Other names include document routing and percolate queries.
In contrast, prospective search starts with the user's queries, gathers the information in a targeted way, indexes it, and then provides results as they arrive.
That is, a query produces results when new information that matches it is added to the corpus.
Sometimes Ping Servers are used to gather notification of changes to websites so that the information received is as fresh as possible.
Prospective search is well suited to queries where the results change over time, such as the current news, blogs, and trends.