netcdf-c/docs/internal.dox

/** \file

\internal

\page nc_dispatch Internal Dispatch Table Architecture

\tableofcontents

This document describes the architecture and details of the netCDF
internal dispatch mechanism. 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 every function
in the netcdf-c API. 
Subsequent netcdf API calls are then 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 list of supported dispatch tables will grow over time.
To date, at least the following dispatch tables are supported.
- netcdf classic files (netcdf-3)
- netcdf enhanced files (netcdf-4)
- DAP2 to netcdf-3
- DAP4 to netcdf-4
- PnetCDF (parallel I/O for classic files -- version 1,2, or 5)
- HDF4 SD files

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.

\section adding_dispatch 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.

\subsection dispatch_configure_ac Defining configure.ac flags

Define a _–-enable_ flag and an _AM_CONFIGURE_ flag in _configure.ac_.
For our example, we assume the option "--enable-ncm" and the AM_CONFIGURE
flag "ENABLE_NCM". If you examine the existing _configure.ac_ and see how,
for example, _DAP2_ is defined, then it should be clear how to do it for
your code.

\subsection dispatch_namespace 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" in order to avoid name conflicts outside the netcdf-c library.

\subsection dispatch_netcdf_h 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.
\code
    \c \#define NC_FORMATX_NCM  7
\endcode

Add any format specific new error codes.
\code
    \c \#define NC_ENCM  (?)
\endcode

\subsection dispatch_ncdispatch Extend include/ncdispatch.h

Modify the file _include/ncdispatch.h_ to
add format specific data and functions; note the use of our NCM namespace.
\code
    #ifdef ENABLE_NCM
    extern NC_Dispatch* NCM_dispatch_table;
    extern int NCM_initialize(void);
    #endif
\endcode

\subsection dispatch_define_code 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 _libdap4/ncd4dispatch.[ch]_ for examples.

Similarly, it is best to take existing _Makefile.am_ and _CMakeLists.txt_
files (from _libdap4_ for example) and modify them.

\subsection dispatch_lib 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.
\code
     if ENABLE_NCM
        libnetcdf_la_LIBADD += $(top_builddir)/libsrcm/libnetcdfm.la
     endif
\endcode

\subsection dispatch_init Extend library initialization

Modify the _NC_initialize_ function in _liblib/nc_initialize.c_ by adding
appropriate references to the NCM dispatch function.
\code
     #ifdef ENABLE_NCM
     extern int NCM_initialize(void);
     #endif
     ...
     int NC_initialize(void)
     {
     ...
     #ifdef ENABLE_NCM
         if((stat = NCM_initialize())) return stat;
     #endif
     ...
     }
\endcode

\section dispatch_tests Testing the new dispatch table

Add a directory of tests: _ncm_test_, say. The file _ncm_test/Makefile.am_
will look something like this.
\code
     # 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 = ...
\endcode

\section dispatch_toplevel Top-Level build of the dispatch code

Provide for _libnetcdfm_ to be constructed by adding the following to
the top-level _Makefile.am_.

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

\section choosing_dispatch_table Choosing a Dispatch Table

The dispatch table is chosen in the NC_create and the NC_open
procedures.
This can be, unfortunately, a complex process.
The code for inferring a dispatch table is largely isolated
to the file _libdispatch/dinfermodel.c_, which is invoked
from _NC_create_ or _NC_open_ in _libdispatch/dfile.c_.

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 choice is determined using the function _NC_urlmodel_.

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_.

\section special_dispatch 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.

\subsection create_open_dispatch Create/Open

The create table entry and the open table entry in the dispatch table
have the following signatures respectively.
\code
     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);
\endcode

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

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.

\subsection put_vara_dispatch Accessing Data with put_vara() and get_vara()

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

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

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.

\subsection put_attr_dispatch Accessing Attributes with put_attr() and get_attr()

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

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

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

\subsection pre_def_dispatch 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.

\subsubsection inquiry_functions Inquiry Functions

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

\subsubsection ncdefault_functions 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.

\subsubsection read_only_functions 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

\subsubsection classic_functions 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

\section dispatch_layer 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.
\code
\c \#ifdef USE_HDF4
...
\c \#endif /*USE_HDF4*/
\endcode

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

\subsection header_changes Header File Changes in include Directory

\subsubsection netcdf_h_file The netcdf.h File

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

\code
\c \#define NC_FORMATX_NC_HDF4   (3)
\c \#define NC_FORMAT_NC_HDF4   NC_FORMATX_NC_HDF4
\endcode

\subsubsection ncdispatch_h_file The ncdispatch.h File

In ncdispatch.h we have the following:

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

\subsubsection netcdf_meta_h_file The netcdf_meta.h File

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

\code
\c \#define NC_HAS_HDF4      1 /*!< hdf4 support. */
\endcode

\subsubsection hdf4dispatch_h_file 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.

\subsection liblib_init Initialization Code Changes in liblib Directory

The file _nc_initialize.c_ is modified to include the following:
\code
#ifdef USE_HDF4
extern int HDF4_initialize(void);
extern int HDF4_finalize(void);
#endif
\endcode

\subsection libdispatch_changes Dispatch Code Changes in libdispatch Directory

\subsubsection dfile_c_changes Changes to 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:

\code
      case NC_FORMATX_NC_HDF4:
         dispatcher = HDF4_dispatch_table;
         break;
\endcode

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

\subsection libhdf4_dispatch_code Dispatch Code in libhdf4

\subsubsection hdf4dispatch_c_table Dispatch Table in hdf4dispatch.c

The file _hdf4dispatch.c_ contains the definition of the HDF4 dispatch
table. It looks like this:
\code
/* 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_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,
};
\endcode

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

\subsubsection hdf4_reading_code 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.

\subsection model_infer Inferring the Dispatch Table

As mentioned above, the dispatch table is inferred using the following
information:
1. The mode argument
2. The file path/URL
3. The file contents (when available)

The primary function for doing this inference is in the file
_libdispatch/dinfermodel.c_ via the API in _include/ncmodel.h_.
The term _model_ is used here to include (at least) the following
information (see the structure type _NCmodel_ in _include/ncmodel.h_).

1. format -- this is an NC_FORMAT_XXX value defining the file format
   as seen by the user program.
2. version -- the specific version of the format.
3. iosp -- this is and NC_IOSP_XXX value describing internal protocols to use.
4. impl -- this is an NC_FORMATX_XXX value defining, in effect, the
   dispatch table to use.

For example, if the format was NC_FORMAT_CLASSIC, then the client
will see the netcdf-3 data model, as modified by the version. If the
version was 5, for example, then that indicates the file format
is actually NC_FORMAT_64BIT_DATA, which is a variant of the netcdf-3
format.

The _iosp_ provides information about how the protocol the
dispatch table will use to access the actual dataset. If the iosp
is NC_IOSP_S3RAW, then it indicates that the dispatcher, NC_FORMATX_NC3,
for example, will access the dataset using the Amazon S3 REST API.

The construction of the model is primarily carried out by the function
_NC_infermodel()_. It is given the following parameters:
1. path -- (IN) absolute file path or URL
2. omodep -- (IN/OUT) the set of mode flags given to _NC_open_ or _NC_create_.
3. iscreate -- (IN) distinguish open from create.
4. useparallel -- (IN) indicate if parallel IO can be used.
5. params -- (IN/OUT) arbitrary data dependent on the mode and path.
6. model -- (IN/OUT) place to store inferred model information (e.g. format
   or version).
7. newpathp -- (OUT) sometimes, it is necessary to rewrite the path.

As a rule, these values are used in the this order to infer the model.
1. file contents -- highest precedence
2. url (if it is one) -- using the protocol and fragment arguments
3. mode flags
4. default format -- lowest precedence
 
*/