.. _topics-practices:

================
Common Practices
================

This section documents common practices when using Scrapy. These are things
that cover many topics and don't often fall into any other specific section.

.. _run-from-script:

Run Scrapy from a script
========================

You can use the :ref:`API <topics-api>` to run Scrapy from a script, instead of
the typical way of running Scrapy via ``scrapy crawl``.

Remember that Scrapy is built on top of the Twisted
asynchronous networking library, so you need to run it inside the Twisted reactor.

The first utility you can use to run your spiders is
:class:`scrapy.crawler.CrawlerProcess`. This class will start a Twisted reactor
for you, configuring the logging and setting shutdown handlers. This class is
the one used by all Scrapy commands.

Here's an example showing how to run a single spider with it.

::

    import scrapy
    from scrapy.crawler import CrawlerProcess

    class MySpider(scrapy.Spider):
        # Your spider definition
        ...

    process = CrawlerProcess({
        'USER_AGENT': 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)'
    })

    process.crawl(MySpider)
    process.start() # the script will block here until the crawling is finished

Make sure to check :class:`~scrapy.crawler.CrawlerProcess` documentation to get
acquainted with its usage details.

If you are inside a Scrapy project there are some additional helpers you can
use to import those components within the project. You can automatically import
your spiders passing their name to :class:`~scrapy.crawler.CrawlerProcess`, and
use ``get_project_settings`` to get a :class:`~scrapy.settings.Settings`
instance with your project settings.

What follows is a working example of how to do that, using the `testspiders`_
project as example.

::

    from scrapy.crawler import CrawlerProcess
    from scrapy.utils.project import get_project_settings

    process = CrawlerProcess(get_project_settings())

    # 'followall' is the name of one of the spiders of the project.
    process.crawl('followall', domain='scrapinghub.com')
    process.start() # the script will block here until the crawling is finished

There's another Scrapy utility that provides more control over the crawling
process: :class:`scrapy.crawler.CrawlerRunner`. This class is a thin wrapper
that encapsulates some simple helpers to run multiple crawlers, but it won't
start or interfere with existing reactors in any way.

Using this class the reactor should be explicitly run after scheduling your
spiders. It's recommended you use :class:`~scrapy.crawler.CrawlerRunner`
instead of :class:`~scrapy.crawler.CrawlerProcess` if your application is
already using Twisted and you want to run Scrapy in the same reactor.

Note that you will also have to shutdown the Twisted reactor yourself after the
spider is finished. This can be achieved by adding callbacks to the deferred
returned by the :meth:`CrawlerRunner.crawl
<scrapy.crawler.CrawlerRunner.crawl>` method.

Here's an example of its usage, along with a callback to manually stop the
reactor after `MySpider` has finished running.

::

    from twisted.internet import reactor
    import scrapy
    from scrapy.crawler import CrawlerRunner
    from scrapy.utils.log import configure_logging

    class MySpider(scrapy.Spider):
        # Your spider definition
        ...

    configure_logging({'LOG_FORMAT': '%(levelname)s: %(message)s'})
    runner = CrawlerRunner()

    d = runner.crawl(MySpider)
    d.addBoth(lambda _: reactor.stop())
    reactor.run() # the script will block here until the crawling is finished

.. seealso:: `Twisted Reactor Overview`_.

.. _run-multiple-spiders:

Running multiple spiders in the same process
============================================

By default, Scrapy runs a single spider per process when you run ``scrapy
crawl``. However, Scrapy supports running multiple spiders per process using
the :ref:`internal API <topics-api>`.

Here is an example that runs multiple spiders simultaneously:

::

    import scrapy
    from scrapy.crawler import CrawlerProcess

    class MySpider1(scrapy.Spider):
        # Your first spider definition
        ...

    class MySpider2(scrapy.Spider):
        # Your second spider definition
        ...

    process = CrawlerProcess()
    process.crawl(MySpider1)
    process.crawl(MySpider2)
    process.start() # the script will block here until all crawling jobs are finished

Same example using :class:`~scrapy.crawler.CrawlerRunner`:

::

    import scrapy
    from twisted.internet import reactor
    from scrapy.crawler import CrawlerRunner
    from scrapy.utils.log import configure_logging

    class MySpider1(scrapy.Spider):
        # Your first spider definition
        ...

    class MySpider2(scrapy.Spider):
        # Your second spider definition
        ...

    configure_logging()
    runner = CrawlerRunner()
    runner.crawl(MySpider1)
    runner.crawl(MySpider2)
    d = runner.join()
    d.addBoth(lambda _: reactor.stop())

    reactor.run() # the script will block here until all crawling jobs are finished

Same example but running the spiders sequentially by chaining the deferreds:

::

    from twisted.internet import reactor, defer
    from scrapy.crawler import CrawlerRunner
    from scrapy.utils.log import configure_logging

    class MySpider1(scrapy.Spider):
        # Your first spider definition
        ...

    class MySpider2(scrapy.Spider):
        # Your second spider definition
        ...

    configure_logging()
    runner = CrawlerRunner()

    @defer.inlineCallbacks
    def crawl():
        yield runner.crawl(MySpider1)
        yield runner.crawl(MySpider2)
        reactor.stop()

    crawl()
    reactor.run() # the script will block here until the last crawl call is finished

.. seealso:: :ref:`run-from-script`.

.. _distributed-crawls:

Distributed crawls
==================

Scrapy doesn't provide any built-in facility for running crawls in a distribute
(multi-server) manner. However, there are some ways to distribute crawls, which
vary depending on how you plan to distribute them.

If you have many spiders, the obvious way to distribute the load is to setup
many Scrapyd instances and distribute spider runs among those.

If you instead want to run a single (big) spider through many machines, what
you usually do is partition the urls to crawl and send them to each separate
spider. Here is a concrete example:

First, you prepare the list of urls to crawl and put them into separate
files/urls::

    http://somedomain.com/urls-to-crawl/spider1/part1.list
    http://somedomain.com/urls-to-crawl/spider1/part2.list
    http://somedomain.com/urls-to-crawl/spider1/part3.list

Then you fire a spider run on 3 different Scrapyd servers. The spider would
receive a (spider) argument ``part`` with the number of the partition to
crawl::

    curl http://scrapy1.mycompany.com:6800/schedule.json -d project=myproject -d spider=spider1 -d part=1
    curl http://scrapy2.mycompany.com:6800/schedule.json -d project=myproject -d spider=spider1 -d part=2
    curl http://scrapy3.mycompany.com:6800/schedule.json -d project=myproject -d spider=spider1 -d part=3

.. _bans:

Avoiding getting banned
=======================

Some websites implement certain measures to prevent bots from crawling them,
with varying degrees of sophistication. Getting around those measures can be
difficult and tricky, and may sometimes require special infrastructure. Please
consider contacting `commercial support`_ if in doubt.

Here are some tips to keep in mind when dealing with these kinds of sites:

* rotate your user agent from a pool of well-known ones from browsers (google
  around to get a list of them)
* disable cookies (see :setting:`COOKIES_ENABLED`) as some sites may use
  cookies to spot bot behaviour
* use download delays (2 or higher). See :setting:`DOWNLOAD_DELAY` setting.
* if possible, use `Google cache`_ to fetch pages, instead of hitting the sites
  directly
* use a pool of rotating IPs. For example, the free `Tor project`_ or paid
  services like `ProxyMesh`_. An open source alternative is `scrapoxy`_, a
  super proxy that you can attach your own proxies to.
* use a highly distributed downloader that circumvents bans internally, so you
  can just focus on parsing clean pages. One example of such downloaders is
  `Crawlera`_

If you are still unable to prevent your bot getting banned, consider contacting
`commercial support`_.

.. _Tor project: https://www.torproject.org/
.. _commercial support: http://scrapy.org/support/
.. _ProxyMesh: http://proxymesh.com/
.. _Google cache: http://www.googleguide.com/cached_pages.html
.. _testspiders: https://github.com/scrapinghub/testspiders
.. _Twisted Reactor Overview: https://twistedmatrix.com/documents/current/core/howto/reactor-basics.html
.. _Crawlera: https://scrapinghub.com/crawlera
.. _scrapoxy: http://scrapoxy.io/