# AutoThrottle extension

This is an extension for automatically throttling crawling speed based on load of both the Scrapy server and the website you are crawling.

## Design goals

1. be nicer to sites instead of using default download delay of zero
2. automatically adjust scrapy to the optimum crawling speed, so the user doesn’t have to tune the download delays and concurrent requests to find the optimum one. The user only needs to specify the maximum concurrent requests it allows, and the extension does the rest.

## How it works

In Scrapy, the download latency is measured as the time elapsed between establishing the TCP connection and receiving the HTTP headers.

Note that these latencies are very hard to measure accurately in a cooperative multitasking environment because Scrapy may be busy processing a spider callback, for example, and unable to attend downloads. However, these latencies should still give a reasonable estimate of how busy Scrapy (and ultimately, the server) is, and this extension builds on that premise.

## Throttling algorithm

This adjusts download delays and concurrency based on the following rules:

1. spiders always start with one concurrent request and a download delay of `AUTOTHROTTLE_START_DELAY`
2. when a response is received, the download delay is adjusted to the average of previous download delay and the latency of the response.

Note

The AutoThrottle extension honours the standard Scrapy settings for concurrency and delay. This means that it will never set a download delay lower than `DOWNLOAD_DELAY` or a concurrency higher than `CONCURRENT_REQUESTS_PER_DOMAIN` (or `CONCURRENT_REQUESTS_PER_IP`, depending on which one you use).

## Settings

The settings used to control the AutoThrottle extension are:

- `AUTOTHROTTLE_ENABLED`
- `AUTOTHROTTLE_START_DELAY`
- `AUTOTHROTTLE_MAX_DELAY`
- `AUTOTHROTTLE_DEBUG`
- `CONCURRENT_REQUESTS_PER_DOMAIN`
- `CONCURRENT_REQUESTS_PER_IP`
- `DOWNLOAD_DELAY`

For more information see Throttling algorithm.

### AUTOTHROTTLE_ENABLED

Default: `False`

Enables the AutoThrottle extension.

### AUTOTHROTTLE_START_DELAY

Default: `5.0`

The initial download delay (in seconds).

### AUTOTHROTTLE_MAX_DELAY

Default: `60.0`

The maximum download delay (in seconds) to be set in case of high latencies.

### AUTOTHROTTLE_DEBUG

Default: `False`

Enable AutoThrottle debug mode which will display stats on every response received, so you can see how the throttling parameters are being adjusted in real time.