Hi Glynn, > An RPC wrapper would move the execution of GRASS functions into a new > process (i.e. a server). If the call generates a fatal error, the > server dies, the client detects this and reports an error rather than > a result. > > The main problem with this is that any error loses the entire state > held on the server, e.g. any open maps no longer exist.
Exactly! Hence the wrapper will not only do simple function wrapping, it will also implement a higher level interface: * map metadata read access as single client function will do on the server side: open map; read metadata; close map; send data * fast raster and vector map read access as single client function will do on server side: open map; read requested map into memory based on bounding box and render resolution; close map; send data * vector editing: keep a single vector map in update state open in a dedicated grass vector editing process. In case of a fatal error, this map may be lost or corrupted. It is important that this process will not be used for other purposes than single vector map editing. * vector analysis functionality that need to keep vector maps open in read only state for fast topological access. Information's about the open vector maps (i. e. the position of the next line to be read) may be lost in case the server process terminates. * Same for raster map analysis The client will detect if the server was terminated and will raise an exception. In QGIS that will be the data provider classes. The dedicated raster display module in QGIS is a nice starting point to implement a RPC function that will read a raster map as RGB color array based on extent and resolution information into a byte array that will be send via thrift (using thrift binary protocol) to the client. The RPC interface should only support meaningful functions to be used in a GUI, that's its only purpose. It should not be used to implement processing algorithms or GRASS modules. But it will in addition provide capabilities to use several processes to read different chunks of the same map in the GUI, hence parallel read only map access. This can be nicely done using a pool of GRASS server processes. > Using setjmp/longjmp would be more useful, but it would require some > effort to add clean-up code to functions so that core data structures > aren't left in an inconsistent state. There are plenty of functions using global variables in GRASS. Also, persistent applications may change the location and therefore the projection several times on run-time. Are the GRASS libraries able to handle this correctly? The suggested RPC server processes can be restarted in this case. Best regards Soeren _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
