The crawled pages and the pending crawl pages are saved to mongodb by default.
If you want to change to other databases, you could implement your own class inherited from DbBase(Default I use mongodb, so class MongoDbQueue is my instance.).
After you have your own DbBase class, add it to the variable DB_CLASS_NAME in file project_settings.py, and also you should provide your DB_CONNECTION_STRING and DB_REPOSITORY_NAME to connect to the database.
In current version, only links (<a> tags in html) will be saved to the database. If you want to customized the behavior of the spiders, you can also implement your own HTMLParser class to replace HtmlResolver in my code. Don't forget to replace your own parser name to HTML_RESOLVER_NAME in file project_settings.py
I start to learn python a week ago, maybe made some stupid mistakes and have some bugs. I'll be very appreciate if you can help me to improve my code, thanks.