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link-filters.rst

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Link Filters

ACHE allows one to customize which domains and paths within a domain should be crawled. This can be done by configuring link filters using regular expressions (regex) or wildcard patterns. Regex filters are evaluated using Java's regular expression rules, and wildcard filters accept only the special character *, which matches any character.

Link Filters are composed of two lists of patterns:

  • whitelists - patterns for URLs that are allowed to be followed, i.e., any URL that doesn't match the patterns is discarded.
  • blacklists - patterns for URLs that are NOT allowed to be followed, i.e., any URL that matches the patterns is discarded.

Links filters can have global or per-domain scope. Global filters are evaluated against all links, whereas per-domain filters are evaluated only against URLs that belong to the specified domain (only top-private domain level). There are two ways to configure link filters:

  • .yml file <link_filters_yml>: Allows to configure global and per-domain link filters using YAML.
  • .txt files <link_filters_txt>: Allows to configure only regex-based global link filters.

Configuring using YAML

ACHE automatically searches for a file named link_filters.yml in the same directory of the ache.yml file. This file can contain a single global entry and one entry per domain. Each entry should specify a type (regex or wildcard) and a list of "whitelist" and "blacklist" patterns, as shown in the example bellow:

global:
  type: wildcard
  whitelist:
    - "http://*allowed*"
  blacklist:
    - "*bad-bad-pattern*"
www.example1.com:
  type: wildcard
  blacklist:
    - http://www.example1.com/*disallowed*.html
  whitelist:
    - http://www.example1.com/*allowed*
www.example2.com:
  type: regex
  blacklist:
    - http:\/\/www\.example2\.com\/disallowed[0-9]+\.html
  whitelist:
    - http:\/\/www\.example2\.com\/allowed[0-9]+\.html

Configuring using .txt files

This is the old way to configure link filters. Only regex-based "global" filters can be configured, i.e., filters that are applied to all URLs. To configure a link filter, you will need to create text files containing one regular expression per line. All regular expressions loaded are evaluated against all links found on the web pages crawled in other to determine whether the crawler should accept or reject them. For whitelist filters, ACHE will automatically search for a file named link_whitelist.txt, whereas for blacklist filters the file name is link_blacklist.txt. These files should be placed under the same directory as the ache.yml. These files are loaded once during crawler start-up. The link filter files should look like this:

https?:\/\/www\.example\.com\/some_path\/.*
https?:\/\/www\.another-example\.com\/some_path\/.*