How Search organises information
Before you search, web crawlers gather information from across hundreds of billions of web pages and organise it in the Search index.
The fundamentals of Search
The crawling process begins with a list of web addresses from past crawls and sitemaps provided by website owners. As our crawlers visit these websites, they use links on those sites to discover other pages. The software pays special attention to new sites, changes to existing sites and dead links. Computer programs determine which sites to crawl, how often and how many pages to fetch from each site.
We offer webmaster tools to give site owners granular choices about how Google crawls their site: they can provide detailed instructions about how to process pages on their sites, can request a recrawl or can opt out of crawling altogether using a file called 'robots.txt'. Google never accepts payment to crawl a site more frequently – we provide the same tools to all websites to ensure the best possible results for our users.
Finding information by crawling
The web is like an ever-growing library with billions of books and no central filing system. We use software known as web crawlers to discover publicly available web pages. Crawlers look at web pages and follow links on those pages, much like you would if you were browsing content on the web. They go from link to link and bring data about those web pages back to Google’s servers.
Organising information by indexing
When crawlers find a web page, our systems render the content of the page, just as a browser does. We take note of key signals – from keywords to website freshness – and we keep track of it all in the Search index.
The Google Search index contains hundreds of billions of web pages and is well over 100,000,000 gigabytes in size. It’s like the index in the back of a book – with an entry for every word seen on every webpage that we index. When we index a webpage, we add it to the entries for all of the words it contains.
With the Knowledge Graph, we’re continuing to go beyond keyword matching to better understand the people, places and things that you care about. To do this, we not only organise information about web pages but other types of information too. Today, Google Search can help you search text from millions of books from major libraries, find travel times from your local public transport providers or help you navigate data from public sources like the World Bank.