"How search engines work
Creating and maintaining an inverted index is the central problem when building an efficient keyword search engine. To index a document, you must first scan it to produce a list of postings. Postings describe occurrences of a word in a document; they generally include the word, a document ID, and possibly the location(s) or frequency of the word within the document.
If you think of the postings as tuples of the form <word, document-id>, a set of documents will yield a list of postings sorted by document ID. But in order to efficiently find documents that contain specific words, you should instead sort the postings by word (or by both word and document, which will make multiword searches faster). In this sense, building a search index is basically a sorting problem. The search index is a list of postings sorted by word."
and
"Incremental versus batch indexing: Some search engines only support batch indexing; once they create an index for a set of documents, adding new documents becomes difficult without reindexing all the documents. Incremental indexing allows easy adding of documents to an existing index. For some applications, like those that handle live data feeds, incremental indexing is critical."
http://www.javaworld.com/javaworld/j...15-lucene.html