retrier, useful for retrying failed crawls, nullifies the :retries option.
:modifier	module	Crawler.Fetcher.Modifier	Custom modifier, useful for adding custom request headers or options.
:scraper	module	Crawler.Scraper	Custom scraper, useful for scraping content as soon as the parser parses it.
:parser	module	Crawler.Parser	Custom parser, useful for handling parsing differently or to add extra functionalities.
:encode_uri	boolean	false	When set to true apply the URI.encode to the URL to be crawled.
:queue	pid	nil	You can pass in an OPQ pid so that multiple crawlers can share the same queue.

Custom Modules

It is possible to swap in your custom logic as shown in the configurations section. Your custom modules need to conform to their respective behaviours:

Retrier

See Crawler.Fetcher.Retrier.

Crawler uses ElixirRetry's exponential backoff strategy by default.

defmodule CustomRetrier do
  @behaviour Crawler.Fetcher.Retrier.Spec
end

URL Filter

See Crawler.Fetcher.UrlFilter.

defmodule CustomUrlFilter do
  @behaviour Crawler.Fetcher.UrlFilter.Spec
end

Scraper

See Crawler.Scraper.

defmodule CustomScraper do
  @behaviour Crawler.Scraper.Spec
end

Parser

See Crawler.Parser.

defmodule CustomParser do
  @behaviour Crawler.Parser.Spec
end

Modifier

See Crawler.Fetcher.Modifier.

defmodule CustomModifier do
  @behaviour Crawler.Fetcher.Modifier.Spec
end

Pause / Resume / Stop Crawler

Crawler provides pause/1, resume/1 and stop/1, see below.

{:ok, opts} = Crawler.crawl("https://elixir-lang.org")

Crawler.running?(opts) # => true

Crawler.pause(opts)

Crawler.running?(opts) # => false

Crawler.resume(opts)

Crawler.running?(opts) # => true

Crawler.stop(opts)

Crawler.running?(opts) # => false

Please note that when pausing Crawler, you would need to set a large enough :timeout (or even set it to :infinity) otherwise parser would timeout due to unprocessed links.

Multiple Crawlers

It is possible to start multiple crawlers sharing the same queue.

{:ok, queue} = OPQ.init(worker: Crawler.Dispatcher.Worker, workers: 2)

Crawler.crawl("https://elixir-lang.org", queue: queue)
Crawler.crawl("https://github.com", queue: queue)

Find All Scraped URLs

Crawler.Store.all_urls() # => ["https://elixir-lang.org", "https://google.com", ...]

Examples

Google Search + Github

This example performs a Google search, then scrapes the results to find Github projects and output their name and description.

See the source code.

You can run the example by cloning the repo and run the command:

mix run -e "Crawler.Example.GoogleSearch.run()"

API Reference

Please see https://hexdocs.pm/crawler.

Changelog

Please see CHANGELOG.md.

Copyright and License

Copyright (c) 2016 Fred Wu

This work is free. You can redistribute it and/or modify it under the terms of the MIT License.