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Frontera API

This section documents the Frontera core API, and is intended for developers of middlewares and backends.

Frontera API / Manager

The main entry point to Frontera API is the :class:`FrontierManager <frontera.core.manager.FrontierManager>` object, passed to middlewares and backend through the from_manager class method. This object provides access to all Frontera core components, and is the only way for middlewares and backend to access them and hook their functionality into Frontera.

The :class:`FrontierManager <frontera.core.manager.FrontierManager>` is responsible for loading the installed middlewares and backend, as well as for managing the data flow around the whole frontier.

Loading from settings

Although :class:`FrontierManager <frontera.core.manager.FrontierManager>` can be initialized using parameters the most common way of doing this is using :doc:`Frontera Settings <frontera-settings>`.

This can be done through the :attr:`from_settings <frontera.core.manager.FrontierManager.from_settings>` class method, using either a string path:

>>> from frontera import FrontierManager
>>> frontier = FrontierManager.from_settings('')

or a :class:`BaseSettings <frontera.settings.BaseSettings>` object instance:

>>> from frontera import FrontierManager, Settings
>>> settings = Settings()
>>> settings.MAX_PAGES = 0
>>> frontier = FrontierManager.from_settings(settings)

It can also be initialized without parameters, in this case the frontier will use the :ref:`default settings <frontier-default-settings>`:

>>> from frontera import FrontierManager, Settings
>>> frontier = FrontierManager.from_settings()

Frontier Manager

.. autoclass:: frontera.core.manager.FrontierManager


    .. autoattribute:: frontera.core.manager.FrontierManager.request_model
    .. autoattribute:: frontera.core.manager.FrontierManager.response_model
    .. autoattribute:: frontera.core.manager.FrontierManager.backend
    .. autoattribute:: frontera.core.manager.FrontierManager.middlewares
    .. autoattribute:: frontera.core.manager.FrontierManager.test_mode
    .. autoattribute:: frontera.core.manager.FrontierManager.max_requests
    .. autoattribute:: frontera.core.manager.FrontierManager.max_next_requests
    .. autoattribute:: frontera.core.manager.FrontierManager.auto_start
    .. autoattribute:: frontera.core.manager.FrontierManager.settings
    .. autoattribute:: frontera.core.manager.FrontierManager.iteration
    .. autoattribute:: frontera.core.manager.FrontierManager.n_requests
    .. autoattribute:: frontera.core.manager.FrontierManager.finished

    **API Methods**

    .. automethod:: frontera.core.manager.FrontierManager.start
    .. automethod:: frontera.core.manager.FrontierManager.stop
    .. automethod:: frontera.core.manager.FrontierManager.add_seeds
    .. automethod:: frontera.core.manager.FrontierManager.get_next_requests
    .. automethod:: frontera.core.manager.FrontierManager.page_crawled
    .. automethod:: frontera.core.manager.FrontierManager.request_error

    **Class Methods**

    .. automethod:: frontera.core.manager.FrontierManager.from_settings

Starting/Stopping the frontier

Sometimes, frontier components need to perform initialization and finalization operations. The frontier mechanism to notify the different components of the frontier start and stop is done by the :attr:`start() <frontera.core.manager.FrontierManager.start>` and :attr:`stop() <frontera.core.manager.FrontierManager.stop>` methods respectively.

By default :attr:`auto_start <frontera.core.manager.FrontierManager.auto_start>` frontier value is activated, this means that components will be notified once the :class:`FrontierManager <frontera.core.manager.FrontierManager>` object is created. If you need to have more fine control of when different components are initialized, deactivate :attr:`auto_start <frontera.core.manager.FrontierManager.auto_start>` and manually call frontier API :attr:`start() <frontera.core.manager.FrontierManager.start>` and :attr:`stop() <frontera.core.manager.FrontierManager.stop>` methods.


Frontier :attr:`stop() <frontera.core.manager.FrontierManager.stop>` method is not automatically called when :attr:`auto_start <frontera.core.manager.FrontierManager.auto_start>` is active (because frontier is not aware of the crawling state). If you need to notify components of frontier end you should call the method manually.

Frontier iterations

Once frontier is running, the usual process is the one described in the :ref:`data flow <frontier-data-flow>` section.

Crawler asks the frontier for next pages using the :attr:`get_next_requests() <frontera.core.manager.FrontierManager.get_next_requests>` method. Each time the frontier returns a non empty list of pages (data available), is what we call a frontier iteration.

Current frontier iteration can be accessed using the :attr:`iteration <frontera.core.manager.FrontierManager.iteration>` attribute.

Finishing the frontier

Crawl can be finished either by the Crawler or by the Frontera. Frontera will finish when a maximum number of pages is returned. This limit is controlled by the :attr:`max_requests <frontera.core.manager.FrontierManager.max_requests>` attribute (:setting:`MAX_REQUESTS` setting).

If :attr:`max_requests <frontera.core.manager.FrontierManager.max_requests>` has a value of 0 (default value) the frontier will continue indefinitely.

Once the frontier is finished, no more pages will be returned by the :attr:`get_next_requests <frontera.core.manager.FrontierManager.get_next_requests>` method and :attr:`finished <frontera.core.manager.FrontierManager.finished>` attribute will be True.

Component objects

.. autoclass:: frontera.core.components.Component


    .. autoattribute::

    **Abstract methods**

    .. automethod:: frontera.core.components.Component.frontier_start
    .. automethod:: frontera.core.components.Component.frontier_stop
    .. automethod:: frontera.core.components.Component.add_seeds
    .. automethod:: frontera.core.components.Component.page_crawled
    .. automethod:: frontera.core.components.Component.request_error

    **Class Methods**

    .. automethod:: frontera.core.components.Component.from_manager

Test mode

In some cases while testing, frontier components need to act in a different way than they usually do (for instance :ref:`domain middleware <frontier-domain-middleware>` accepts non valid URLs like 'A1' or 'B1' when parsing domain urls in test mode).

Components can know if the frontier is in test mode via the boolean :attr:`test_mode <frontera.core.manager.FrontierManager.test_mode>` attribute.

Other ways of using the frontier

Communication with the frontier can also be done through other mechanisms such as an HTTP API or a queue system. These functionalities are not available for the time being, but hopefully will be included in future versions.