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.

Activating a downloader middleware

To activate a downloader middleware component, add it to the 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 DOWNLOADER_MIDDLEWARES setting is merged with the 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 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 DOWNLOADER_MIDDLEWARES_BASE and enabled by default) you must define it in your project’s 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:

class scrapy.contrib.downloadermiddleware.DownloaderMiddleware
process_request(request, spider)

This method is called for each request that goes through the download middleware.

process_request() should return either None, a Response object, or a 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 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 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 IgnoreRequest exception, the entire request will be dropped completely and its callback never called.

Parameters:
  • request (Request object) – the request being processed
  • spider (BaseSpider object) – the spider for which this request is intended
process_response(request, response, spider)

process_response() should return a Response object or raise a IgnoreRequest exception.

If it returns a Response (it could be the same given response, or a brand-new one), that response will continue to be processed with the process_response() of the next middleware in the pipeline.

If it returns an IgnoreRequest exception, the response will be dropped completely and its callback never called.

Parameters:
  • request (is a Request object) – the request that originated the response
  • reponse – the response being processed
  • spider (BaseSpider object) – the spider for which this response is intended
process_exception(request, exception, spider)

Scrapy calls process_exception() when a download handler or a process_request() (from a downloader middleware) raises an exception.

process_exception() should return either None, Response or 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 Response object, the response middleware kicks in, and won’t bother calling any other exception middleware.

If it returns a 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 process_exception() middleware the same as returning Response would do.

Parameters:
  • request (is a Request object) – the request that generated the exception
  • exception (an Exception object) – the raised exception
  • spider (BaseSpider object) – the spider for which this request is intended

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 downloader middleware usage guide.

For a list of the components enabled by default (and their orders) see the DOWNLOADER_MIDDLEWARES_BASE setting.

CookiesMiddleware

class scrapy.contrib.downloadermiddleware.cookies.CookiesMiddleware

This middleware enables working with sites that require cookies, such as those that use sessions. It keeps track of cookies sent by web servers, and send them back on subsequent requests (from that spider), just like web browsers do.

The following settings can be used to configure the cookie middleware:

COOKIES_ENABLED

Default: True

Whether to enable the cookies middleware. If disabled, no cookies will be sent to web servers.

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 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

class scrapy.contrib.downloadermiddleware.defaultheaders.DefaultHeadersMiddleware

This middleware sets all default requests headers specified in the DEFAULT_REQUEST_HEADERS setting.

DownloadTimeoutMiddleware

class scrapy.contrib.downloadermiddleware.downloadtimeout.DownloadTimeoutMiddleware

This middleware sets the download timeout for requests specified in the DOWNLOAD_TIMEOUT setting.

HttpAuthMiddleware

class scrapy.contrib.downloadermiddleware.httpauth.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 ...

HttpCacheMiddleware

class scrapy.contrib.downloadermiddleware.httpcache.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:

You can change the storage backend with the HTTPCACHE_STORAGE setting. Or you can also implement your own backend.

DBM storage backend (default)

New in version 0.13.

A DBM storage backend is also available for the HTTP cache middleware. To use it (instead of the default filesystem backend) set HTTPCACHE_STORAGE to scrapy.contrib.httpcache.DbmCacheStorage.

By default, it uses the anydbm module, but you can change it with the HTTPCACHE_DBM_MODULE setting.

File system backend

By default, the 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 middleware settings

The HttpCacheMiddleware can be configured through the following settings:

HTTPCACHE_ENABLED

New in version 0.11.

Default: False

Whether the HTTP cache will be enabled.

Changed in version 0.11: Before 0.11, HTTPCACHE_DIR was used to enable cache.

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.

Changed in version 0.11: Before 0.11, zero meant cached requests always expire.

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: Default structure of Scrapy projects.

HTTPCACHE_IGNORE_HTTP_CODES

New in version 0.10.

Default: []

Don’t cache response with these HTTP codes.

HTTPCACHE_IGNORE_MISSING

Default: False

If enabled, requests not found in the cache will be ignored instead of downloaded.

HTTPCACHE_IGNORE_SCHEMES

New in version 0.10.

Default: ['file']

Don’t cache responses with these URI schemes.

HTTPCACHE_STORAGE

Default: 'scrapy.contrib.downloadermiddleware.httpcache.FilesystemCacheStorage'

The class which implements the cache storage backend.

HTTPCACHE_DBM_MODULE

New in version 0.13.

Default: 'anydbm'

The database module to use in the DBM storage backend. This setting is specific to the DBM backend.

HttpCompressionMiddleware

class scrapy.contrib.downloadermiddleware.httpcompression.HttpCompressionMiddleware

This middleware allows compressed (gzip, deflate) traffic to be sent/received from web sites.

ChunkedTransferMiddleware

class scrapy.contrib.downloadermiddleware.chunked.ChunkedTransferMiddleware

This middleware adds support for chunked transfer encoding

HttpProxyMiddleware

New in version 0.8.

class scrapy.contrib.downloadermiddleware.httpproxy.HttpProxyMiddleware

This middleware sets the HTTP proxy to use for requests, by setting the proxy meta value to Request objects.

Like the Python standard library modules urllib and urllib2, it obeys the following enviroment variables:

  • http_proxy
  • https_proxy
  • no_proxy

RedirectMiddleware

class scrapy.contrib.downloadermiddleware.redirect.RedirectMiddleware

This middleware handles redirection of requests based on response status and meta-refresh html tag.

The urls which the request goes through (while being redirected) can be found in the redirect_urls Request.meta key.

The RedirectMiddleware can be configured through the following settings (see the settings documentation for more info):

If Request.meta contains the dont_redirect key, the request will be ignored by this middleware.

RedirectMiddleware settings

REDIRECT_ENABLED

New in version 0.13.

Default: True

Whether the Redirect middleware will be enabled.

REDIRECT_MAX_TIMES

Default: 20

The maximum number of redirections that will be follow for a single request.

REDIRECT_MAX_METAREFRESH_DELAY

Default: 100

The maximum meta-refresh delay (in seconds) to follow the redirection.

RetryMiddleware

class scrapy.contrib.downloadermiddleware.retry.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 RetryMiddleware can be configured through the following settings (see the settings documentation for more info):

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.

If Request.meta contains the dont_retry key, the request will be ignored by this middleware.

RetryMiddleware Settings

RETRY_ENABLED

New in version 0.13.

Default: True

Whether the Retry middleware will be enabled.

RETRY_TIMES

Default: 2

Maximum number of times to retry, in addition to the first download.

RETRY_HTTP_CODES

Default: [500, 503, 504, 400, 408]

Which HTTP response codes to retry. Other errors (DNS lookup issues, connections lost, etc) are always retried.

RobotsTxtMiddleware

class scrapy.contrib.downloadermiddleware.robotstxt.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 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

class scrapy.contrib.downloadermiddleware.stats.DownloaderStats

Middleware that stores stats of all requests, responses and exceptions that pass through it.

To use this middleware you must enable the DOWNLOADER_STATS setting.

UserAgentMiddleware

class scrapy.contrib.downloadermiddleware.useragent.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.