Tools to Build Clients

1. Tools to Build Clients

2. Toolkits based on C/C++ • Libdap • C++ library, the reference implementation of DAP2 • Ocapi • C library, client side only • Libnc-dap • Build clients using netCDF • Loaddap • Build clients using either Matlab or IDL

3. Two types of Clients • Built using DAP implementations • Complete access to all data sets served • Well matched to network I/O operations • Can be confusing to use - not intended for casual use • Built using non-DAP APIs • Easy APIs to use because many people are familiar with them - designed to be suited to application domains • Not well matched to all data sets - data model mis-match between the API and the DAP2

4. Libdap C++ • Basic use model: open a ‘connection’ to a server • Get the data set’s metadata • Access data • Close connection

5. Libdap Connect Class • Provides the interface for most client access • The Connect constructor method is used to ‘connect’ to a URL • Four methods are used to get the DAS, DDS, DDX and Data responses • Other methods control miscellaneous features: • Configure client-side caching • Set username and password credentials • Establish whether the client can process compressed responses • Get the server/protocol version given a virtual connection has been established

6. Libdap example • Look at libdap’s Connect.h source file for the complete interface to Connect • The getdap client uses Connect • Look at the libdap source file getdap.cc • ‘name’ below is the URL minus any CE

7. Get a Response - How getdap gets the DDS The request. ‘expr’ is the constraint expression passed in using ‘-c’. The DDS object is a value-result parameter Getdap just prints the DDS using the DD::print() method

8. Getdap access data in much the same way… The data request. Note that the DDS is a value-result parameter here too. However the variables in this DDS are loaded with data The print_data function handles access to the data values

9. More about using libdap • Responses from the ‘get’ methods of Connect are libdap container classes • DAS: Semantic metadata; access values using methods bound to the DAS class • DDS: Syntactic metadata; access name and type information for the dataset • Data: Also returned using a DDS, but now the variables hold data

10. About Connect’s Get Methods • DDS -vs- Data: The syntactic metadata is represented using a DDS object with variables that are instances of classes which specialize BaseType • Or, think of the DDS/Variables objects as a parse tree typical of a compiler or interpreter • This tree is used to record/express the variables’ types and their relationships

11. Where data resides • In the DDS ‘parse tree’ data can be held in the variables. • Accessor methods can be used to read values out. • These Accessors behave like most data read functions (stdio’s read(), netCDF’s get_var(), et c.) providing data in blocks of memory.

12. Client APIs Summary • C++ is hard(er) to use for many people, but the basic mode of operation for libdap (C++) and Ocapi ( C) is the same: • Open • Read • Close • The read operation used for our interfaces are very similar to those used with netCDF, HDF4, HDF5 • Grab the metadata, look at the types, allocate memory, read data per variable. • DAP APIs (libdap, Ocapi) provide a way to read several variables at once (optimized for network I/O), but the concepts are really the same.