How do search engines crawl, index, and retrieve information?

How do search engines crawl, index, and retrieve information?

Web search involves discovering content, organizing information about it, and selecting useful results for a query. These stages explain why a page can exist online without appearing in a particular search.

Google's explanation of search describes three broad stages: crawling, indexing, and serving results.

Crawling discovers and fetches pages

Automated programs find URLs through sources such as links and previously known pages, then request content. Access restrictions, server problems, and discovery gaps can affect whether a page is fetched.

Imagine publishing a tutorial with no links from anywhere else. A search engine may not discover it immediately. A sitemap or a link from a known page can help with discovery, but neither guarantees indexing.

Indexing organizes what was found

After fetching a page, the search system analyzes its content and other signals. It may identify the main text, understand relationships between pages, and choose how to represent the material in an index.

The index is an organized structure used to search efficiently. It is not the same thing as a language model's trained parameters.

Retrieval and ranking serve a query

When someone searches, the system selects potentially relevant indexed material and ranks results using many signals. It usually does not crawl the entire web from scratch for every query.

For example, a query about JavaScript closures should surface useful pages about that concept, even though the web contains many unrelated uses of the word closure.

Discovery, indexing, and ranking are distinct outcomes. A page can be crawled but not indexed, or indexed but not selected for a particular query. Search results are therefore a useful route to evidence, not a complete inventory of everything that exists.