Designing a web search engine like Google is a complex task that involves multiple components such as a web crawler, an indexer, and a query processor, among others.
1. **Web Crawler**: A crawler, also known as a spider or bot, is a program that browses the web in a methodical and automated manner. This process is called web crawling or spidering.
A web crawler starts with a list of URLs to visit, called the seeds. As the crawler visits these URLs, it identifies all the hyperlinks in the page and adds them to the list of URLs to visit. URLs from the frontier are recursively visited according to a set of policies. If the crawler is performing archiving of websites it copies and saves the information as it goes.
The basic algorithm for a web crawler is as follows:
procedure WebCrawler(SeedUrl)
add SeedUrl to the frontier
while frontier is not empty
Url = get URL from frontier
contents = fetch(Url)
add Url to visited URLs list
for all links in contents
if link is not in visited URLs list
add link to frontier2. **Indexer**: After the web crawler has retrieved the pages, they need to be processed and indexed.
First, the page content is transformed - this could involve removing HTML tags, JavaScript, CSS and any other unnecessary content.
Then, the remaining text is tokenized, i.e., split into individual words, and these words are processed (e.g., case folding, stemming).
Finally, the tidied-up content is added to the index. An index might often be implemented as an inverted index, a data structure storing a mapping from content, such as words or numbers, to its locations in a document or a set of documents.
3. **Query Processor**: This is the component that processes user’s search queries against the index and returns a list of results.
The query processor might analyze the user’s input (e.g., parsing the query, apply synonyms), and then it will use the index to find relevant documents. The list of documents is then ranked based on various factors (e.g., content of page, PageRank).
Google uses a ranking algorithm called PageRank, which is calculated like this:
PR(A) = (1-d) + d (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn))where:
- ‘PR(A)‘ is the PageRank of page A
- ‘PR(Ti)‘ is the PageRank of pages Ti which link to page A
- ‘C(Ti)‘ is the number of outbound links on page Ti
- ‘d‘ is a damping factor which can be set between 0 and 1.
Designing and implementing a search engine is a very complex task and requires good understanding of various algorithms and data structures. This answer can just give a glimpse into the main concepts involved in a simplified manner.