How Search Engines Work: Crawling, Indexing, Ranking & More

How do search engines determine what results to show? Search engines employ web crawlers to gather content from the internet and send that content to be indexed. When a search is conducted, the algorithms run a query on that indexed content and select search results based on hundreds of criteria, including associations, quality, and content relevance.

The role of a search engine is similar to that of a librarian. Search engines organize a massive repository of internet content and serve it to billions of users every day. This guide outlines the process: crawling, indexing, and the rest, up to ranking and penalties.

What is the process that search engines execute?

Crawling, indexing, and ranking are the steps search engines execute to process the entire data of the internet. The first, crawling, is the automated process of discovering the digital content. The second, indexing, entails analyzing and storing content in a database. The third, ranking, is the process of serving the most relevant and stored content in response to a search request.

1. Crawling

What is it? Crawling is the automated process of discovering usable content across the internet by web crawlers (also known as spiders or bots). This content can be in the form of text or different media types and can also be previously distributed web pages. This process can be likened to the librarian’s task of gathering resources that haven’t yet been placed on a shelf.

Why it matters: From crawling, everything else in the search engine builds on. If searching doesn’t crawl to find the content to build the index, then the searching engine cannot yield searching results.

How it works: Crawlers gather content to be searched and noted in the index, and at the same time, search for new content to be included. Crawling takes lots of resources and, therefore, search engines use algorithms to decide which sites to crawl, how often to crawl them, and how many pages to explore during each visit.

Common crawling tools:

Tool	Purpose
Google Search Console	Track crawl progress and troubleshoot crawling issues.
Screaming Frog	Simulate how different crawlers would move through your site.
Robots.txt Tester	Test and validate your crawling rules.

Ways to optimize for crawling:

Robots.txt file: This file informs crawlers of which parts of the site they can visit, allowing you to direct crawlers to pages you want to be indexed, and away from pages you want to be excluded, like admin pages.
XML Sitemap: Provide a list of all the content on your site to help crawlers find everything they need. This works like a city map. You can help various search engines further by submitting them to Google Search Console and Bing Webmaster Tools.
Good internal link structure: Internal linking works like connecting all the parts of a city. You should aim for 3 to 5 relevant internal links on each page.
Implement canonical tags: These function similarly to a road sign, indicating to crawlers where the officially original content resides, and it is useful if your site features duplicate content or pages that are almost a duplicate.
Update content frequently: Updating or adding content signals to users and search engines that the site is active and crawlers will be more likely to return to the site for new updates.

Troubleshooting crawling errors:

Scan your robots.txt file: This file can be set, often by accident, to restrict crawlers’ access to your whole site. Make sure your configurations are set correctly and can be validated by a validator tool.
Correct HTTP network errors: These errors indicate to crawlers that there isn’t content for them to access. See these errors using a crawler like Screaming Frog or the Pages report in Search Console.
Address Server errors: Firewall settings, traffic spikes and hosting issues can block access for crawlers. If the issues are on your end, see your server configuration and your hosting plan.

2. Indexing

Definition: Indexing is the search engine’s way of organizing content it has crawled, making it searchable and available. Not everything gets indexed. To protect the quality of search results, duplicate content or low-quality content that has been crawled is ignored, as well as content that has been marked as “noindex.” This is likened to a librarian organizing the content on a library’s shelf.

Why it matters: Quality control of content is done during indexing. Content is evaluated for its quality and eligibility to be made searchable by adding it to the search engine’s database.

Process: Indexing reviews the crawled data. This includes page usability, page images and videos, and the canonical URL and title. This determines if a page can be indexed.

Indexing tools:

Tool	Purpose
Google Search Console	Track indexing status and troubleshoot Google indexing issues
Bing Webmaster Tools	Track indexing status and troubleshoot Bing indexing issues

How to optimize for indexing:

Create original content: unique and original content that is informative to users is preferred by search engines because it provides a solution to a problem.
Use meta tags: The title tag and meta description are the title and summary of the page, respectively. Title tags can be a max of 60 characters and meta descriptions can be a max of 150 characters.
Use header tags: Header tags (H1, H2, and H3) break down page content, and using one H1 tag per page content assists search engines.
Use multimedia: Search engines rely on descriptive alt text to understand images and videos. Relevant media can help search engines better understand a page.
Design a usable site: a site that is easy to navigate and allows all users to access it is a site that search engines can index.

Canonical tags also apply to optimizations for indexing.

Indexing problem solutions:

Ensure content can be indexed: Check that content is not blocked by a robots.txt file, a noindex tag, or is missing from the sitemap.
Ensure content is not duplicated: Near-duplicate content can be solved by creating different canonical tags, different redirects or rewritten pages. Tools for this can be Search Console or Screaming Frog.
Audit for content quality: Consider Google’s helpful content guidelines to analyze your content against others in search results for originality and helpfulness.
Audit for content usability: Google’s Mobile-Friendly Test and other similar tools can help you check your content for usability and access on all devices.

3. Ranking

Definition: Ranking is the process by which search engines display a list of webpages as results for a search query. Ranking assigns a value to each webpage using hundreds of different criteria in highly sophisticated algorithms to return results that are the most relevant to the search. It is most comparable to a research librarian that not only knows their library well, but also knows which book is most relevant for the user.

Why it matters: Ranking is the process by which search engines display a list of webpages as results for a search query. Ranking assigns a value to each webpage using hundreds of different criteria in highly sophisticated algorithms to return results that are the most relevant to the search. It is most comparable to a research librarian that not only knows their library well, but also knows which book is most relevant for the user.

Process: The entire ranking process is completed in a matter of milliseconds. The moment someone enters a search, the engine has already begun to comb its stored index for the most relevant and quality pages. It considers many factors, including as the device a user is on and numerous content factors like title tags, before returning its list of results.

Common ranking tools:

Tool	Purpose
Screaming Frog	Identify and prioritize SEO issues or opportunities
Google PageSpeed Insights	Find opportunities to improve page speed
Keywords Everywhere	Research keyword targeting opportunities

How to optimize for ranking:

Focus on certain search queries: Use thorough keyword research to naturally include the search query in multiple content facets, such as title tags and headers.
Focus on certain locations: For search queries that have a geographic component, such as “SEO agency in Mumbai,” ensure your content is focused on that geography.
Study intent behind queries: To write better content, review what is ranking for your target queries, and look for ways to give users content that is better and more focused.
Examine ranking factors: Though search engines don’t provide an exhaustive list, there are confirmed factors that are worth prioritizing such as HTTPS, page speed, and content quality.

Ranking issues require understanding crawling and indexing. This includes your robots.txt file, sitemap, canonical tags, internal linking, meta tags, headers, multimedia, and usability.

Troubleshooting ranking issues:

Analyze intent behind queries: This can change. Check current results for queries you target and ensure your content meets user intent the same or better as competition.
Check keyword metrics: Google Trends and similar tools show popularity metrics for keywords. This can help explain competitive and ranking issues.

The same crawling and indexing issues that are checked for ranking apply here too. Confirm your robots.txt file isn’t blocking pages, you don’t have any HTTP errors, server issues, or duplicate content, and usability is acceptable.

4. Penalties

Definition: A site ranking in search results is demoted or removed from a search engine’s index when a site is found to be in clear violation of the search engine’s spam policy and content ranking manipulation. This is similar to a librarian removing a book that shouldn’t have been on the shelf.

Why it is important: To keep improving index quality and search results, spam sites need to be eliminated and avoid ranking wasteful resources for high quality and valuable relevance sites.

How it works: Search engines combine multiple automated processes, human reviewers, and user-reported spam to discover policy breaches. When instances are found, they may take manual action on the search results or lower the ranking of affected content, or choose to not rank it at all.

Common tools for tracking penalties:

Tool	Purpose
Google Search Console	Understand and resolve Google penalties
Bing Webmaster Tools	Bing Webmaster Tools Verify, understand, and resolve Bing penalties

Practices to avoid (to avoid penalties):

Cloaking: Displaying different content to search engines from what the user sees, with the malicious intent to improve the ranking of a page.
Hidden text: Text that is visible to search engine crawlers, but is made invisible to the users of a page, is done to keyword to page content.
Keyword Stuffing: Excessively filling a page with a list of keywords that lowers the quality and readability of the content.
Link Spam: Purchasing backlinks, rather than earning them by producing content of value.

Troubleshooting penalties:

Once a penalty is confirmed: Review the provided documentation (i.e. Manual Actions report in Google Search Console) to reveal the cause of the penalty and clear the penalty.
Once a penalty is unconfirmed: Return to the previously outlined indexing and ranking approaches and look for typical offender practices; keyword stuffing and purchased links contain the cause of the penalty.

As a worst case scenario, an entire site can be blacklisted from a search engine. This outcome can be easily avoided by eliminating the previously documented practices.

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