A sort of specialized category within NoSQL is search engines. These services specialize in indexing data and are often used in concert with an application database. For instance, you might be building an e-commerce website. You could query your product database when a visitor wants to search products, but you'd have to take on much of the work of figuring out partial searches, relevancy, boosting, multi-field search, and other challenges.

Many people start out thinking of search engines as being associated with search crawlers—Google and Bing have crawlers that visit websites and index content, titles, links, and other relevant data. With most NoSQL search engines, they don't include a crawler mechanism. Instead, we'll insert documents for indexing as they are created, giving us greater control over how they are indexed and reducing the complexity and computing overhead involved in building a search crawler.

In fact, many application development teams...