Filter and update functions in CouchDB?

What are these functions?

Where can I read about how they work?

The CouchDB definitive guide doesn't explain it, weird开发者_运维知识库.

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From the CouchDB definitive guide:

There are other design document functions that are being introduced at the time of this writing, including _update and _filter that we aren’t covering in depth here. Filter functions are covered in Chapter 20, Change Notifications. Imagine a web service that POSTs an XML blob at a URL of your choosing when particular events occur. PayPal’s instant payment notification is one of these. With an _update handler, you can POST these directly in CouchDB and it can parse the XML into a JSON document and save it. The same goes for CSV, multi-part form, or any other format.

More info on update from the wiki:

http://wiki.apache.org/couchdb/Document_Update_Handlers