View Source Crawler

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


  • 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.


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


Crawler.crawl("", 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


OptionTypeDefault ValueDescription
:assetslist[]Whether to fetch any asset files, available options: "css", "js", "images".
:save_tostringnilWhen provided, the path for saving crawled pages.
:workersinteger10Maximum number of concurrent workers for crawling.
:intervalinteger0Rate limit control - number of milliseconds before crawling more pages, defaults to 0 which is effectively no rate limit.
:max_depthsinteger3Maximum nested depth of pages to crawl.
:max_pagesinteger:infinityMaximum amount of pages to crawl.
:timeoutinteger5000Timeout value for fetching a page, in ms. Can also be set to :infinity, useful when combined with Crawler.pause/1.
:storemodulenilModule for storing the crawled page data and crawling metadata, defaults to nil. You can also set it to Crawler.Store or your own, see Crawler.Store.add_page_data/3 for implementation details.
:forcebooleanfalseForce crawling URLs even if they have already been crawled, useful if you want to refresh the crawled data.
:scopetermnilSimilar to :force, but you can pass a custom :scope to determine how Crawler should perform on links already seen.
:user_agentstringCrawler/x.x.x (...)User-Agent value sent by the fetch requests.
:url_filtermoduleCrawler.Fetcher.UrlFilterCustom URL filter, useful for restricting crawlable domains, paths or content types.
:retriermoduleCrawler.Fetcher.RetrierCustom fetch retrier, useful for retrying failed crawls.
:modifiermoduleCrawler.Fetcher.ModifierCustom modifier, useful for adding custom request headers or options.
:scrapermoduleCrawler.ScraperCustom scraper, useful for scraping content as soon as the parser parses it.
:parsermoduleCrawler.ParserCustom parser, useful for handling parsing differently or to add extra functionalities.
:encode_uribooleanfalseWhen set to true apply the URI.encode to the URL to be crawled.
:queuepidnilYou 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:


See Crawler.Fetcher.Retrier.

Crawler uses ElixirRetry's exponential backoff strategy by default.

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

URL Filter

See Crawler.Fetcher.UrlFilter.

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


See Crawler.Scraper.

defmodule CustomScraper do
  @behaviour Crawler.Scraper.Spec


See Crawler.Parser.

defmodule CustomParser do
  @behaviour Crawler.Parser.Spec


See Crawler.Fetcher.Modifier.

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

Pause / Resume / Stop Crawler

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

{:ok, opts} = Crawler.crawl("")

Crawler.running?(opts) # => true


Crawler.running?(opts) # => false


Crawler.running?(opts) # => true


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("", queue: queue)
Crawler.crawl("", queue: queue)

Find All Scraped URLs

Crawler.Store.all_urls() # => ["", "", ...]


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 ""

API Reference

Please see


Please see

Copyright (c) 2016 Fred Wu

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