Crawler best practices

Introduction Automatic clients, colloquially referred to as crawlers and bots, are used to access web resources, including indexing for search engines or, more recently, training data for new artificial intelligence (AI) applications. As crawling activity increases, automatic clients must behave appropriately and respect the constraints of the resources they access. This includes clearly documenting how they can be identified and how their behavior can be influenced. Therefore, crawler operators are asked to follow the best practices for crawling outlined in this document. For the purposes of this document, a crawler is an automated HTTP client that retrieves resources across one or more web sites without direct human initiation of individual requests. A crawler discovers URIs during retrieval and schedules them for later processing. It relies on algorithmic prioritization and protocol-level instructions such as the Robots Exclusion Protocol to govern its behavior. To further assist website owners, it should also be considered to create a central registry where website owners can look up well-behaved crawlers. Note that while self-declared research crawlers, including privacy and malware discovery crawlers, and contractual crawlers are welcome to adopt these practices, due to the nature of their relationship with sites, they may exempt themselves from any of the Crawler Best Practices with a documented rationale.

Recommended Best Practices The following best practices SHOULD be followed and are already applied by a vast majority of large-scale crawlers on the Internet:

Crawlers MUST support and respect the Robots Exclusion Protocol.
Crawlers MUST be easily identifiable through their user agent string.
Crawlers MUST not interfere with the regular operation of a site.
Crawlers MUST support caching directives.
Crawlers MUST expose the ranges they are crawling from in a standardized format.
Crawlers MUST expose a page that explains how the crawling can be blocked, whether the page is rendered, and how the crawled data is used.

Crawlers MUST respect the Robots Exclusion Protocol All well behaved-crawlers MUST support the REP as defined in to allow site owners to opt out from crawling. Especially if the website chooses not to use a robots.txt file as defined by the REP, crawlers further need to respect the X-robots-tag in the HTTP header.

Crawlers MUST be easily identifiable through their user agent string As outlined in (Robots Exclusion Protocol; REP), the HTTP request header User-Agent SHOULD clearly identify the crawler, usually by including a URL that hosts the crawler's description. For example: This is already a widely accepted practice among crawler operators. To remain compliant, crawler operators MUST include unique identifiers for their crawlers in the case-insensitive User-Agent header, such as "contains 'googlebot' and 'https://url/...'". Additionally, the name SHOULD clearly identify both the crawler owner and its purpose as much as reasonably possible.

Crawlers MUST not interfere with the normal operation of a site Depending on a site's setup (computing resources and software efficiency) and its size, crawling may slow down the site or even take it offline altogether. Crawler operators MUST ensure that their crawlers are equipped with back-out logic that relies on at least the standard signals defined by , preferably also additional heuristics such as a change in the relative response time of the server. Therefore, crawlers SHOULD log already visited URLs, the number of requests sent to each resource, and the respective HTTP status codes in the responses, especially if errors occur, to prevent repeatedly crawling the same source. Using the same data, crawlers SHOULD, on a best effort basis, crawl the site at times of the day when the site is estimated to have fewer human visitors. Generally, crawlers SHOULD avoid sending multiple requests to the same resources at the same time and SHOULD limit the crawling speed to prevent server overload, if possible, following the limits outlined in . Additionally, resources SHOULD NOT be re-crawled too often. Ideally, crawlers SHOULD restrict the depth of crawling and the number of requests per resource to prevent loops. Crawlers SHOULD NOT attempt to bypass authentication or other access restrictions, such as when login is required, CAPTCHAs are in use, or content is behind a paywall, unless explicitly agreed upon with the website owner. Crawlers SHOULD primarily access resources using HTTP GET requests, resorting to other methods (e.g., POST, PUT) only if there is a prior agreement with the publisher or if the publisher's content management system automatically makes those calls through client side code (e.g., JavaScript). Generally, the load caused by executing client side code should be carefully considered or even avoided whenever possible.

Crawlers MUST support caching directives HTTP caching removes the need of repeated access from crawlers to the same URL.

Crawlers MUST expose the IP ranges they use for crawling To complement the , crawler operators SHOULD publish the IP ranges they have allocated for crawling in format, and keep this information reasonably up-to-date, according to the specification. The resource containing the IP addresses MUST be linked from the page describing the crawler using the client-ranges relation. To facilitate efficient machine discovery. This relation SHOULD be provided via an HTTP Link header or as a <link> element in the page's HTML metadata section (as defined in ). For example: ]]>

Crawlers MUST explain how the crawled data is used and how the crawler can be blocked Crawlers MUST be easily identifiable through their user-agent string, and they SHOULD explain how the data they collect will be used. In practice, this is usually done via the documentation page linked in the crawler's user agent. Additionally, the documentation page SHOULD include a contact address for the crawler owner. The documentation SHOULD also provide an example file to block the crawler and a method for verifying files. If the crawler has exempted itself of these best practices, the documentation page SHOULD describe the reason for that. All endpoints hosting identification, documentation, and IP range data must be publicly and highly available, and served with minimal latency for programmatic access.