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.. _topics-downloader-middleware:
=====================
Downloader Middleware
=====================
The downloader middleware is a framework of hooks into Scrapy's
request/response processing. It's a light, low-level system for globally
altering Scrapy's requests and responses.
.. _topics-downloader-middleware-setting:
Activating a downloader middleware
==================================
To activate a downloader middleware component, add it to the
:setting:`DOWNLOADER_MIDDLEWARES` setting, which is a dict whose keys are the
middleware class paths and their values are the middleware orders.
Here's an example::
DOWNLOADER_MIDDLEWARES = {
'myproject.middlewares.CustomDownloaderMiddleware': 543,
}
The :setting:`DOWNLOADER_MIDDLEWARES` setting is merged with the
:setting:`DOWNLOADER_MIDDLEWARES_BASE` setting defined in Scrapy (and not meant to
be overridden) and then sorted by order to get the final sorted list of enabled
middlewares: the first middleware is the one closer to the engine and the last
is the one closer to the downloader.
To decide which order to assign to your middleware see the
:setting:`DOWNLOADER_MIDDLEWARES_BASE` setting and pick a value according to
where you want to insert the middleware. The order does matter because each
middleware performs a different action and your middleware could depend on some
previous (or subsequent) middleware being applied.
If you want to disable a built-in middleware (the ones defined in
:setting:`DOWNLOADER_MIDDLEWARES_BASE` and enabled by default) you must define it
in your project's :setting:`DOWNLOADER_MIDDLEWARES` setting and assign `None`
as its value. For example, if you want to disable the off-site middleware::
DOWNLOADER_MIDDLEWARES = {
'myproject.middlewares.CustomDownloaderMiddleware': 543,
'scrapy.contrib.downloadermiddleware.useragent.UserAgentMiddleware': None,
}
Finally, keep in mind that some middlewares may need to be enabled through a
particular setting. See each middleware documentation for more info.
Writing your own downloader middleware
======================================
Writing your own downloader middleware is easy. Each middleware component is a
single Python class that defines one or more of the following methods:
.. module:: scrapy.contrib.downloadermiddleware
.. class:: DownloaderMiddleware
.. method:: process_request(request, spider)
This method is called for each request that goes through the download
middleware.
:meth:`process_request` should return either ``None``, a
:class:`~scrapy.http.Response` object, or a :class:`~scrapy.http.Request`
object.
If it returns ``None``, Scrapy will continue processing this request, executing all
other middlewares until, finally, the appropriate downloader handler is called
the request performed (and its response downloaded).
If it returns a :class:`~scrapy.http.Response` object, Scrapy won't bother
calling ANY other request or exception middleware, or the appropriate
download function; it'll return that Response. Response middleware is
always called on every Response.
If it returns a :class:`~scrapy.http.Request` object, the returned request will be
rescheduled (in the Scheduler) to be downloaded in the future. The callback of
the original request will always be called. If the new request has a callback
it will be called with the response downloaded, and the output of that callback
will then be passed to the original callback. If the new request doesn't have a
callback, the response downloaded will be just passed to the original request
callback.
If it returns an :exc:`~scrapy.exceptions.IgnoreRequest` exception, the
entire request will be dropped completely and its callback never called.
:param request: the request being processed
:type request: :class:`~scrapy.http.Request` object
:param spider: the spider for which this request is intended
:type spider: :class:`~scrapy.spider.BaseSpider` object
.. method:: process_response(request, response, spider)
:meth:`process_response` should return a :class:`~scrapy.http.Response`
object or raise a :exc:`~scrapy.exceptions.IgnoreRequest` exception.
If it returns a :class:`~scrapy.http.Response` (it could be the same given
response, or a brand-new one), that response will continue to be processed
with the :meth:`process_response` of the next middleware in the pipeline.
If it returns an :exc:`~scrapy.exceptions.IgnoreRequest` exception, the
response will be dropped completely and its callback never called.
:param request: the request that originated the response
:type request: is a :class:`~scrapy.http.Request` object
:param reponse: the response being processed
:type response: :class:`~scrapy.http.Response` object
:param spider: the spider for which this response is intended
:type spider: :class:`~scrapy.spider.BaseSpider` object
.. method:: process_exception(request, exception, spider)
Scrapy calls :meth:`process_exception` when a download handler
or a :meth:`process_request` (from a downloader middleware) raises an
exception.
:meth:`process_exception` should return either ``None``,
:class:`~scrapy.http.Response` or :class:`~scrapy.http.Request` object.
If it returns ``None``, Scrapy will continue processing this exception,
executing any other exception middleware, until no middleware is left and
the default exception handling kicks in.
If it returns a :class:`~scrapy.http.Response` object, the response middleware
kicks in, and won't bother calling any other exception middleware.
If it returns a :class:`~scrapy.http.Request` object, the returned request is
used to instruct an immediate redirection.
The original request won't finish until the redirected
request is completed. This stops the :meth:`process_exception`
middleware the same as returning Response would do.
:param request: the request that generated the exception
:type request: is a :class:`~scrapy.http.Request` object
:param exception: the raised exception
:type exception: an ``Exception`` object
:param spider: the spider for which this request is intended
:type spider: :class:`~scrapy.spider.BaseSpider` object
.. _topics-downloader-middleware-ref:
Built-in downloader middleware reference
========================================
This page describes all downloader middleware components that come with
Scrapy. For information on how to use them and how to write your own downloader
middleware, see the :ref:`downloader middleware usage guide
<topics-downloader-middleware>`.
For a list of the components enabled by default (and their orders) see the
:setting:`DOWNLOADER_MIDDLEWARES_BASE` setting.
.. _cookies-mw:
CookiesMiddleware
-----------------
.. module:: scrapy.contrib.downloadermiddleware.cookies
:synopsis: Cookies Downloader Middleware
.. class:: CookiesMiddleware
This middleware enables working with sites that need cookies. It keeps track
of merging cookies sent by servers, so that they're send in future requests
for that spider, just like a web browser would do.
The following settings can be used to configure the cookie middleware:
* :setting:`COOKIES_DEBUG`
.. setting:: COOKIES_DEBUG
COOKIES_DEBUG
~~~~~~~~~~~~~
Default: ``False``
If enabled, Scrapy will log all cookies sent in requests (ie. ``Cookie``
header) and all cookies received in responses (ie. ``Set-Cookie`` header).
Here's an example of a log with :setting:`COOKIES_DEBUG` enabled::
2011-04-06 14:35:10-0300 [diningcity] INFO: Spider opened
2011-04-06 14:35:10-0300 [diningcity] DEBUG: Sending cookies to: <GET http://www.diningcity.com/netherlands/index.html>
Cookie: clientlanguage_nl=en_EN
2011-04-06 14:35:14-0300 [diningcity] DEBUG: Received cookies from: <200 http://www.diningcity.com/netherlands/index.html>
Set-Cookie: JSESSIONID=B~FA4DC0C496C8762AE4F1A620EAB34F38; Path=/
Set-Cookie: ip_isocode=US
Set-Cookie: clientlanguage_nl=en_EN; Expires=Thu, 07-Apr-2011 21:21:34 GMT; Path=/
2011-04-06 14:49:50-0300 [diningcity] DEBUG: Crawled (200) <GET http://www.diningcity.com/netherlands/index.html> (referer: None)
[...]
DefaultHeadersMiddleware
------------------------
.. module:: scrapy.contrib.downloadermiddleware.defaultheaders
:synopsis: Default Headers Downloader Middleware
.. class:: DefaultHeadersMiddleware
This middleware sets all default requests headers specified in the
:setting:`DEFAULT_REQUEST_HEADERS` setting.
DownloadTimeoutMiddleware
-------------------------
.. module:: scrapy.contrib.downloadermiddleware.downloadtimeout
:synopsis: Download timeout middleware
.. class:: DownloadTimeoutMiddleware
This middleware sets the download timeout for requests specified in the
:setting:`DOWNLOAD_TIMEOUT` setting.
HttpAuthMiddleware
------------------
.. module:: scrapy.contrib.downloadermiddleware.httpauth
:synopsis: HTTP Auth downloader middleware
.. class:: HttpAuthMiddleware
This middleware authenticates all requests generated from certain spiders
using `Basic access authentication`_ (aka. HTTP auth).
To enable HTTP authentication from certain spiders, set the ``http_user``
and ``http_pass`` attributes of those spiders.
Example::
class SomeIntranetSiteSpider(CrawlSpider):
http_user = 'someuser'
http_pass = 'somepass'
name = 'intranet.example.com'
# .. rest of the spider code omitted ...
.. _Basic access authentication: http://en.wikipedia.org/wiki/Basic_access_authentication
HttpCacheMiddleware
-------------------
.. module:: scrapy.contrib.downloadermiddleware.httpcache
:synopsis: HTTP Cache downloader middleware
.. class:: HttpCacheMiddleware
This middleware provides low-level cache to all HTTP requests and responses.
Every request and its corresponding response are cached. When the same
request is seen again, the response is returned without transferring
anything from the Internet.
The HTTP cache is useful for testing spiders faster (without having to wait for
downloads every time) and for trying your spider offline, when an Internet
connection is not available.
Scrapy ships with two storage backends for the HTTP cache middleware:
* :ref:`httpcache-fs-backend`
* :ref:`httpcache-dbm-backend`
You can change the storage backend with the :setting:`HTTPCACHE_STORAGE`
setting. Or you can also implement your own backend.
.. _httpcache-fs-backend:
File system backend (default)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
By default, the :class:`HttpCacheMiddleware` uses a file system storage with the following structure:
Each request/response pair is stored in a different directory containing
the following files:
* ``request_body`` - the plain request body
* ``request_headers`` - the request headers (in raw HTTP format)
* ``response_body`` - the plain response body
* ``response_headers`` - the request headers (in raw HTTP format)
* ``meta`` - some metadata of this cache resource in Python ``repr()`` format
(grep-friendly format)
* ``pickled_meta`` - the same metadata in ``meta`` but pickled for more
efficient deserialization
The directory name is made from the request fingerprint (see
``scrapy.utils.request.fingerprint``), and one level of subdirectories is
used to avoid creating too many files into the same directory (which is
inefficient in many file systems). An example directory could be::
/path/to/cache/dir/example.com/72/72811f648e718090f041317756c03adb0ada46c7
.. _httpcache-dbm-backend:
DBM storage backend
~~~~~~~~~~~~~~~~~~~
.. versionadded:: 0.13
A DBM_ storage backend is also available for the HTTP cache middleware. To use
it (instead of the default filesystem backend) set :setting:`HTTPCACHE_STORAGE`
to ``scrapy.contrib.httpcache.DbmCacheStorage``.
By default, it uses the anydbm_ module, but you can change it with the
:setting:`HTTPCACHE_DBM_MODULE` setting.
Settings
~~~~~~~~
The :class:`HttpCacheMiddleware` can be configured through the following
settings:
.. setting:: HTTPCACHE_ENABLED
HTTPCACHE_ENABLED
^^^^^^^^^^^^^^^^^
.. versionadded:: 0.11
Default: ``False``
Whether the HTTP cache will be enabled.
.. versionchanged:: 0.11
Before 0.11, :setting:`HTTPCACHE_DIR` was used to enable cache.
.. setting:: HTTPCACHE_EXPIRATION_SECS
HTTPCACHE_EXPIRATION_SECS
^^^^^^^^^^^^^^^^^^^^^^^^^
Default: ``0``
Expiration time for cached requests, in seconds.
Cached requests older than this time will be re-downloaded. If zero, cached
requests will never expire.
.. versionchanged:: 0.11
Before 0.11, zero meant cached requests always expire.
.. setting:: HTTPCACHE_DIR
HTTPCACHE_DIR
^^^^^^^^^^^^^
Default: ``'httpcache'``
The directory to use for storing the (low-level) HTTP cache. If empty, the HTTP
cache will be disabled. If a relative path is given, is taken relative to the
project data dir. For more info see: :ref:`topics-project-structure`.
.. setting:: HTTPCACHE_IGNORE_HTTP_CODES
HTTPCACHE_IGNORE_HTTP_CODES
^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. versionadded:: 0.10
Default: ``[]``
Don't cache response with these HTTP codes.
.. setting:: HTTPCACHE_IGNORE_MISSING
HTTPCACHE_IGNORE_MISSING
^^^^^^^^^^^^^^^^^^^^^^^^
Default: ``False``
If enabled, requests not found in the cache will be ignored instead of downloaded.
.. setting:: HTTPCACHE_IGNORE_SCHEMES
HTTPCACHE_IGNORE_SCHEMES
^^^^^^^^^^^^^^^^^^^^^^^^
.. versionadded:: 0.10
Default: ``['file']``
Don't cache responses with these URI schemes.
.. setting:: HTTPCACHE_STORAGE
HTTPCACHE_STORAGE
^^^^^^^^^^^^^^^^^
Default: ``'scrapy.contrib.downloadermiddleware.httpcache.FilesystemCacheStorage'``
The class which implements the cache storage backend.
.. setting:: HTTPCACHE_DBM_MODULE
HTTPCACHE_DBM_MODULE
^^^^^^^^^^^^^^^^^^^^
.. versionadded:: 0.13
Default: ``'anydbm'``
The database module to use in the :ref:`DBM storage backend
<httpcache-dbm-backend>`. This setting is specific to the DBM backend.
HttpCompressionMiddleware
-------------------------
.. module:: scrapy.contrib.downloadermiddleware.httpcompression
:synopsis: Http Compression Middleware
.. class:: HttpCompressionMiddleware
This middleware allows compressed (gzip, deflate) traffic to be
sent/received from web sites.
HttpProxyMiddleware
-------------------
.. module:: scrapy.contrib.downloadermiddleware.httpproxy
:synopsis: Http Proxy Middleware
.. versionadded:: 0.8
.. class:: HttpProxyMiddleware
This middleware sets the HTTP proxy to use for requests, by setting the
``proxy`` meta value to :class:`~scrapy.http.Request` objects.
Like the Python standard library modules `urllib`_ and `urllib2`_, it obeys
the following enviroment variables:
* ``http_proxy``
* ``https_proxy``
* ``no_proxy``
.. _urllib: http://docs.python.org/library/urllib.html
.. _urllib2: http://docs.python.org/library/urllib2.html
RedirectMiddleware
------------------
.. module:: scrapy.contrib.downloadermiddleware.redirect
:synopsis: Redirection Middleware
.. class:: RedirectMiddleware
This middleware handles redirection of requests based on response status and
meta-refresh html tag.
.. reqmeta:: redirect_urls
The urls which the request goes through (while being redirected) can be found
in the ``redirect_urls`` :attr:`Request.meta <scrapy.http.Request.meta>` key.
The :class:`RedirectMiddleware` can be configured through the following
settings (see the settings documentation for more info):
* :setting:`REDIRECT_MAX_METAREFRESH_DELAY` - Maximum meta-refresh delay that a page is allowed to have for redirection.
* :setting:`REDIRECT_MAX_TIMES` - Maximum number of redirects to perform on a request.
* :setting:`REDIRECT_PRIORITY_ADJUST` - Adjusts the redirected request priority by this amount.
.. reqmeta:: dont_redirect
If :attr:`Request.meta <scrapy.http.Request.meta>` contains the
``dont_redirect`` key, the request will be ignored by this middleware.
RetryMiddleware
---------------
.. module:: scrapy.contrib.downloadermiddleware.retry
:synopsis: Retry Middleware
.. class:: RetryMiddleware
A middlware to retry failed requests that are potentially caused by
temporary problems such as a connection timeout or HTTP 500 error.
Failed pages are collected on the scraping process and rescheduled at the
end, once the spider has finished crawling all regular (non failed) pages.
Once there are no more failed pages to retry, this middleware sends a signal
(retry_complete), so other extensions could connect to that signal.
The :class:`RetryMiddleware` can be configured through the following
settings (see the settings documentation for more info):
* :setting:`RETRY_TIMES` - how many times to retry a failed page
* :setting:`RETRY_HTTP_CODES` - which HTTP response codes to retry
About HTTP errors to consider:
You may want to remove 400 from RETRY_HTTP_CODES, if you stick to the
HTTP protocol. It's included by default because it's a common code used
to indicate server overload, which would be something we want to retry.
.. reqmeta:: dont_retry
If :attr:`Request.meta <scrapy.http.Request.meta>` contains the ``dont_retry``
key, the request will be ignored by this middleware.
.. _topics-dlmw-robots:
RobotsTxtMiddleware
-------------------
.. module:: scrapy.contrib.downloadermiddleware.robotstxt
:synopsis: robots.txt middleware
.. class:: RobotsTxtMiddleware
This middleware filters out requests forbidden by the robots.txt exclusion
standard.
To make sure Scrapy respects robots.txt make sure the middleware is enabled
and the :setting:`ROBOTSTXT_OBEY` setting is enabled.
.. warning:: Keep in mind that, if you crawl using multiple concurrent
requests per domain, Scrapy could still download some forbidden pages
if they were requested before the robots.txt file was downloaded. This
is a known limitation of the current robots.txt middleware and will
be fixed in the future.
DownloaderStats
---------------
.. module:: scrapy.contrib.downloadermiddleware.stats
:synopsis: Downloader Stats Middleware
.. class:: DownloaderStats
Middleware that stores stats of all requests, responses and exceptions that
pass through it.
To use this middleware you must enable the :setting:`DOWNLOADER_STATS`
setting.
UserAgentMiddleware
-------------------
.. module:: scrapy.contrib.downloadermiddleware.useragent
:synopsis: User Agent Middleware
.. class:: UserAgentMiddleware
Middleware that allows spiders to override the default user agent.
In order for a spider to override the default user agent, its `user_agent`
attribute must be set.
.. _DBM: http://en.wikipedia.org/wiki/Dbm
.. _anydbm: http://docs.python.org/library/anydbm.html
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