As to the question of how well they work, the eight products we tested have some big customer names, strong staying power ... and not a whole lot of differentiation. You'll find some variation when looking at advanced federated search features that typically involve indexing systems like databases and CMSs, but beware of paying a premium: The biggest bang for your buck is in indexing file and Web servers. The advanced features vendors will try to sell you yield only marginally better results.
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How It Works
At the heart of most search systems are three core pieces: A crawling/indexing engine, a query engine and a ranking/relevancy engine. Search vendors have varying names and different ways of segmenting these three pieces, but the paradigm is the same in the end. We did find an exception: X1 does not provide advanced search algorithms, such as fuzzy search or word stemming, or relevancy ranking. It approaches enterprise search with a unique, and more easily secured, desktop client-server architecture.
The crawling/indexing engine is responsible for retrieving documents and data from a source, say a database, file server or CMS, and placing the information into a data structure that can be searched efficiently. In most cases, the data structure is an inverted index. The crawling/indexing engine is also responsible for creating document caches, which are used for creating document "summaries" that are displayed on search-result pages.
The query engine searches for occurrences of keywords in the index and creates a list of documents that contain them. The relevancy/ranking engine is responsible for ordering documents such that, hopefully, those most useful to the user are at the top of the list.