Web crawler reference

A crawl is a process, associated with an engine, by which the web crawler discovers web content, and extracts and indexes that content into the engine as search documents.

During a crawl, the web crawler stays within user-defined domains, starting from specific entry points, and it discovers additional content according to crawl rules. Operators and administrators manage crawls through the App Search dashboard.

Each crawl also respects instructions within robots.txt files and sitemaps, and instructions embedded within HTML files using canonical link tags, robots meta tags, and nofollow links.

Operators can monitor crawls using the ID and status of each crawl, and using the web crawler events logs.

Each engine may run one active crawl at a time, but multiple engines can run crawls concurrently.

By default, each re-crawl is a full crawl. Partial crawls may be requested through the API.

To begin each crawl, the web crawler populates a crawl queue with the entry points for the crawl and the URLs provided by any sitemaps. It then begins fetching and parsing each page in the queue. The crawler handles each page according to its HTTP response status code.

As the web crawler discovers additional URLs, it uses the crawl rules from the crawl configuration, the directives from any robots.txt files, and instructions embedded within the content to determine which of those URLs it is allowed to crawl. The crawler adds the allowed URLs to the crawl queue.

If a page is not linked from other pages, the web crawler will not discover it. Refer to the web of pages in the following image. The crawler will not discover the page outlined in red, unless an admin adds its URL as an entry point for the domain or includes the URL in a sitemap:

The web crawler continues fetching and adding to the crawl queue until the URL queue is empty, the crawler hits a resource limit, or the crawl fails unexpectedly.

The crawler logs detailed events while it crawls, which allow monitoring and auditing of the content discovery process.

Content extraction is the process where the web crawler transforms an HTML document into a search document. And indexing is the process where the crawler puts the search document into an App Search engine for searching.

During content extraction, the web crawler handles duplicate documents. The crawler indexes duplicate web documents as a single search document.

The web crawler extracts each web document into a predefined schema before indexing it as a search document. Custom field extraction is supported via meta tags and attributes.

During a crawl, the web crawler uses HTTP response status codes and robots meta tags to determine which documents it is allowed to index.

By default, the web crawler identifies groups of duplicate web documents and stores each group as a single App Search document within your engine. Within the App Search document, the fields url and additional_urls represent all the URLs where the web crawler discovered the document’s content (or a sample of URLs if more than 100). The url field represents the canonical URL, which you can explicitly manage using canonical URL link tags.

The crawler identifies duplicate content intelligently, ignoring insignificant differences such as navigation, whitespace, style, and scripts. More specifically, the crawler combines the values of specific fields, and it hashes the result to create a unique "fingerprint" to represent the content of the web document.

The web crawler then checks your engine for an existing document with the same content hash. If it doesn’t find one, it saves a new document to the engine. If it does exist, the crawler updates the existing document instead of saving a new one. The crawler adds to the document the additional URL at which the content was discovered.

You can manage which fields the web crawler uses to create the content hash. You can also disable this feature and allow duplicate documents.

Manage these settings for each domain within the web crawler UI. See Manage duplicate document handling.

Set the default fields for all domains using the following configuration setting: crawler.extraction.default_deduplication_fields.

Inject HTML data attributes into your web pages to instruct the web crawler to include or exclude particular sections from extracted content. For example, use this feature to exclude navigation and footer content when crawling, or to exclude sections of content only intended for screen readers.

For all pages that contain HTML tags with a data-elastic-include attribute, the crawler will only index content within those tags.

For all pages that contain HTML tags with a data-elastic-exclude attribute, the crawler will skip those tags from content extraction. You can nest data-elastic-include and data-elastic-exclude tags, too.

The web crawler will still crawl any links that appear inside excluded sections as long as the configured crawl rules allow them.

A simple content exclusion rule example:

<body>
  <p>This is your page content, which will be indexed by the App Search web crawler.
  <div data-elastic-exclude>Content in this div will be excluded from the search index</div>
</body>

In this more complex example with nested exclusion and inclusion rules, the web crawler will only extract "test1 test3 test5 test7" from the page.

<body>
  test1
  <div data-elastic-exclude>
    test2
    <p data-elastic-include>
      test3
      <span data-elastic-exclude>
        test4
        <span data-elastic-include>test5</span>
      </span>
    </p>
    test6
  </div>
  test7
</body>

The web crawler must also delete documents from an engine to keep its documents in sync with the corresponding web content.

During a crawl, the web crawler uses HTTP response status codes to determine which documents to delete. However, this process cannot delete stale documents in the engine that are no longer linked to on the web.

Therefore, at the conclusion of each crawl, the web crawler begins an additional "purge" crawl, which fetches URLs that exist in the engine, but have not been seen on the web during recent crawls. The crawler deletes from the engine all documents that respond with 4xx and 3xx responses during the phase.

You can purge documents on demand by using a process crawl. A process crawl removes from your engine all documents that are no longer allowed by your crawl rules.

A domain is a website or other internet realm that is the target of a crawl. Each domain belongs to a crawl, and each crawl has one or more domains.

See Manage domains to manage domains for a crawl.

A crawl always stays within its domains when discovering content. It cannot discover and index content outside of its domains.

Each domain has a domain URL that identifies the domain using a protocol and hostname. The domain URL must not include a path.

Each unique combination of protocol and hostname is a separate domain. Each of the following is its own domain:

Each domain has:

An entry point is a path within a domain that serves as a starting point for a crawl. Each entry point belongs to a domain, and each domain has one or more entry points.

See Manage entry points to manage entry points for a domain.

Use entry points to instruct the crawler to fetch URLs it would otherwise not discover. For example, if you’d like to index a page that isn’t linked from other pages, add the page’s URL as an entry point.

Ensure the entry points for each domain are allowed by the domain’s crawl rules, and the directives within the domain’s robots.txt file. The web crawler will not fetch entry points that are disallowed by crawl rules or robots.txt directives.

You can also inform the crawler of URLs using sitemaps.

A sitemap is an XML file, associated with a domain, that informs web crawlers about pages within that domain. XML elements within the sitemap identify specific URLs that are available for crawling.

These elements are similar to entry points. You can therefore choose to submit URLs to the Enterprise Search web crawler using sitemaps, entry points, or a combination of both.

You may prefer using sitemaps over entry points for any of the following reasons:

Use sitemaps to inform the web crawler of pages you think are important, or pages that are isolated and not linked from other pages. However, be aware the web crawler will visit only those pages from the sitemap that are allowed by the domain’s crawl rules and robots.txt file directives.

A crawl rule is a crawler instruction to allow or disallow specific paths within a domain. Each crawl rule belongs to a domain, and each domain has one or more crawl rules.

See Manage crawl rules to manage crawl rules for a domain. After modifying your crawl rules, you can re-apply the rules to your existing documents without waiting for a full re-crawl. See Process crawl.

During content discovery, the web crawler discovers new URLs and must determine which it is allowed to follow. Each URL has a domain (e.g. https://example.com) and a path (e.g. /category/clothing or /c/Credit_Center).

The web crawler looks up the crawl rules for the domain, and applies the path to the crawl rules to determine if the path is allowed or disallowed. The crawler evaluates the crawl rules in order. The first matching crawl rule determines the policy for the newly discovered URL.

Each crawl rule has a path pattern, a rule, and a policy. To evaluate each rule, the web crawler compares a newly discovered path to the path pattern, using the logic represented by the rule, resulting in a policy.

The policy for each URL is also affected by directives in robots.txt files. The web crawler will crawl only those URLs that are allowed by crawl rules and robots.txt directives.

A robots.txt file is a plain text file, associated with a domain, that provides instructions to web crawlers. The instructions within the file, also called directives, communicate which paths within that domain are disallowed (and allowed) for crawling.

The directives within a robots.txt file are similar to crawl rules. You can therefore choose to manage inclusion and exclusion through robots.txt files, crawl rules, or a combination of both. The Enterprise Search web crawler will crawl only those paths that are allowed by the crawl rules for the domain and the directives within the robots.txt file for the domain.

You may prefer using a robots.txt file over crawl rules for any of the following reasons:

You can also use a robots.txt file to specify sitemaps for a domain.

A canonical URL link tag is an HTML element you can embed within pages that duplicate the content of other pages. The canonical URL link tag specifies the canonical URL for that content.

The canonical URL is stored on the App Search document in the url field, while the additional_urls field contains other URLs where the crawler discovered the same content. If your site contains pages that duplicate the content of other pages, use canonical URL link tags to explicitly manage which URL is stored in the url field of the resulting App Search document.

Template:

<link rel="canonical" href="{CANONICAL_URL}">

Example:

<link rel="canonical" href="https://example.com/categories/dresses/starlet-red-medium">

Robots meta tags are HTML elements you can embed within pages to prevent the crawler from following links or indexing content.

Template:

<meta name="robots" content="{DIRECTIVES}">

Supported directives:

Examples:

<meta name="robots" content="noindex">
<meta name="robots" content="nofollow">
<meta name="robots" content="noindex, nofollow">

With meta tags and data attributes you can extract custom fields from your HTML pages.

Template:

<head>
  <meta class="elastic" name="{FIELD_NAME}" content="{FIELD_VALUE}">
</head>
<body>
  <div data-elastic-name="{FIELD_NAME}">{FIELD_VALUE}</div>
</body>

The crawled document for this example

<head>
  <meta class="elastic" name="product_price" content="99.99">
</head>
<body>
  <h1 data-elastic-name="product_name">Printer</h1>
</body>

will include 2 additional fields

{
  "product_price": "99.99",
  "product_name": "Printer"
}

For compatibility with Elastic Site Search, the crawler also supports the class value swiftype on meta elements and the data attribute data-swiftype-name on elements within the body.

Nofollow links are HTML links that instruct the crawler to not follow the URL.

The web crawler will not follow links that include rel="nofollow" (i.e. will not add links to the crawl queue). The web crawler logs a url_discover_denied event for each link.

The link does not prevent the web crawler from indexing the page in which it appears.

Template:

<a rel="nofollow" href="{LINK_URL}">{LINK_TEXT}</a>

Example:

<a rel="nofollow" href="/admin/categories">Edit this category</a>

Each crawl has a status, which quickly communicates its state.

See View crawl status to view the status for a crawl.

All crawl statuses:

A partial crawl is a web crawl that runs with custom configuration to narrow the scope of the crawl. Configuration for a partial crawl is provided at the time of the crawl request, replacing any globally configured settings.

Partial crawls offer the user powerful customization to meet a variety of use cases. Consider an engine with the following domains and entry points:

A typical crawl for this engine would crawl all three domains, initiating a url queue with seed URLs:

['http://www.awesome-recipes.com/quick', 'http://www.awesome-recipes.com/vegetarian', 'http://www.awesome-recipes.com/slow-cooker', 'http://www.awesome-workouts.com/', 'http://www.awesome-reflections.com/']

Under the http://www.awesome-workouts.com domain, the crawler discovers a sitemap and adds multiple urls to the queue. The crawler visits each url in the queue, enqueuing additional urls as it discovers and traverses any available HTML links. Through this process, the crawler discovers hundreds of web pages, drilling into linked content up to the default depth of 10 traversals.

While running full crawls at regular intervals account for the bulk of this engine’s maintenance, partial crawls allow an administrator to quickly visit and update specific content ad-hoc.

Scenarios include:

Partial crawls further streamline a web crawl by skipping the "purge" phase and, in certain cases, ignoring sitemaps defined in robots.txt. Robots.txt sitemaps are ignored by a partial crawl if seed URLs and/or sitemap URLs are included in the partial crawl configuration.

At this time, partial crawls can only be requested through the API. For a full list of partial crawl capabilities, visit the API reference.

A process crawl is an operation that re-processes the documents in your engine using the current crawl rules. Essentially, it re-applies your crawl rules to your existing documents, removing the documents whose URLs are no longer allowed by the current crawl rules.

After updating your crawl rules, you can use a process crawl to remove unwanted documents without waiting for a full re-crawl to remove them. You can request process crawls through the web crawler UI or API.

Using the UI, you can request a process crawl for all domains or a specific domain. See Re-apply crawl rules.

Using the API, you can request a process crawl for all domains or a subset of domains. See Process crawls.

It is recommended to cancel any active web crawls before requesting a process crawl. A web crawl that runs concurrently with a process crawl may continue to index fresh documents with out of date configuration; the process crawl will only apply to documents indexed at the time of the request.

Using the API, you can also complete a "dry run" process crawl. When "dry run" is enabled, the process crawl will report which documents would be deleted by a normal process crawl, but it does not delete any documents from your engine. Use a dry run to get quick feedback after modifying crawl rules through the web crawler API. If the results of the dry run include documents you do not want to remove, further modify your crawl rules. Repeat this process until you get the expected results.

The web crawler can crawl domains protected by HTTP Authentication.

Use the web crawler API to provide the crawler with the information required to crawl each domain. Create or update each domain with the auth parameter. Refer to the following API references:

Related: HTTP proxy

The web crawler can crawl content through an HTTP proxy, with or without Basic authentication. Refer to the following guide: Crawl a private network using a web crawler on Elastic Cloud.

Configure Enterprise Search with the necessary information about the HTTP proxy. Refer to the following configuration settings reference: Crawler HTTP proxy settings.

Related: HTTP authentication

The following sections describe how the web crawler handles HTTP response status codes for HTML documents and robots.txt files.

The web crawler indexes search documents using the following schema. All fields are strings or arrays of strings.

In addition to these predefined fields, you can also extract custom fields via meta tags and attributes.

The web crawler logs many events while discovering, extracting, and indexing web content.

To view these events, see View web crawler events logs.

For a complete reference of all events, see Web crawler events logs reference.

Operators can configure several web crawler settings.

See Elastic crawler within the Enterprise Search configuration documentation.