Crawling & Indexing 

Crawling and indexing work in tandem. Crawlers gather data from web pages, and this data is then processed, organized, and stored in an index. When a user submits a search query, the search engine's query processing component uses this index to identify and rank the most relevant documents, which are then presented as search results.

Crawling 

Crawling is a fundamental process in the operation of search engines and web indexing. It involves the systematic and automated exploration of the internet by specialized programs called web crawlers, spiders, or bots. The primary purpose of crawling is to discover, collect, and index web pages so that they can be later retrieved and displayed in search engine results when users enter relevant queries.

Here's how the crawling process typically works:

• Seed URLs: The crawling process begins with a set of initial URLs, often referred to as "seed URLs." These seed URLs are usually a small selection of well-known and authoritative websites or pages. Crawlers start by fetching and analyzing these initial URLs.
• Following Links: After analyzing the content of a seed URL, the web crawler extracts all the hyperlinks present on that page. These hyperlinks lead to other web pages on the same site or external websites. The crawler adds these new URLs to a queue or list for further exploration.
• Recursion: The crawler follows this process recursively. It continues to fetch pages, extract links, and add new URLs to the queue. This recursive process can lead to the discovery of a vast number of web pages across the internet.
• Respecting Robots.txt: Web crawlers are typically programmed to respect the rules specified in the robots.txt file of a website. This file provides instructions to crawlers about which parts of the site can be crawled and indexed and which parts should be excluded. It helps ensure that the crawler doesn't access sensitive or restricted areas of a website.
• Throttling and Politeness: To avoid overwhelming web servers with requests and causing server strain, web crawlers often employ techniques like "throttling" and "politeness." Throttling involves limiting the number of requests made per second or minute to a single website, while politeness involves respecting a website's rules for crawl frequency.
• Duplicate Content Handling: Crawlers must identify and handle duplicate content effectively. Duplicate content can arise from multiple URLs leading to the same page or through other means. Search engines aim to index only one version of a page to avoid redundancy.
• Indexing: Once a web page is fetched and analyzed by the crawler, the information is passed to the search engine's indexing system. The indexing system stores the content and metadata of the page in a searchable database.
• Revisiting and Updating: Web crawlers revisit previously crawled web pages at regular intervals to check for updates or changes. This ensures that the search engine's index remains up-to-date.

Web crawling is a continuous and automated process that allows search engines to keep their indexes current and comprehensive. It forms the foundation of how search engines retrieve and display relevant search results to users when they enter queries, making it a critical component of the search engine ecosystem.

Indexing 

Indexing is a crucial process in the operation of search engines, following the crawling process. After web crawlers collect data from web pages across the internet, the collected information is indexed, organized, and stored in a structured manner in a database. This indexed data forms the foundation for search engines to quickly retrieve and display relevant search results when users enter queries. Here's how the indexing process works:

• Data Extraction: Once a web crawler fetches a web page, it extracts various pieces of information from the page's content, including text, images, links, metadata (such as title and meta description), and other relevant data.
• Tokenization: The extracted text content is broken down into individual words or terms, a process known as tokenization. This step involves removing punctuation, converting text to lowercase, and splitting it into meaningful tokens.
• Stopword Removal: Common words like "and," "the," "of," and other stopwords are typically removed from the text, as they do not carry significant meaning and can take up storage space.
• Stemming and Lemmatization: Stemming and lemmatization are techniques used to reduce words to their root forms. For example, "running" might be reduced to "run." This helps indexers treat variations of words as the same term, improving search accuracy.
• Inverted Index: The processed and tokenized data is then organized into an inverted index. An inverted index is a data structure that maps terms (words or tokens) to the web pages or documents in which they appear. Each term is associated with a list of document IDs or URLs where it occurs.
• Weighting and Ranking: Some indexing systems assign weights or scores to terms based on factors like their frequency in a document, their importance within the document, and their overall relevance to the topic. These weights help search engines rank search results based on relevance when users enter queries.
• Storage: The indexed data, including the inverted index and associated metadata, is stored in a highly optimized and efficient database. This database allows for rapid retrieval of relevant documents when a user performs a search.
• Updating: Search engines regularly update their indexes by re-crawling websites and incorporating new or changed content. This ensures that the index remains current and reflects the most recent information available on the web.
• Query Processing: When a user submits a search query, the search engine's query processing component uses the index to identify the most relevant documents matching the query. This involves looking up the query terms in the inverted index and ranking the documents based on their relevance scores.
• Result Display: The search engine then displays a list of search results to the user, typically with titles, snippets, and URLs. These results are ranked in descending order of relevance, as determined by the search engine's ranking algorithm.

Indexing is a critical step that enables search engines to provide fast and accurate search results to users. It allows search engines to retrieve relevant documents quickly from the vast amount of data available on the internet, making it an essential component of the search engine process.

Working of Google crawler, index, Rank algorithm and SERPs

In summary, crawling is the process of collecting data from the web, while indexing is the process of organizing and storing that data for efficient retrieval. Together, these processes enable search engines to provide users with timely and relevant search results.