Crawling Strategies

ACHE has several configuration options to control the crawling strategy, i.e., which links the crawler should follow and priority of each link.


Scope refers to the ability of the crawler only follow links that point to the same “host”. If the crawler is configured to use the “seed scope”, it will only follow links that belong to the same host of the URLs included in the seeds file. You can enable scope adding the following line to ache.yml:

link_storage.link_strategy.use_scope: true

For example, if the scope is enabled and the seed file contains the following URLs:

then the crawler will only follow links within the domains and Links to any other domains will be ignored.

Hard-focus vs. Soft-focus

The focus mode (hard vs. soft) is another way to prune the search space, i.e, discard links that will not lead to relevant pages. Relevant pages tend to cluster in connected components, therefore the crawler can ignore all links from irrelevant pages to reduce the amount of links that should be considered for crawling. In “hard-focus mode”, the crawler will ignore all links from irrelevant pages. In “soft-focus mode”, the crawler will not ignore links from irrelevant pages, and will rely solely on the link classifier to define which links should be followed and their priority. The hard-focus mode can be enabled (or disabled) using the following setting in ache.yml:

target_storage.hard_focus: true

When the hard focus mode is disabled, the number of discovered links will grow quickly, so the use of a link classifier (described bellow) is highly recommended to define the priority that links should be crawled.

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