Running a Focused Crawl¶
A focused (or topical) crawler crawls the Web in search of pages that belong to a given topic (or domain). To run a focused crawl using ACHE, you need to provide:
- Samples of relevant and irrelevant pages: ACHE analyzes these pages, and learns a classification model that is able to determine the relevance of the pages retrieved during the crawl.
- A list of seed URLs, which will be the starting point for the crawl. The seeds are just HTTP or HTTPS links of pages that are relevant to the topic – they can be the same as relevant pages supplied during model generation. ACHE will start crawling the Web from these links.
- A config file that provides settings/options that are used during the crawl.
In what follows, we provide details on how to run a focused crawl using ACHE.
- Build a Classification Model
Ideally, there should be as many relevant pages as irrelevant ones for training a good classification model. You should try to get as close as possible to a ratio of irrelevant to relevant pages of 1:1. If it’s hard to collect relevant pages, a ratio of 5:1 would suffice. Another good practice is to collect samples of relevant and irrelevant pages for every web site. Collecting only relevant pages from a web site may lead to a model that classifies any page from that web site as relevant, which may not be appropriate.
In order to collect the relevant and irrelevant pages, you can use a web-based system like the Domain Discovery Tool (DDT). DDT provides tools that streamline the process of gathering the training pages and building a model for ACHE. Alternatively, you can build the training set manually and build the model using ACHE’s command line tool
ache buildModel, as we describe below.
You do NOT need to follow these instructions if you’re using the Domain Discovery Tool (DDT). From DDT, you can create and save the model in an appropriate folder. Then, you can provide this folder as an input to ACHE.
As you collect the training data, make sure the HTML files are organized in a directory with relevant pages in a folder named
positiveand the irrelevant ones in a folder named
negative). See the directory config/sample_training_data in ACHE’s repository for an example.
Optionaly, you can provide a file containing stopwords to the model-builder as well. Stopwords are words which contain little or no information about the context of the page. Some examples include: “the”, “at”, and “a”. A sample file is included in the repository as well at config/sample_config/stoplist.txt. If you don’t provide a stopwords file, a default list of common english stopwords bundled into ACHE will be used.
Finally, type in the terminal:
ache buildModel -c <Path of Stoplist> -t <Path of training data folder> -o <Path to save model at>
This command should take a while to execute, depending on how many training samples you collected. In the end, it will print accuracy statistics about the derived model.
- Find Seeds
ACHE needs a list of URLs that will serve as starting points for the crawl. You can use as seeds the list of relevant pages generated in the previous step. A sample seed file can be found at config/sample.seeds. You can use the SeedFinder to help with this step as well. The SeedFinder uses commercial search engines to automatically discover a large set of seed URLs.
- Create Configuration File
To run ACHE, a configuration file is needed that contains the settings and rules the crawler will adhere to. A config file sample is provided in config/config_focused_crawl/ache.yml containing the following important parameters:
link_storage.max_pages_per_domain- The maximum number of pages to download per domain. This is useful to deal with crawler traps and redirects.
link_storage.link_strategy.use_scope- Whether the crawler should crawl the websites provided as seeds only. This needs to be set to
falsein a focused crawl.
link_storage.online_learning.enabled- Whether to enable relearning of the link classifier while the crawler is running.
link_storage.online_learning.type- Type of online learning to use. More details here.
link_storage.scheduler.host_min_access_interval- Time delay between sending subsequent requests to the same host. Should not be too small to avoid overloading web servers.
- Run ACHE
Finally, when you have created the model, the config file ache.yml, and the seeds file, you can run ACHE in the terminal:
ache startCrawl -c <Path of Config Folder> -o <Path of Output Folder> -s <Path of Seeds File> -m <Path of Model Folder>
ACHE will continue crawling until interrupted using
For large crawls, you should consider using
nohupfor running the process in background:
nohup ache startCrawl -c <Path of Config Folder> -o <Path of Output Folder> -s <Path of Seeds File> -m <Path of Model Folder> > crawler-log.txt &
If you are running ACHE from Docker, you should use the following command:
docker run -v $CONFIG:/config -v $DATA:/data -p 8080:8080 vidanyu/ache:latest startCrawl -c /config/ -s /config/seeds.txt -o /data/ -m /config/model/
$DATAis the directory where the crawled data will be stored, and
$CONFIGis the directory where the
seeds.txtfile, and a directory named
model(which contains the page classification model) are located.