SharePoint 2010 search : How it works???

SharePoint 2010 search architecture is made up of the following components

• Crawler
• Indexing Engine
• Query Engine
• Search User Interface 
• Query Object Model

Architecturally every SharePoint 2010 Seacrh component is part of its query architecture and the crawling architecture. The query architecture includes query components, index partitions, and property databases. At the same time Crawl architecture includes components like Crawl component, Crawl database and Property database.


The server which has Web server role in your farm hosts Search Web Parts and Web Part pages for answering search queries. These web components interacts with the user in issuing search requests and accepting search response as results. 


The application server configured for having query components serves query results to web servers. It also holds part of / entire search index. It process the query issued by web server and forwards query requests to one query component for each index partition and then merges results to display to users. The query processor is also responsible for security trimming, duplicate results detection, and gathering and displaying properties.  Application server with crawl components Runs one or more crawler components. 


Database server Hosts search-related databases like Property DB, Crawl DB and Search Admin DB.


The crawl process works as given below

1. When the Full crawl process starts, The Start address of the search source moved to queue. 
2. The Protocol for earch determined and selects the requires Connector. 
3. iFilter opens files and Content index created on crawl server. 
4. Then the Index moved in batches to query server and the relevant Data written to Crawl and Property databases.

The query flow is follows

1. When A search is performed by a user, the WFE serving the call uses the associated search service application proxy to connect to a server running the Query and Site Settings Service also known as the Query Processor.  It uses WCF for this communication. 
2. The QP will connect to the following components to gather results merges\security trims and return results back to WFE: Query Component (holds entire index or partition of an index) Property Store DB (holds metadata\properties of indexed content) Search Admin DB (holds Security Descriptors\Configuration data). 
3. Then WFE displays search results to the user