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app/models/formats/README

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+Format classes are required to expose a constructor with no arguments,
+a getFileExtension() and a sendResponse(opts, callback) method.
+
+The ``opts`` parameter contains:
+
+ sink        Output stream to send the reponse to
+ sql         SQL query requested by the user
+ skipfields  Comma separate list of fields to skip from output
+             really only needed with "SELECT *" queries
+ gn          Name of the geometry column (for formats requiring one)
+ dp          Number of decimal points of precision for geometries (if used)
+ database    Name of the database to connect to
+ user_id     Identifier of the user
+ filename    Name to use for attachment disposition
+ 
+The ``callback`` parameter is a function that is invoked when the
+format object finished with sending the result to the sink.
+If an error occurs the callback is invoked with an Error argument.