netcdf-c/docs/dispatch.md

Internal Dispatch Table Architecture

Internal Dispatch Table Architecture

\tableofcontents

Introduction

The netcdf-c library uses an internal dispatch mechanism as the means for wrapping the netcdf-c API around a wide variety of underlying storage and stream data formats. As of last check, the following formats are supported and each has its own dispatch table.

Warning: some of the listed function signatures may be out of date and the specific code should be consulted to see the actual parameters.

Format	Directory	NC_FORMATX Name
NetCDF-classic	libsrc	NC_FORMATX_NC3
NetCDF-enhanced	libhdf5	NC_FORMATX_NC_HDF5
HDF4	libhdf4	NC_FORMATX_NC_HDF4
PNetCDF	libsrcp	NC_FORMATX_PNETCDF
DAP2	libdap2	NC_FORMATX_DAP2
DAP4	libdap4	NC_FORMATX_DAP4
UDF0	N.A.	NC_FORMATX_UDF0
UDF1	N.A.	NC_FORMATX_UDF1
NCZarr	libnczarr	NC_FORMATX_NCZARR

Note that UDF0 and UDF1 allow for user-defined dispatch tables to be implemented.

The idea is that when a user opens or creates a netcdf file, a specific dispatch table is chosen. A dispatch table is a struct containing an entry for (almost) every function in the netcdf-c API. During execution, netcdf API calls are channeled through that dispatch table to the appropriate function for implementing that API call. The functions in the dispatch table are not quite the same as those defined in netcdf.h. For simplicity and compactness, some netcdf.h API calls are mapped to the same dispatch table function. In addition to the functions, the first entry in the table defines the model that this dispatch table implements. It will be one of the NC_FORMATX_XXX values. The second entry in the table is the version of the dispatch table. The rule is that previous entries may not be removed, but new entries may be added, and adding new entries increases the version number.

The dispatch table represents a distillation of the netcdf API down to a minimal set of internal operations. The format of the dispatch table is defined in the file libdispatch/ncdispatch.h. Every new dispatch table must define this minimal set of operations.

Adding a New Dispatch Table

In order to make this process concrete, let us assume we plan to add an in-memory implementation of netcdf-3.

Defining configure.ac flags

Define a –-enable flag option for configure.ac. For our example, we assume the option "--enable-ncm" and the internal corresponding flag "enable_ncm". If you examine the existing configure.ac and see how, for example, --enable_dap2 is defined, then it should be clear how to do it for your code.

Defining a "name space"

Choose some prefix of characters to identify the new dispatch system. In effect we are defining a name-space. For our in-memory system, we will choose "NCM" and "ncm". NCM is used for non-static procedures to be entered into the dispatch table and ncm for all other non-static procedures. Note that the chosen prefix should probably start with "nc" or "NC" in order to avoid name conflicts outside the netcdf-c library.

Extend include/netcdf.h

Modify the file include/netcdf.h to add an NC_FORMATX_XXX flag by adding a flag for this dispatch format at the appropriate places.

  #define NC_FORMATX_NCM  7

Add any format specific new error codes.

#define NC_ENCM  (?)

Extend include/ncdispatch.h

Modify the file include/ncdispatch.h to add format specific data and initialization functions; note the use of our NCM namespace.

    #ifdef ENABLE_NCM
    extern NC_Dispatch* NCM_dispatch_table;
    extern int NCM_initialize(void);
    #endif

Define the dispatch table functions

Define the functions necessary to fill in the dispatch table. As a rule, we assume that a new directory is defined, libsrcm, say. Within this directory, we need to define Makefile.am and CMakeLists.txt. We also need to define the source files containing the dispatch table and the functions to be placed in the dispatch table -– call them ncmdispatch.c and ncmdispatch.h. Look at libsrc/nc3dispatch.[ch] or libnczarr/zdispatch.[ch] for examples.

Similarly, it is best to take existing Makefile.am and CMakeLists.txt files (from libsrcp for example) and modify them.

Adding the dispatch code to libnetcdf

Provide for the inclusion of this library in the final libnetcdf library. This is accomplished by modifying liblib/Makefile.am by adding something like the following.

     if ENABLE_NCM
        libnetcdf_la_LIBADD += $(top_builddir)/libsrcm/libnetcdfm.la
     endif

Extend library initialization

Modify the NC_initialize function in liblib/nc_initialize.c by adding appropriate references to the NCM dispatch function.

     #ifdef ENABLE_NCM
     extern int NCM_initialize(void);
     #endif
     ...
     int NC_initialize(void)
     {
     ...
     #ifdef ENABLE_NCM
         if((stat = NCM_initialize())) return stat;
     #endif
     ...
     }

Finalization is handled in an analogous fashion.

Testing the new dispatch table

Add a directory of tests: ncm_test, say. The file ncm_test/Makefile.am will look something like this.

     # These files are created by the tests.
     CLEANFILES = ...
     # These are the tests which are always run.
     TESTPROGRAMS = test1 test2 ...
     test1_SOURCES = test1.c ...
     ...
     # Set up the tests.
     check_PROGRAMS = $(TESTPROGRAMS)
     TESTS = $(TESTPROGRAMS)
     # Any extra files required by the tests
     EXTRA_DIST = ...

Top-Level build of the dispatch code

Provide for libnetcdfm to be constructed by adding the following to the top-level Makefile.am.

     if ENABLE_NCM
     NCM=libsrcm
     NCMTESTDIR=ncm_test
     endif
     ...
     SUBDIRS = ... $(DISPATCHDIR)  $(NCM) ... $(NCMTESTDIR)

Choosing a Dispatch Table

The dispatch table is ultimately chosen by the function NC_infermodel() in libdispatch/dinfermodel.c. This function is invoked by the NC_create and the NC_open procedures. This can be, unfortunately, a complex process. The detailed operation of NC_infermodel() is defined in the companion document in docs/dinternal.md.

In any case, the choice of dispatch table is currently based on the following pieces of information.

1. The mode argument – this can be used to detect, for example, what kind of file to create: netcdf-3, netcdf-4, 64-bit netcdf-3, etc. Using a mode flag is the most common mechanism, in which case netcdf.h needs to be modified to define the relevant mode flag.

2. The file path – this can be used to detect, for example, a DAP url versus a normal file system file. If the path looks like a URL, then the fragment part of the URL is examined to determine the specific dispatch function.

3. The file contents - when the contents of a real file are available, the contents of the file can be used to determine the dispatch table. As a rule, this is likely to be useful only for nc_open.

4. If the file is being opened vs being created.

5. Is parallel IO available?

The NC_infermodel function returns two values.

1. model - this is used by nc_open and nc_create to choose the dispatch table.
2. newpath - in some case, usually URLS, the path may be rewritten to include extra information for use by the dispatch functions.

Special Dispatch Table Signatures.

The entries in the dispatch table do not necessarily correspond to the external API. In many cases, multiple related API functions are merged into a single dispatch table entry.

Create/Open

The create table entry and the open table entry in the dispatch table have the following signatures respectively.

     int (*create)(const char *path, int cmode,
                size_t initialsz, int basepe, size_t *chunksizehintp,
                int useparallel, void* parameters,
                struct NC_Dispatch* table, NC* ncp);

     int (*open)(const char *path, int mode,
              int basepe, size_t *chunksizehintp,
              int use_parallel, void* parameters,
              struct NC_Dispatch* table, NC* ncp);

The key difference is that these are the union of all the possible create/open signatures from the include/netcdfXXX.h files. Note especially the last three parameters. The parameters argument is a pointer to arbitrary data to provide extra info to the dispatcher. The table argument is included in case the create function (e.g. *NCM_create_) needs to invoke other dispatch functions. The very last argument, ncp, is a pointer to an NC instance. The raw NC instance will have been created by libdispatch/dfile.c and is passed to e.g. open with the expectation that it will be filled in by the dispatch open function.

Accessing Data with put_vara() and get_vara()

     int (*put_vara)(int ncid, int varid, const size_t *start, const size_t *count,
                          const void *value, nc_type memtype);

     int (*get_vara)(int ncid, int varid, const size_t *start, const size_t *count,
                     void *value, nc_type memtype);

Most of the parameters are similar to the netcdf API parameters. The last parameter, however, is the type of the data in memory. Additionally, instead of using an "int islong" parameter, the memtype will be either ::NC_INT or ::NC_INT64, depending on the value of sizeof(long). This means that even netcdf-3 code must be prepared to encounter the ::NC_INT64 type.

Accessing Attributes with put_attr() and get_attr()

     int (*get_att)(int ncid, int varid, const char *name,
                         void *value, nc_type memtype);

     int (*put_att)(int ncid, int varid, const char *name, nc_type datatype, size_t len,
                    const void *value, nc_type memtype);

Again, the key difference is the memtype parameter. As with put/get_vara, it used ::NC_INT64 to encode the long case.

Pre-defined Dispatch Functions

It is sometimes not necessary to implement all the functions in the dispatch table. Some pre-defined functions are available which may be used in many cases.

Inquiry Functions

Many of The netCDF inquiry functions operate from an in-memory model of metadata. Once a file is opened, or a file is created, this in-memory metadata model is kept up to date. Consequenty the inquiry functions do not depend on the dispatch layer code. These functions can be used by all dispatch layers which use the internal netCDF enhanced data model.

• NC4_inq
• NC4_inq_type
• NC4_inq_dimid
• NC4_inq_dim
• NC4_inq_unlimdim
• NC4_inq_att
• NC4_inq_attid
• NC4_inq_attname
• NC4_get_att
• NC4_inq_varid
• NC4_inq_var_all
• NC4_show_metadata
• NC4_inq_unlimdims
• NC4_inq_ncid
• NC4_inq_grps
• NC4_inq_grpname
• NC4_inq_grpname_full
• NC4_inq_grp_parent
• NC4_inq_grp_full_ncid
• NC4_inq_varids
• NC4_inq_dimids
• NC4_inq_typeids
• NC4_inq_type_equal
• NC4_inq_user_type
• NC4_inq_typeid

NCDEFAULT get/put Functions

The mapped (varm) get/put functions have been implemented in terms of the array (vara) functions. So dispatch layers need only implement the vara functions, and can use the following functions to get the and varm functions:

• NCDEFAULT_get_varm
• NCDEFAULT_put_varm

For the netcdf-3 format, the strided functions (nc_get/put_vars) are similarly implemented in terms of the vara functions. So the following convenience functions are available.

• NCDEFAULT_get_vars
• NCDEFAULT_put_vars

For the netcdf-4 format, the vars functions actually exist, so the default vars functions are not used.

Read-Only Functions

Some dispatch layers are read-only (ex. HDF4). Any function which writes to a file, including nc_create(), needs to return error code ::NC_EPERM. The following read-only functions are available so that these don't have to be re-implemented in each read-only dispatch layer:

• NC_RO_create
• NC_RO_redef
• NC_RO__enddef
• NC_RO_sync
• NC_RO_set_fill
• NC_RO_def_dim
• NC_RO_rename_dim
• NC_RO_rename_att
• NC_RO_del_att
• NC_RO_put_att
• NC_RO_def_var
• NC_RO_rename_var
• NC_RO_put_vara
• NC_RO_def_var_fill

Classic NetCDF Only Functions

There are two functions that are only used in the classic code. All other dispatch layers (except PnetCDF) return error ::NC_ENOTNC3 for these functions. The following functions are provided for this purpose:

• NOTNC3_inq_base_pe
• NOTNC3_set_base_pe

HDF4 Dispatch Layer as a Simple Example

The HDF4 dispatch layer is about the simplest possible dispatch layer. It is read-only, classic model. It will serve as a nice, simple example of a dispatch layer.

Note that the HDF4 layer is optional in the netCDF build. Not all users will have HDF4 installed, and those users will not build with the HDF4 dispatch layer enabled. For this reason HDF4 code is guarded as follows.

#ifdef USE_HDF4
...
#endif /*USE_HDF4*/

Code in libhdf4 is only compiled if HDF4 is turned on in the build.

The netcdf.h File

In the main netcdf.h file, we have the following:

#define NC_FORMATX_NC_HDF4   (3)

The ncdispatch.h File

In ncdispatch.h we have the following:

#ifdef USE_HDF4
extern NC_Dispatch* HDF4_dispatch_table;
extern int HDF4_initialize(void);
extern int HDF4_finalize(void);
#endif

The netcdf_meta.h File

The netcdf_meta.h file allows for easy determination of what features are in use. For HDF4, It contains the following, set by configure:

...
#define NC_HAS_HDF4      0 /*!< HDF4 support. */
...

The hdf4dispatch.h File

The file hdf4dispatch.h contains prototypes and macro definitions used within the HDF4 code in libhdf4. This include file should not be used anywhere except in libhdf4.

Initialization Code Changes in liblib Directory

The file nc_initialize.c is modified to include the following:

#ifdef USE_HDF4
extern int HDF4_initialize(void);
extern int HDF4_finalize(void);
#endif

Changes to libdispatch/dfile.c

In order for a dispatch layer to be used, it must be correctly determined in functions NC_open() or NC_create() in libdispatch/dfile.c. HDF4 has a magic number that is detected in NC_interpret_magic_number(), which allows NC_open to automatically detect an HDF4 file.

Once HDF4 is detected, the model variable is set to NC_FORMATX_NC_HDF4, and later this is used in a case statement:

      case NC_FORMATX_NC_HDF4:
         dispatcher = HDF4_dispatch_table;
         break;

This sets the dispatcher to the HDF4 dispatcher, which is defined in the libhdf4 directory.

Dispatch Table in libhdf4/hdf4dispatch.c

The file hdf4dispatch.c contains the definition of the HDF4 dispatch table. It looks like this:

/* This is the dispatch object that holds pointers to all the
 * functions that make up the HDF4 dispatch interface. */
static NC_Dispatch HDF4_dispatcher = {
NC_FORMATX_NC_HDF4,
NC_DISPATCH_VERSION,
NC_RO_create,
NC_HDF4_open,
NC_RO_redef,
NC_RO__enddef,
NC_RO_sync,
...
NC_NOTNC4_set_var_chunk_cache,
NC_NOTNC4_get_var_chunk_cache,
...
};

Note that most functions use some of the predefined dispatch functions. Functions that start with NC_RO* are read-only, they return ::NC_EPERM. Functions that start with NOTNC4* return ::NC_ENOTNC4.

Only the functions that start with NC_HDF4* need to be implemented for the HDF4 dispatch layer. There are 6 such functions:

• NC_HDF4_open
• NC_HDF4_abort
• NC_HDF4_close
• NC_HDF4_inq_format
• NC_HDF4_inq_format_extended
• NC_HDF4_get_vara

HDF4 Reading Code

The code in hdf4file.c opens the HDF4 SD dataset, and reads the metadata. This metadata is stored in the netCDF internal metadata model, allowing the inq functions to work.

The code in hdf4var.c does an nc_get_vara() on the HDF4 SD dataset. This is all that is needed for all the nc_get_* functions to work.

Appendix A. Changing NC_DISPATCH_VERSION

When new entries are added to the struct NC_Dispatch type located in include/netcdf_dispatch.h.in it is necessary to do two things.

1. Bump the NC_DISPATCH_VERSION number
2. Modify the existing dispatch tables to include the new entries. It if often the case that the new entries do not mean anything for a given dispatch table. In that case, the new entries may be set to some variant of NC_RO_XXX or NC_NOTNC4_XXX NC_NOTNC3_XXX.

Modifying the dispatch version requires two steps:

1. Modify the version number in netcdf-c/configure.ac, and
2. Modify the version number in netcdf-c/CMakeLists.txt.

The two should agree in value.

NC_DISPATCH_VERSION Incompatibility

When dynamically adding a dispatch table -- in nc_def_user_format (see libdispatch/dfile.c) -- the version of the new table is compared with that of the built-in NC_DISPATCH_VERSION; if they differ, then an error is returned from that function.

Point of Contact

Author: Dennis Heimbigner
Email: dmh at ucar dot edu
Initial Version: 12/22/2021
Last Revised: 12/22/2021