MARKLOGIC SERVER

interest? Alerting gives you that. How about when an earthquake occurs in a certain city?
Define the city as a geospatial query in the reverse index and check for matching latitude/
longitude data when earthquakes occur.
Without reverse query indexing, for each new document or set of documents, you'd
have to loop over all of your queries to see which ones match. As the number of queries
increases, this simplistic approach becomes increasingly inefficient.
You can also use reverse queries for rule-based classification. MarkLogic includes an SVM
(support vector machine) classifier. Details on the SVM classifier are beyond the scope
of this book, but suffice to say it's based on document training sets and finding similarity
between document term vectors. Reverse queries provide a rule-based alternative to
training-based classifiers. You define each classification group as a cts:query. That
query must be satisfied for membership. With reverse query, each new or modified
document can be placed quickly into the right classification group or groups.
Perhaps the most interesting and mind-bending use case for reverse queries is for
matchmaking. You can match for carpools (driver/rider), employment (job/resume),
medication (patient/drug), search security (document/user), love (man/woman or a
mixed pool), or even battle (target/shooter). For matchmaking you represent each entity
as a document. Within that document, you define the facts about the entity itself and
that entity's preferences about other documents that should match it, serialized as a
cts:query. With a reverse query and a forward query used in combination, you can
do an efficient bi-directional match, finding pairs of entities that match each
other's criteria.
A REVERSE QUERY CARPOOL MATCH
Let's use the carpool example to make the idea concrete. You have a driver, a nonsmoking
woman driving from San Ramon to San Carlos, leaving at 8:00am, who listens
to rock, pop, and hip-hop, and wants $10 for gas. She requires a female passenger
within five miles of her start and end points. You have a passenger, a woman who will
pay up to $20. Starting at "3001 Summit View Drive, San Ramon, CA 94582" and
traveling to "400 Concourse Drive, Belmont, CA 94002." She requires a non-smoking
car and won't listen to country music. A matchmaking query can match these two
women to each other, as well as any other matches across potentially millions of people,
in sub-second time.
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