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Server-side Functions for Geo-spatial Selection

Learn about server-side functions and their importance in geo-spatial selection operations. Find out how these functions make data access more efficient and user-friendly.

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Server-side Functions for Geo-spatial Selection

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  1. Server-side Functions for Geo-spatial Selection James Gallagher 22 Feb 2007 OPeNDAP Developer’s Workshop Feb 21-23 2007

  2. Why build server-side functions? • Some operations are much more efficient to do before sending the data. • Some operations are possible only after performing several data accesses. • But the real reason… Hide the mechanics of an operation behind an interface. OPeNDAP Developer’s Workshop Feb 21-23 2007

  3. Why is ‘Hiding’ Important for Data? • Operations such as geo-referenced selection are different for each dataset/format convention. • An interface can • combine knowledge of several conventions • be specialized • return better error messages than generic operators OPeNDAP Developer’s Workshop Feb 21-23 2007

  4. Server-side functions in Hyrax • Four* server-side functions: • version(): Get info on installed functions • geogrid(): Given a Grid variable and a latitude-longitude box, return the data within that box in a Grid • geoarray(): Similar to geogrid() but for an Array • linear_scale(): Scale data using y=mx+b • These are in libdap 3.7.5 *Really 3.1 since version() is a placeholder OPeNDAP Developer’s Workshop Feb 21-23 2007

  5. Geo-spatial selection for a Grid variable • How does geogrid() make getting data easier? OPeNDAP Developer’s Workshop Feb 21-23 2007

  6. First, the ‘old’ way… • A user/client looks at the Grid’s map vectors - technically a metadata request • Based on the values in the maps, they figure out the start and stop indexes • They use those indexes to make the real request • Summary: The request requires two data accesses, plus scanning the map vectors to determine the indexes OPeNDAP Developer’s Workshop Feb 21-23 2007

  7. geogrid() • geogrid() arguments: • The dataset Grid name; • A latitude/longitude bounding box and • Zero or more relational expressions • Evaluation of the Map vectors is still the ‘algorithm,’ but it’s done by the server, hidden from the user/client OPeNDAP Developer’s Workshop Feb 21-23 2007

  8. The geogrid() Function /data/nc/coads_climatology.nc "geogrid(SST,50,-70,48,-68,\"1000<TIME<2000\")" The data: Grid { Array: Float32 SST[TIME = 2][COADSY = 3][COADSX = 3]; Maps: Float64 TIME[TIME = 2]; Float64 COADSY[COADSY = 3]; Float64 COADSX[COADSX = 3]; } SST = { Array: {{{-1e+34, -1e+34, -1e+34},{-1e+34, -1e+34, -1e+34},{-1e+34, -1e+34, -1e+34}}, {{-1e+34, -1e+34, -1e+34},{-1e+34, -1e+34, -1e+34},{-1e+34, -1e+34, -1e+34}}} Maps: {1096.485, 1826.97}, {47, 49, 51}, {-71, -69, -67} }; OPeNDAP Developer’s Workshop Feb 21-23 2007

  9. The geoarray() Function • Intent: Provide the same sort of geo-spatial selection capability for Arrays as for Grids. • Current version: User/client must supply the geo-spatial extent of the data • RSN* version: The function will use extent information when present in the dataset • A future version will also be savvy about different projections *Really Soon Now OPeNDAP Developer’s Workshop Feb 21-23 2007

  10. The linear_scale() Function • Perform simple linear scaling (y=mx+b) • Use COARDS attributes if present • Provide a way for clients to tell the function which coefficients to use OPeNDAP Developer’s Workshop Feb 21-23 2007

  11. The version()function • Returns information on the functions installed and their versions • Returns two responses: • version() human-readable text • version(xml) returns xml • This is really just the first attempt at a discovery system for these functions! OPeNDAP Developer’s Workshop Feb 21-23 2007

  12. How they work • The function is run by the constraint evaluator • When a handler is initialized, it loads the functions it understands • There is now a set of ‘generic’ functions in libdap; a handler can override those OPeNDAP Developer’s Workshop Feb 21-23 2007

  13. Implementing a Server-side function • Functions take four arguments: • A list of arguments which may be constants or variables in the data set • A count of those arguments • A DDS (really DDX) for the data set • The string that names the dataset • Once called, a server-side function can call any method is libdap. • The function must return one of the libdap data types – that’s the function’s return value. OPeNDAP Developer’s Workshop Feb 21-23 2007

  14. What else do I need to know to write one? • You should also understand how libdap evaluates constraint expressions • How the libdap read() methods operate • How to access attribute values from a from the libdap 3.x DDS object (which is actually the binary representation of the DDX) OPeNDAP Developer’s Workshop Feb 21-23 2007

  15. Future directions • We need feedback on • The functions/features implemented so far • The function loading scheme • Discovery OPeNDAP Developer’s Workshop Feb 21-23 2007

  16. Features • Are there specific conventions that should be supported? • Other functions? • Things the functions do that they shouldn’t OPeNDAP Developer’s Workshop Feb 21-23 2007

  17. Function loading/evaluation • Now all of the functions are compiled into libdap • Should functions be loaded from a shared library at run time? Configured in the BES? • Should functions be an option for metadata (DDX) requests too? (we do support constraints for ‘syntax’ metadata requests) OPeNDAP Developer’s Workshop Feb 21-23 2007

  18. Discovery • There must be a way to discover functions, true of false? • The current version() function is really only a placeholder • What are the requirements for discovery? • Should we use the Data response? Error response? Something new? OPeNDAP Developer’s Workshop Feb 21-23 2007

  19. Vexing questions • Server-side functions seem like a very powerful tool, but they get little use, why? • Assuming these are to be driven by the community, how do we sustain that? OPeNDAP Developer’s Workshop Feb 21-23 2007

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