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15. Using Nonrelational Databases > _changes: Abusing CouchDB as a Message Queu...
CouchDB provides a change notification API (called _changes) that allows clients a great deal of
flexibility in defining how to react in response to data flows.
Briefly, _changes works like
so:
-
An HTTP connection is opened to the
_changesURL for the CouchDB database of interest. -
Wait. When any change occurs in the database in question (any document creation, deletion, or update), a JSON map is sent to the client describing the affected document ID and revision.
-
If you have elected to be notified of changes continuously, repeat from 2.
The simplest of all _changes
usage via Clojure might be using Clutch’s watch-changes function to echo all change
notifications to *out*. Here, we’ll
create a new database for our _changes
experimentation, set up a Clutch watch function for that database, and add
a few documents to see what happens:
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