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Throttling

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Sending too many requests too quickly can overwhelm websites. :ref:`Throttling <basic-throttling>` and :ref:`backoff <backoff>` aim to prevent that.

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Concurrency and delay

Requests are throttled on a per-domain basis by default [1]. This allows efficient crawling of multiple sites simultaneously.

Each domain and subdomain is treated separately: requests to books.toscrape.com and quotes.toscrape.com each have their own throttling limits, as do toscrape.com and books.toscrape.com.

The main throttling :ref:`settings <topics-settings>` are:

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```
.. setting:: THROTTLING_SCOPE_CONCURRENCY
```
:setting:`THROTTLING_SCOPE_CONCURRENCY` (default: 1)

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Default maximum number of simultaneous requests per :ref:`throttling scope <throttling-scopes>`. Requests are grouped by domain by default, so this is the maximum number of simultaneous requests per domain.

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It defines a number of “slots” per scope. Each slot can send 1 request at a time: it sends a request, waits for the response, then sends the next request, and so on.
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```
.. setting:: DOWNLOAD_DELAY
```
:setting:`DOWNLOAD_DELAY` (default: 1 (:ref:`fallback <default-settings>`: 0))

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Minimum seconds between any two requests to the same domain.

To target a specific number of requests per minute (RPM) per domain, set this to 60 / RPM. For example, DOWNLOAD_DELAY = 1.0 for 60 RPM, or DOWNLOAD_DELAY = 2.0 for 30 RPM.

When configuring these settings, note that:

:setting:`CONCURRENT_REQUESTS` caps :setting:`THROTTLING_SCOPE_CONCURRENCY`.

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If DOWNLOAD_DELAY ≥ response time, concurrency is effectively 1, because the next request to the domain is not sent until the delay elapses, by which time the previous response has already arrived.

[1]

(1, 2)

You can :ref:`customize <throttling-scopes>` how requests are grouped for throttling, but domain-based throttling works well in most cases. For more complex domain grouping strategies, see :ref:`alternative-domain-throttling`.

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.. setting:: THROTTLING_SCOPES

Per-domain throttling

The :setting:`THROTTLING_SCOPES` setting allows you to customize throttling behavior for specific domains [1].

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It is a dict that maps scope names to :class:`~scrapy.throttling.ThrottlingScopeConfig` dicts. It is empty by default.

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:command:`startproject` scaffolds an entry for the testing websites used during the :ref:`tutorial <intro-tutorial>`, so that they are crawled faster while the :ref:`conservative defaults <basic-throttling>` still apply to other domains:

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.. code-block:: python

    THROTTLING_SCOPES = {
        "books.toscrape.com": {"concurrency": 16, "delay": 0.1},
        "quotes.toscrape.com": {"concurrency": 16, "delay": 0.1},
    }

Additional keys like "jitter" and "backoff" can be used here and are covered later on.

Backoff

When servers respond with rate limiting errors (like HTTP 429) or network timeouts occur, request rate is automatically reduced using exponential backoff.

The key settings are:

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```
.. setting:: BACKOFF_HTTP_CODES
```
:setting:`BACKOFF_HTTP_CODES` (default: [429, 502, 503, 504, 520, 521, 522, 523, 524])

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HTTP response status codes that trigger backoff. Can be overridden per scope with the http_codes key (see :ref:`per-scope-backoff`).

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```
.. setting:: BACKOFF_DELAY_FACTOR
```
:setting:`BACKOFF_DELAY_FACTOR` (default: 2.0)

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Factor by which the delay of a scope is multiplied on each backoff step (2×, 4×, 8×, etc.).
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```
.. setting:: BACKOFF_MAX_DELAY
```
:setting:`BACKOFF_MAX_DELAY` (default: 300.0)

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Maximum delay, in seconds, that backoff can reach. Also caps :ref:`Retry-After <retry-after>` and :ref:`RateLimit-Reset <rate-limiting-headers>` delays.

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See :ref:`throttling-settings` for :setting:`BACKOFF_EXCEPTIONS`, :setting:`BACKOFF_JITTER`, :setting:`BACKOFF_MIN_DELAY` and :setting:`BACKOFF_WINDOW`.

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How backoff works

Every :ref:`throttling scope <throttling-scopes>` keeps a current delay that starts at its configured "delay" (:setting:`DOWNLOAD_DELAY` by default).

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A backoff trigger is a response whose status code is in :setting:`BACKOFF_HTTP_CODES` or a download exception whose type is in :setting:`BACKOFF_EXCEPTIONS`. Both can be :ref:`overridden per scope <per-scope-backoff>` (through the http_codes and exceptions keys), so the same response or exception may trigger backoff for one scope and not for another. On each trigger:

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The delay grows exponentially:
```
delay = min(BACKOFF_MAX_DELAY, max(BACKOFF_MIN_DELAY, delay * BACKOFF_DELAY_FACTOR))
```
:setting:`BACKOFF_JITTER` is then applied so that requests that backed off together do not retry in lockstep.

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If the response carries a :ref:`Retry-After or RateLimit-Reset <rate-limiting-headers>` value, the scope is also held back until that time (capped at :setting:`BACKOFF_MAX_DELAY`) before its next request. This is a one-time gate, on top of the exponential step above: it honors the header for the next request without turning a short header value into a long-standing delay for every later request.

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Recovery is linear: after a scope goes a full :setting:`BACKOFF_WINDOW` without a new trigger, its delay drops by one :setting:`BACKOFF_DELAY_FACTOR` step toward the configured value, and keeps dropping one step per quiet window until it is back to the configured value. A new trigger resets the countdown.

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This exponential-increase / linear-decrease pattern, similar to TCP congestion control, makes a scope back off quickly when a server is unhappy and return to full speed gradually once it recovers. To keep a scope hovering around a target rate instead of repeatedly probing and backing off, enable :ref:`rampup <rampup>`.

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Backoff triggers are detected by the :class:`~scrapy.downloadermiddlewares.backoff.BackoffMiddleware`, a built-in :ref:`downloader middleware <topics-downloader-middleware>` enabled by default. Any component can also trigger backoff programmatically for arbitrary scopes — e.g. based on the response body of a specific site — through :meth:`crawler.throttler.back_off() <scrapy.throttling.ThrottlingManagerProtocol.back_off>`.

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Per-scope backoff configuration

The global BACKOFF_* settings can be overridden per scope with the "backoff" key of a :setting:`THROTTLING_SCOPES` entry, an instance of :class:`~scrapy.throttling.BackoffConfig`:

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.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "example.com": {
            "backoff": {
                "http_codes": [429, 503],
                "exceptions": ["builtins.IOError"],
                "delay_factor": 1.2,
                "max_delay": 180.0,
                "min_delay": 5.0,
                "jitter": [0.01, 0.33],
            },
        },
    }

Every key overrides the matching global BACKOFF_* setting for that scope (http_codes overrides :setting:`BACKOFF_HTTP_CODES`, exceptions overrides :setting:`BACKOFF_EXCEPTIONS`, delay_factor overrides :setting:`BACKOFF_DELAY_FACTOR`, and so on), and any key left out falls back to it. So a scope can, for example, treat an extra status code as a backoff trigger, or stop treating one of the defaults as a trigger, independently of every other scope.

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Rampup

When using APIs that charge per request, like web scraping APIs, you often want to maximize throughput while staying within rate limits. To do that, set "rampup" to True in :setting:`THROTTLING_SCOPES`:

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.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "api.toscrape.com": {
            "rampup": True,
        },
    }

Rampup increases concurrency or lowers delay as needed based on the following setting:

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```
.. setting:: RAMPUP_BACKOFF_TARGET
```
:setting:`RAMPUP_BACKOFF_TARGET` (default: 1)

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Target number of backoff responses per :setting:`BACKOFF_WINDOW` that :ref:`rampup <rampup>` aims for when probing the rate limit of a scope.

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For every :setting:`BACKOFF_WINDOW` that stays below :setting:`RAMPUP_BACKOFF_TARGET` backoff triggers, rampup increases throughput one step: it first lowers the delay, and once the delay reaches its minimum it raises the concurrency limit of the scope. Windows that reach or exceed the target do not ramp up; the rate is reduced by normal :ref:`backoff <backoff>` (which grows the delay) instead. Rampup only ever probes upward, so the rate settles around the most throughput a scope allows while triggering fewer than :setting:`RAMPUP_BACKOFF_TARGET` rate-limit responses per window.

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Rampup behavior can be fine-tuned per scope by giving "rampup" a dict instead of True:

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.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "api.toscrape.com": {
            "rampup": {
                "backoff_target": 1,  # overrides RAMPUP_BACKOFF_TARGET
                "delay_factor": 0.5,  # multiply the delay by this on each ramp-up step
                "min_delay": 0.05,  # do not ramp the delay below this
            },
        },
    }

Rate limiting headers

Servers may include Retry-After or RateLimit-Reset headers to indicate when you should make your next request. These headers are respected automatically during :ref:`backoff <backoff>`: the scope's next request is held back until the indicated time (capped at :setting:`BACKOFF_MAX_DELAY`), on top of the usual exponential backoff step.

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.. seealso:: :setting:`REDIRECT_MAX_DELAY`

robots.txt

Crawl-Delay is a non-standard robots.txt directive that indicates a number of seconds to wait between requests.

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.. setting:: THROTTLING_ROBOTSTXT_OBEY

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.. setting:: THROTTLING_ROBOTSTXT_MAX_DELAY

If :setting:`ROBOTSTXT_OBEY` and :setting:`THROTTLING_ROBOTSTXT_OBEY` are True (default), valid Crawl-Delay directives override :setting:`THROTTLING_SCOPE_CONCURRENCY` and :setting:`DOWNLOAD_DELAY`. Concurrency is set to 1 and the delay is raised to at least the Crawl-Delay value (a larger configured delay is kept), capped at :setting:`THROTTLING_ROBOTSTXT_MAX_DELAY` (default: 60.0).

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If :setting:`THROTTLING_SCOPES` defines a different concurrency or delay, it will be respected, but a warning will be logged about the discrepancy with Crawl-Delay. Set ignore_robots_txt to True to silence this warning.

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Delaying a scope programmatically

You can delay a :ref:`throttling scope <throttling-scopes>` on demand through :meth:`crawler.throttler.delay_scope() <scrapy.throttling.ThrottlingManagerProtocol.delay_scope>`:

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.. code-block:: python

    crawler.throttler.delay_scope("example.com", 30.0)

This holds back the scope's next request for at least the given number of seconds and registers a :ref:`backoff <backoff>` trigger. Like a Retry-After header, it is a one-time delay rather than a permanent one (the scope's delay also grows by one backoff step and then recovers); call it again, e.g. on each matching response, to keep a scope slowed down for longer.

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It is useful to react to situations that :ref:`automatic backoff <backoff>` cannot detect on its own, such as a soft block that comes back as a 200 response. For example, a spider callback can slow down the whole domain when it detects a maintenance page, and reschedule the current request:

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.. code-block:: python

    from scrapy import Request, Spider
    from scrapy.utils.httpobj import urlparse_cached


    class MySpider(Spider):
        name = "myspider"
        start_urls = ["https://example.com/"]

        def parse(self, response):
            if "under maintenance" in response.text:
                scope = urlparse_cached(response).netloc
                self.crawler.throttler.delay_scope(scope, 600.0)
                yield response.request.replace(dont_filter=True)
                return
            # Normal parsing follows.

Unlike :ref:`untrusted delays <rate-limiting-headers>`, this delay is not capped at :setting:`BACKOFF_MAX_DELAY`.

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Per-request throttling

Sometimes you need different throttling behavior for individual requests or for request groups that are not tied to a specific domain.

For example, you might want to throttle API endpoints differently than web pages on the same domain, group requests by content type (images vs HTML), or apply different throttling based on request priority.

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.. reqmeta:: throttling_scopes

Use the throttling_scopes request metadata to assign requests to custom throttling groups:

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.. code-block:: python

    Request("https://api.example/", meta={"throttling_scopes": "api"})

You can also assign multiple throttling groups to a single request:

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.. code-block:: python

    Request("https://api.example/users", meta={"throttling_scopes": {"api", "users"}})

You can then use the :setting:`THROTTLING_SCOPES` setting to customize throttling for such requests:

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.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "api": {"concurrency": 2},
        "users": {"delay": 5.0},
    }

Note

These custom throttling groups persist through redirects. For redirect-aware throttling assignment, see :ref:`custom-throttling-scopes`.

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.. reqmeta:: throttling_delay

Delaying a single request

To hold a single request for a fixed number of seconds before it is sent, regardless of its scopes, set the throttling_delay request metadata key:

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.. code-block:: python

    Request("https://example.com/slow", meta={"throttling_delay": 5.0})

The delay is applied once, the first time the request reaches the throttling gate.

throttling_delay defines only the earliest time the request may be sent, not the exact time: once the delay elapses, the request still competes with every other pending request for its scopes. If you want it sent as soon as its delay elapses, give it a higher :attr:`~scrapy.Request.priority` too:

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.. code-block:: python

    Request("https://example.com/slow", meta={"throttling_delay": 5.0}, priority=1)

Without a higher priority, a backlog of requests ahead of it in a FIFO queue could keep it waiting well past the configured delay; a higher priority puts it at the front of the queue, so it goes out right after its delay.

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.. reqmeta:: dont_throttle

Excluding a request from throttling state

Some requests (authentication flows, one-off API calls, file downloads) should not influence throttling state even if they get a :setting:`BACKOFF_HTTP_CODES` response or raise a :setting:`BACKOFF_EXCEPTIONS` exception. Set the :reqmeta:`dont_throttle` request metadata key to True to process such a request normally without letting its outcome trigger :ref:`backoff <backoff>`:

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.. code-block:: python

    Request("https://example.com/login", meta={"dont_throttle": True})

Throttling scopes

Throttling scopes represent aspects of requests that can be throttled independently.

Customizing throttling scopes

There are 2 ways to customize throttling scopes.

To configure existing scopes, use the :setting:`THROTTLING_SCOPES` setting. Its keys are scope names and its values are :class:`~scrapy.throttling.ThrottlingScopeConfig` dicts, which accept the following keys:

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concurrency (:class:`int`): System Message: ERROR/3 (<stdin>, line 470); backlink
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Maximum number of concurrent requests for the scope. Defaults to :setting:`THROTTLING_SCOPE_CONCURRENCY`.

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delay (:class:`float`): System Message: ERROR/3 (<stdin>, line 474); backlink
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Minimum seconds between requests for the scope. Defaults to :setting:`DOWNLOAD_DELAY`.

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jitter (:class:`float` or 2-:class:`list`): System Message: ERROR/3 (<stdin>, line 480); backlink
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Magnitude of the random variation applied to delay; the per-scope override of :setting:`RANDOMIZE_DOWNLOAD_DELAY`. 0 disables it, 0.5 means ±50% (the default when :setting:`RANDOMIZE_DOWNLOAD_DELAY` is on), and a [low, high] pair multiplies the delay by 1 + uniform(low, high).

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quota (:class:`float`): System Message: ERROR/3 (<stdin>, line 483); backlink
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Maximum :ref:`quota <throttling-quotas>` consumed per window.

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window (:class:`float`): System Message: ERROR/3 (<stdin>, line 486); backlink
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Quota window in seconds. Defaults to :setting:`THROTTLING_WINDOW`.

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rampup (:class:`bool` or :class:`dict`): System Message: ERROR/3 (<stdin>, line 489); backlink
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Enables :ref:`rampup <rampup>` for the scope.

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backoff (:class:`~scrapy.throttling.BackoffConfig`): System Message: ERROR/3 (<stdin>, line 492); backlink
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Per-scope :ref:`backoff overrides <per-scope-backoff>`.

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manager (:class:`str` or :class:`type`): System Message: ERROR/3 (<stdin>, line 496); backlink
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Import path or class of a :ref:`custom scope manager <custom-throttling-scope-managers>` for the scope.

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ignore_robots_txt (:class:`bool`): System Message: ERROR/3 (<stdin>, line 500); backlink
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Silences the warning logged when this configuration is more aggressive than a :ref:`robots.txt Crawl-delay <crawl-delay>`.

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.. setting:: THROTTLING_MANAGER

To change how scopes are assigned (or anything beyond per-scope settings), set :setting:`THROTTLING_MANAGER` (default: :class:`~scrapy.throttling.ThrottlingManager`) to a :ref:`component <topics-components>` that implements the :class:`~scrapy.throttling.ThrottlingManagerProtocol` protocol (or its import path as a string):

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.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_MANAGER = "myproject.throttling.MyThrottlingManager"

Handling of multiple throttling scopes

When a request has multiple throttling scopes, it is not sent until all of its throttling scopes allow it.

Throttling quotas

When different requests can consume different amounts of a throttling scope, you can express this using throttling quotas.

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.. setting:: THROTTLING_WINDOW

Use the :setting:`THROTTLING_WINDOW` setting (default: 60.0) or the "window" key in the :setting:`THROTTLING_SCOPES` setting to define the time window after which throttling quotas are reset.

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Then use the :setting:`THROTTLING_SCOPES` setting to define the throttling quotas for each throttling scope:

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.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "api.toscrape.com": {
            "quota": 500.0,
        },
    }

Then, in the :reqmeta:`throttling_scopes` request metadata key or in the return value of the :meth:`~scrapy.throttling.ThrottlingManagerProtocol.get_scopes` method, define a :class:`dict` where keys are throttling scopes and values are :class:`float` values that indicate the expected quota consumption (it does not need to be exact).

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Customizing throttling scope managers

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.. setting:: THROTTLING_SCOPE_MANAGER

The :setting:`THROTTLING_SCOPE_MANAGER` setting (default: :class:`~scrapy.throttling.ThrottlingScopeManager`) is a :ref:`component <topics-components>` that implements the :class:`~scrapy.throttling.ThrottlingScopeManagerProtocol` (or its import path as a string):

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.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPE_MANAGER = "myproject.throttling.MyThrottlingScopeManager"

For each throttling scope, an instance of this class is created to manage any gradual :ref:`backoff <backoff>` or :ref:`rampup <rampup>` required at run time.

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You can implement your own throttling scope manager if you wish to change the backoff or rampup behavior beyond what settings allow.

You can also define a custom throttling scope manager for a specific throttling scope by setting the "manager" key in the :setting:`THROTTLING_SCOPES` setting:

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.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "api.toscrape.com": {
            "manager": "myproject.throttling.MyThrottlingScopeManager",
        },
    }

Most custom scope managers subclass the default :class:`~scrapy.throttling.ThrottlingScopeManager` and override only the methods whose behavior they want to change; implementing the :class:`~scrapy.throttling.ThrottlingScopeManagerProtocol` from scratch is also supported. For example, this manager disables exponential :ref:`backoff <backoff>`, so a scope relies solely on its configured delay and quota:

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.. code-block:: python
    :caption: ``myproject/throttling.py``

    from scrapy.throttling import ThrottlingScopeManager


    class FixedWindowScopeManager(ThrottlingScopeManager):
        def record_backoff(self, *args, **kwargs):
            pass  # never back off

Throttling-aware scheduling

By default, throttling is enforced at the engine, where a request waiting on its :ref:`throttling scopes <throttling-scopes>` holds a concurrency slot. In a crawl that mixes heavily-throttled scopes with unthrottled ones, this can let throttled requests starve unthrottled ones that could be sent right away (head-of-line blocking; Scrapy logs a warning, see :setting:`DELAYED_REQUESTS_WARN_THRESHOLD`).

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:class:`~scrapy.core.scheduler.ThrottlingAwareScheduler` avoids this. To enable it:

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.. code-block:: python
    :caption: ``settings.py``

    SCHEDULER = "scrapy.core.scheduler.ThrottlingAwareScheduler"
    SCHEDULER_PRIORITY_QUEUE = "scrapy.pqueues.ThrottlingAwarePriorityQueue"

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.. autoclass:: scrapy.core.scheduler.ThrottlingAwareScheduler

Examples

Alternative domain throttling

If you are not happy with the :ref:`default throttling scope behavior <basic-throttling>` with regards to domains and subdomains, you can change it.

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Alternative approaches include:

Using the highest-level registrable domain as the throttling scope, e.g. https://books.toscrape.com and https://toscrape.com both get a toscrape.com throttling scope.

This allows to apply the same throttling settings to all subdomains of a registrable domain.

For example:

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.. code-block:: python
    :caption: ``settings.py``

    import tldextract
    from scrapy.throttling import ThrottlingManager, scope_cache
    from scrapy.utils.httpobj import urlparse_cached


    class MyThrottlingManager(ThrottlingManager):
        @scope_cache
        async def get_scopes(self, request):
            extracted = tldextract.extract(request.url)
            if extracted.domain and extracted.suffix:
                return f"{extracted.domain}.{extracted.suffix}"
            return urlparse_cached(request).netloc


    THROTTLING_MANAGER = MyThrottlingManager

Using multiple throttling scopes per request, one per registrable domain and for every higher-level subdomain, e.g. https://books.toscrape.com and https://toscrape.com both get a toscrape.com throttling scope, but https://books.toscrape.com also gets a books.toscrape.com throttling scope.

This allows to apply the same throttling settings to all subdomains of a registrable domain, but also allows applying further restrictions on each or on some subdomains.

For example:

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.. code-block:: python
    :caption: ``settings.py``

    import tldextract
    from scrapy.throttling import ThrottlingManager, scope_cache
    from scrapy.utils.httpobj import urlparse_cached


    class MyThrottlingManager(ThrottlingManager):
        @scope_cache
        async def get_scopes(self, request):
            extracted = tldextract.extract(request.url)
            if not (extracted.domain and extracted.suffix):
                return urlparse_cached(request).netloc
            scopes = set()
            registrable_domain = f"{extracted.domain}.{extracted.suffix}"
            scopes.add(registrable_domain)
            if extracted.subdomain:
                subdomain_parts = extracted.subdomain.split(".")
                for i in range(len(subdomain_parts)):
                    subdomain = ".".join(subdomain_parts[i:])
                    full_domain = f"{subdomain}.{registrable_domain}"
                    scopes.add(full_domain)
            return scopes


    THROTTLING_MANAGER = MyThrottlingManager
    THROTTLING_SCOPES = {
        "toscrape.com": {"concurrency": 32},
        "books.toscrape.com": {"concurrency": 24},
        "quotes.toscrape.com": {"concurrency": 16},
    }

Here books.toscrape.com requests can reach 24 concurrency and quotes.toscrape.com requests can reach 16 concurrency, but never both at the same time, because that would sum 40 concurrency, and toscrape.com requests are limited to 32.

Endpoint-specific throttling

To apply different throttling settings to different endpoints of the same domain and not enforce any common throttling, effectively treating them as different domains:

Implement a :ref:`throttling manager <custom-throttling-scopes>` that sets endpoint-specific throttling scopes for that domain:

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.. code-block:: python

    from scrapy.throttling import ThrottlingManager, scope_cache
    from scrapy.utils.httpobj import urlparse_cached


    class MyThrottlingManager(ThrottlingManager):
        @scope_cache
        async def get_scopes(self, request):
            parsed_url = urlparse_cached(request)
            if parsed_url.netloc != "api.toscrape.com":
                return await super().get_scopes(request)
            return f"{parsed_url.netloc}{parsed_url.path}"

Use the :setting:`THROTTLING_SCOPES` setting to set different throttling settings per endpoint:

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```
.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "api.toscrape.com/fast-endpoint": {"concurrency": 1000, "delay": 0.08},
        "api.toscrape.com/slow-endpoint": {"delay": 5.0},
    }
```

Web scraping API throttling

Imagine you are sending requests to a web scraping API, e.g. to avoid bans. Unless that API provides a Scrapy plugin to make it easier to use, you may want to:

Use the :setting:`THROTTLING_SCOPES` setting to increase concurrency for API requests. For example:

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```
.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "api.toscrape.com": {"concurrency": 1000, "delay": 0.08},
    }
```

Implement a :ref:`throttling manager <custom-throttling-scopes>` that:

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Adds a throttling scope for the URL being scraped.

For example, if you request https://api.toscrape.com/?url=https://example.com, by default it will get a api.toscrape.com throttling scope, but it should also get the example.com throttling scope:

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.. code-block:: python

    from urllib.parse import urlparse

    from scrapy.throttling import add_scope, ThrottlingManager, scope_cache
    from scrapy.utils.httpobj import urlparse_cached
    from w3lib.url import url_query_parameter


    class MyThrottlingManager(ThrottlingManager):
        @scope_cache
        async def get_scopes(self, request):
            scopes = await super().get_scopes(request)
            if urlparse_cached(request).netloc != "api.toscrape.com":
                return scopes
            target_url = url_query_parameter(request.url, "url")
            if not target_url:
                return scopes
            target_domain = urlparse(target_url).netloc
            return add_scope(scopes, target_domain)

Add a :ref:`downloader middleware <topics-downloader-middleware>` that differentiates between exhaustion of the target website and exhaustion of the API itself. The API returns 200 even when the target website rate-limits it, reporting the upstream status in a header; the middleware backs off the target-website scope (not the API scope) in that case, reusing the scope's own :meth:`~scrapy.throttling.ThrottlingScopeManagerProtocol.triggers_backoff_for_status` check:

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.. code-block:: python

    from scrapy.throttling import iter_scopes
    from scrapy.utils.httpobj import urlparse_cached


    class UpstreamBackoffMiddleware:
        def __init__(self, crawler):
            self.throttler = crawler.throttler

        @classmethod
        def from_crawler(cls, crawler):
            return cls(crawler)

        def process_response(self, request, response, spider):
            if urlparse_cached(request).netloc == "api.toscrape.com":
                upstream_status = int(
                    response.headers.get("X-Upstream-Status-Code", b"200")
                )
                scopes = [
                    scope
                    for scope in iter_scopes(self.throttler.get_resolved_scopes(request))
                    if scope != "api.toscrape.com"
                    and self.throttler.get_scope_manager(scope).triggers_backoff_for_status(
                        upstream_status
                    )
                ]
                self.throttler.back_off(scopes)
            return response

Cost-capped throttling

Imagine you are using an API that charges different requests differently, e.g. based on the features used, and you want to limit how much you spend per time window (:setting:`THROTTLING_WINDOW`). You can use :ref:`throttling quotas <throttling-quotas>` for that:

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Implement a :ref:`throttling manager <custom-throttling-scopes>` that:

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Sets a cost throttling scope on each request to some estimation based e.g. on request URL parameters:

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.. code-block:: python

    from scrapy.utils.httpobj import urlparse_cached
    from scrapy.throttling import ThrottlingManager, scope_cache


    class MyThrottlingManager(ThrottlingManager):
        @scope_cache
        async def get_scopes(self, request):
            scopes = await super().get_scopes(request)
            parsed_url = urlparse_cached(request)
            if parsed_url.netloc != "api.toscrape.com":
                return scopes
            return add_scope(scopes, "cost", estimate_request_cost(request))

Add a :ref:`downloader middleware <topics-downloader-middleware>` that reconciles the estimated cost with the actual cost reported by the response, so that the quota tracks real spending:

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.. code-block:: python

    class CostReconcileMiddleware:
        def __init__(self, crawler):
            self.throttler = crawler.throttler

        @classmethod
        def from_crawler(cls, crawler):
            return cls(crawler)

        def process_response(self, request, response, spider):
            if response.headers.get("X-Actual-Cost") is not None:
                estimated = estimate_request_cost(request)
                actual = float(response.headers[b"X-Actual-Cost"])
                # Report the difference between actual and estimated cost.
                self.throttler.reconcile_quota("cost", consumed=actual - estimated)
            return response

Use the :setting:`THROTTLING_SCOPES` setting to set a maximum cost per time window:

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```
.. code-block:: python
    :caption: ``settings.py``

    THROTTLING_SCOPES = {
        "cost": {"quota": 100.0},
    }
```
This will allow you to spend up to 100.0 units of cost per time window (default: 60 seconds) before throttling kicks in.

Per-IP concurrency limiting

A concurrency limit keyed by IP is just a throttling scope whose id is the request's IP, with a concurrency limit. A request then carries two scopes, its domain and its IP, and is only sent when both allow it (see :ref:`multiple-throttling-scopes`).

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Implement a :ref:`throttling manager <custom-throttling-scopes>` that adds the request's IP as a second scope:

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.. code-block:: python

    import socket

    from scrapy.throttling import ThrottlingManager, add_scope, scope_cache
    from scrapy.utils.asyncio import run_in_thread
    from scrapy.utils.httpobj import urlparse_cached


    class IPThrottlingManager(ThrottlingManager):
        @scope_cache
        async def get_scopes(self, request):
            scopes = await super().get_scopes(request)
            host = urlparse_cached(request).hostname
            address = await run_in_thread(socket.gethostbyname, host)
            return add_scope(scopes, address)

Additional settings

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```
.. setting:: BACKOFF_EXCEPTIONS
```
:setting:`BACKOFF_EXCEPTIONS`

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Default:
- :exc:`~scrapy.exceptions.DownloadFailedError`
  
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- :exc:`~scrapy.exceptions.DownloadTimeoutError`
  
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- :exc:`~scrapy.exceptions.ResponseDataLossError`
  
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List of exception classes that trigger backoff when raised while downloading a request. Strings are interpreted as import paths. Can be overridden per scope with the exceptions key (see :ref:`per-scope-backoff`).

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```
.. seealso:: :setting:`RETRY_EXCEPTIONS`
```
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```
.. setting:: BACKOFF_JITTER
```
:setting:`BACKOFF_JITTER` (default: 0.1)

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Random jitter applied to each backoff delay, as a fraction of the delay. With the default value of 0.1 the delay is randomized by ±10%. Overrides :setting:`RANDOMIZE_DOWNLOAD_DELAY` during backoff.

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```
.. setting:: BACKOFF_MIN_DELAY
```
:setting:`BACKOFF_MIN_DELAY` (default: 1.0)

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Delay, in seconds, applied on the first backoff step (and the minimum delay during backoff).
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```
.. setting:: BACKOFF_WINDOW
```
:setting:`BACKOFF_WINDOW` (default: 60.0)

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Time window, in seconds, used by :ref:`backoff <backoff>` and :ref:`rampup <rampup>`. During backoff, a :ref:`throttling scope <throttling-scopes>` must go this many seconds without a new backoff trigger (an HTTP error code from :setting:`BACKOFF_HTTP_CODES` or an exception from :setting:`BACKOFF_EXCEPTIONS`) before its delay decreases by one :setting:`BACKOFF_DELAY_FACTOR` step toward the configured value. A new trigger resets the countdown.

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```
.. setting:: DELAYED_REQUESTS_WARN_THRESHOLD
```
:setting:`DELAYED_REQUESTS_WARN_THRESHOLD` (default: 500)

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Number of requests held back by :ref:`throttling <throttling>` at which Scrapy logs a warning, to help detect throttling configurations that hold back more requests than expected.

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While throttled, requests in the :ref:`scheduler <topics-scheduler>` remain in the scheduler. However, requests sent with :meth:`engine.download() <scrapy.core.engine.ExecutionEngine.download>` bypass the scheduler, including requests sent by some built-in :ref:`components <topics-components>` and :ref:`inline requests <inline-requests>`. When such requests are throttled, they are paused and kept in memory, along with any run time context from the code that is sending them. If they accumulate, they can become a memory issue.

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```
.. setting:: RANDOMIZE_DOWNLOAD_DELAY
```
:setting:`RANDOMIZE_DOWNLOAD_DELAY` (default: True)

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Randomize delays by this factor, e.g. if 0.2 randomize delays between delay*0.8 and delay*1.2.

It can be set to a 2-item list with low and high factors, e.g. [-0.1, 0.3] to randomize delays between delay*0.9 and delay*1.3.

If True, 0.5 (i.e. ±50%) is used as the randomization factor. If False, no randomization is applied.
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```
.. setting:: THROTTLING_DEBUG
```
:setting:`THROTTLING_DEBUG` (default: False)

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Whether to log :ref:`throttling <throttling>` decisions (per-scope delays, backoff steps and recoveries) for debugging.

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```
.. setting:: THROTTLING_SCOPE_LIMIT
```
:setting:`THROTTLING_SCOPE_LIMIT` (default: 100000)

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Maximum number of :ref:`throttling scope <throttling-scopes>` states kept in memory at once, to bound memory usage on broad crawls that touch a large number of scopes (e.g. domains).

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When the limit is exceeded, the least-recently-used idle scopes are evicted (an evicted scope is recreated from its configuration the next time it is needed). Scopes with in-flight requests or in active backoff are never evicted, so the limit may be temporarily exceeded if that many scopes are busy at once. Set to 0 to disable the limit.

This complements :setting:`THROTTLING_SCOPE_MAX_IDLE`, which evicts scopes by inactivity time rather than by count.

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```
.. setting:: THROTTLING_SCOPE_MAX_IDLE
```
:setting:`THROTTLING_SCOPE_MAX_IDLE` (default: 3600.0)

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Seconds of inactivity after which the state of a :ref:`throttling scope <throttling-scopes>` is evicted from memory to bound memory usage on long-running crawls. Set to 0 to never evict. Scopes in active backoff are never evicted.

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API

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.. autoclass:: scrapy.throttling.ThrottlingManagerProtocol
    :members:
    :member-order: bysource

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.. autoclass:: scrapy.throttling.ThrottlingManager

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.. autoclass:: scrapy.throttling.ThrottlingScopeManagerProtocol
    :members:
    :member-order: bysource

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.. autoclass:: scrapy.throttling.ThrottlingScopeManager

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Unknown directive type "autoclass".

.. autoclass:: scrapy.pqueues.ThrottlingAwarePriorityQueue

System Message: ERROR/3 (<stdin>, line 1079)

Unknown directive type "autoclass".

.. autoclass:: scrapy.throttling.ThrottlingScopeConfig

System Message: ERROR/3 (<stdin>, line 1081)

Unknown directive type "autoclass".

.. autoclass:: scrapy.throttling.BackoffConfig

System Message: ERROR/3 (<stdin>, line 1083)

Unknown directive type "autoclass".

.. autoclass:: scrapy.throttling.RampupConfig

System Message: ERROR/3 (<stdin>, line 1085)

Unknown directive type "autofunction".

.. autofunction:: scrapy.throttling.scope_cache

System Message: ERROR/3 (<stdin>, line 1086)

Unknown directive type "autofunction".

.. autofunction:: scrapy.throttling.add_scope

System Message: ERROR/3 (<stdin>, line 1087)

Unknown directive type "autofunction".

.. autofunction:: scrapy.throttling.iter_scopes

</html>

38 KiB Raw Blame History Unescape Escape

Throttling

Concurrency and delay

Per-domain throttling

Backoff

How backoff works

Per-scope backoff configuration

Rampup

Rate limiting headers

robots.txt

Delaying a scope programmatically

Per-request throttling

Delaying a single request

Excluding a request from throttling state

Throttling scopes

Customizing throttling scopes

Handling of multiple throttling scopes

Throttling quotas

Customizing throttling scope managers

Throttling-aware scheduling

Examples

Alternative domain throttling

Endpoint-specific throttling

Web scraping API throttling

Cost-capped throttling

Per-IP concurrency limiting

Additional settings

API

38 KiB

Raw Blame History