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# 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`
- `COOKIES_DEBUG`

#### 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:

  - File system backend (default)
  - DBM storage backend

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

#### File system backend (default)

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

#### DBM storage backend

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.

#### 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):

- `REDIRECT_ENABLED`
- `REDIRECT_MAX_TIMES`
- `REDIRECT_MAX_METAREFRESH_DELAY`

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):

- `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` 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.

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