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.