# 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 user-agent middleware:

    DOWNLOADER_MIDDLEWARES = {
        'myproject.middlewares.CustomDownloaderMiddleware': 543,
        'scrapy.downloadermiddlewares.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

Each middleware component is a Python class that defines one or more of the following methods:

class `scrapy.downloadermiddlewares.``DownloaderMiddleware`
  `process_request`(request, spider)

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

    `process_request()` should either: return `None`, return a `Response` object, return a `Request` object, or raise `IgnoreRequest`.

    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 `process_request()` or `process_exception()` methods, or the appropriate download function; it’ll return that response. The `process_response()` methods of installed middleware is always called on every response.

    If it returns a `Request` object, Scrapy will stop calling process_request methods and reschedule the returned request. Once the newly returned request is performed, the appropriate middleware chain will be called on the downloaded response.

    If it raises an `IgnoreRequest` exception, the `process_exception()` methods of installed downloader middleware will be called. If none of them handle the exception, the errback function of the request (`Request.errback`) is called. If no code handles the raised exception, it is ignored and not logged (unlike other exceptions).

    Parameters:

    - request (`Request` object) – the request being processed
    - spider (`Spider` object) – the spider for which this request is intended

  `process_response`(request, response, spider)

    `process_response()` should either: return a `Response` object, return a `Request` 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 chain.

    If it returns a `Request` object, the middleware chain is halted and the returned request is rescheduled to be downloaded in the future. This is the same behavior as if a request is returned from `process_request()`.

    If it raises an `IgnoreRequest` exception, the errback function of the request (`Request.errback`) is called. If no code handles the raised exception, it is ignored and not logged (unlike other exceptions).

    Parameters:

    - request (is a `Request` object) – the request that originated the response
    - response (`Response` object) – the response being processed
    - spider (`Spider` 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 (including an `IgnoreRequest` exception)

    `process_exception()` should return: either `None`, a `Response` object, or a `Request` object.

    If it returns `None`, Scrapy will continue processing this exception, executing any other `process_exception()` methods of installed middleware, until no middleware is left and the default exception handling kicks in.

    If it returns a `Response` object, the `process_response()` method chain of installed middleware is started, and Scrapy won’t bother calling any other `process_exception()` methods of middleware.

    If it returns a `Request` object, the returned request is rescheduled to be downloaded in the future. This stops the execution of `process_exception()` methods of the middleware the same as returning a response would.

    Parameters:

    - request (is a `Request` object) – the request that generated the exception
    - exception (an `Exception` object) – the raised exception
    - spider (`Spider` 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.downloadermiddlewares.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`
- `COOKIES_DEBUG`

#### Multiple cookie sessions per spider

New in version 0.15.

There is support for keeping multiple cookie sessions per spider by using the `cookiejar` Request meta key. By default it uses a single cookie jar (session), but you can pass an identifier to use different ones.

For example:

    for i, url in enumerate(urls):
        yield scrapy.Request("http://www.example.com", meta={'cookiejar': i},
            callback=self.parse_page)

Keep in mind that the `cookiejar` meta key is not “sticky”. You need to keep passing it along on subsequent requests. For example:

    def parse_page(self, response):
        # do some processing
        return scrapy.Request("http://www.example.com/otherpage",
            meta={'cookiejar': response.meta['cookiejar']},
            callback=self.parse_other_page)

#### 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 [scrapy] INFO: Spider opened
    2011-04-06 14:35:10-0300 [scrapy] DEBUG: Sending cookies to: <GET http://www.diningcity.com/netherlands/index.html>
            Cookie: clientlanguage_nl=en_EN
    2011-04-06 14:35:14-0300 [scrapy] 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 [scrapy] DEBUG: Crawled (200) <GET http://www.diningcity.com/netherlands/index.html> (referer: None)
    [...]

### DefaultHeadersMiddleware

class `scrapy.downloadermiddlewares.defaultheaders.``DefaultHeadersMiddleware`

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

### DownloadTimeoutMiddleware

class `scrapy.downloadermiddlewares.downloadtimeout.``DownloadTimeoutMiddleware`

  This middleware sets the download timeout for requests specified in the `DOWNLOAD_TIMEOUT` setting or `download_timeout` spider attribute.

Note

You can also set download timeout per-request using `download_timeout` Request.meta key; this is supported even when DownloadTimeoutMiddleware is disabled.

### HttpAuthMiddleware

class `scrapy.downloadermiddlewares.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:

      from scrapy.spiders import CrawlSpider

      class SomeIntranetSiteSpider(CrawlSpider):

          http_user = 'someuser'
          http_pass = 'somepass'
          name = 'intranet.example.com'

          # .. rest of the spider code omitted ...

### HttpCacheMiddleware

class `scrapy.downloadermiddlewares.httpcache.``HttpCacheMiddleware`

  This middleware provides low-level cache to all HTTP requests and responses. It has to be combined with a cache storage backend as well as a cache policy.

  Scrapy ships with two HTTP cache storage backends:

  > - Filesystem storage backend (default)
  > - DBM storage backend

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

  Scrapy ships with two HTTP cache policies:

  > - RFC2616 policy
  > - Dummy policy (default)

  You can change the HTTP cache policy with the `HTTPCACHE_POLICY` setting. Or you can also implement your own policy.

  You can also avoid caching a response on every policy using `dont_cache` meta key equals True.

#### Dummy policy (default)

This policy has no awareness of any HTTP Cache-Control directives. 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 Dummy policy 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. The goal is to be able to “replay” a spider run exactly as it ran before.

In order to use this policy, set:

- `HTTPCACHE_POLICY` to `scrapy.extensions.httpcache.DummyPolicy`

#### RFC2616 policy

This policy provides a RFC2616 compliant HTTP cache, i.e. with HTTP Cache-Control awareness, aimed at production and used in continuous runs to avoid downloading unmodified data (to save bandwidth and speed up crawls).

what is implemented:

- Do not attempt to store responses/requests with no-store cache-control directive set
- Do not serve responses from cache if no-cache cache-control directive is set even for fresh responses
- Compute freshness lifetime from max-age cache-control directive
- Compute freshness lifetime from Expires response header
- Compute freshness lifetime from Last-Modified response header (heuristic used by Firefox)
- Compute current age from Age response header
- Compute current age from Date header
- Revalidate stale responses based on Last-Modified response header
- Revalidate stale responses based on ETag response header
- Set Date header for any received response missing it

what is missing:

- Pragma: no-cache support http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.1
- Vary header support http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.6
- Invalidation after updates or deletes http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.10
- ... probably others ..

In order to use this policy, set:

- `HTTPCACHE_POLICY` to `scrapy.extensions.httpcache.RFC2616Policy`

#### Filesystem storage backend (default)

File system storage backend is available for the HTTP cache middleware.

In order to use this storage backend, set:

- `HTTPCACHE_STORAGE` to `scrapy.extensions.httpcache.FilesystemCacheStorage`

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

#### DBM storage backend

New in version 0.13.

A DBM storage backend is also available for the HTTP cache middleware.

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

In order to use this storage backend, set:

- `HTTPCACHE_STORAGE` to `scrapy.extensions.httpcache.DbmCacheStorage`

#### LevelDB storage backend

New in version 0.23.

A LevelDB storage backend is also available for the HTTP cache middleware.

This backend is not recommended for development because only one process can access LevelDB databases at the same time, so you can’t run a crawl and open the scrapy shell in parallel for the same spider.

In order to use this storage backend:

- set `HTTPCACHE_STORAGE` to `scrapy.extensions.httpcache.LeveldbCacheStorage`
- install LevelDB python bindings like `pip install leveldb`

#### 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.extensions.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.

##### HTTPCACHE_POLICY

New in version 0.18.

Default: `'scrapy.extensions.httpcache.DummyPolicy'`

The class which implements the cache policy.

##### HTTPCACHE_GZIP

New in version 0.25.

Default: `False`

If enabled, will compress all cached data with gzip. This setting is specific to the Filesystem backend.

### HttpCompressionMiddleware

class `scrapy.downloadermiddlewares.httpcompression.``HttpCompressionMiddleware`

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

#### HttpCompressionMiddleware Settings

##### COMPRESSION_ENABLED

Default: `True`

Whether the Compression middleware will be enabled.

### ChunkedTransferMiddleware

class `scrapy.downloadermiddlewares.chunked.``ChunkedTransferMiddleware`

  This middleware adds support for chunked transfer encoding

### HttpProxyMiddleware

New in version 0.8.

class `scrapy.downloadermiddlewares.httpproxy.``HttpProxyMiddleware`

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

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

  - `http_proxy`
  - `https_proxy`
  - `no_proxy`

  You can also set the meta key `proxy` per-request, to a value like `http://some_proxy_server:port`.

### RedirectMiddleware

class `scrapy.downloadermiddlewares.redirect.``RedirectMiddleware`

  This middleware handles redirection of requests based on response status.

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):

- `REDIRECT_ENABLED`
- `REDIRECT_MAX_TIMES`

If `Request.meta` has `dont_redirect` key set to True, 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.

### MetaRefreshMiddleware

class `scrapy.downloadermiddlewares.redirect.``MetaRefreshMiddleware`

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

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

- `METAREFRESH_ENABLED`
- `METAREFRESH_MAXDELAY`

This middleware obey `REDIRECT_MAX_TIMES` setting, `dont_redirect` and `redirect_urls` request meta keys as described for `RedirectMiddleware`

#### MetaRefreshMiddleware settings

##### METAREFRESH_ENABLED

New in version 0.17.

Default: `True`

Whether the Meta Refresh middleware will be enabled.

##### REDIRECT_MAX_METAREFRESH_DELAY

Default: `100`

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

### RetryMiddleware

class `scrapy.downloadermiddlewares.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):

- `RETRY_ENABLED`
- `RETRY_TIMES`
- `RETRY_HTTP_CODES`

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` has `dont_retry` key set to True, 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, 502, 503, 504, 400, 408]`

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

### RobotsTxtMiddleware

class `scrapy.downloadermiddlewares.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.

If `Request.meta` has `dont_obey_robotstxt` key set to True the request will be ignored by this middleware even if `ROBOTSTXT_OBEY` is enabled.

### DownloaderStats

class `scrapy.downloadermiddlewares.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.downloadermiddlewares.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.

### AjaxCrawlMiddleware

class `scrapy.downloadermiddlewares.ajaxcrawl.``AjaxCrawlMiddleware`

  Middleware that finds ‘AJAX crawlable’ page variants based on meta-fragment html tag. See https://developers.google.com/webmasters/ajax-crawling/docs/getting-started for more info.

  Note

  Scrapy finds ‘AJAX crawlable’ pages for URLs like `'http://example.com/!#foo=bar'` even without this middleware. AjaxCrawlMiddleware is necessary when URL doesn’t contain `'!#'`. This is often a case for ‘index’ or ‘main’ website pages.

#### AjaxCrawlMiddleware Settings

##### AJAXCRAWL_ENABLED

New in version 0.21.

Default: `False`

Whether the AjaxCrawlMiddleware will be enabled. You may want to enable it for broad crawls.