Prospective search

Prospective search, or persistent search, is a method of searching on the Internet where the query is given first and the information for the results are then acquired. This differs from traditional, or "retrospective", search such as search engines, where the information for the results is acquired and then queried.
Comparison to retrospective search.
Retrospective search starts by gathering the information, indexing it, then letting users query the information. The results don't change until the index is rebuilt. Prospective search starts with the user's queries, gathers the information in a targeted way, indexing it and then providing the results as they arrive. Sometimes Ping Servers are used to gather notification of changes to websites so that the information received is as fresh as possible. Users can be notified in a number of ways of new results.
Prospective search is well suited to queries where the results change over time, such as the current news, blogs and trends.