netcdf-c/NUG/DAP4.dox

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\page dap4 DAP4 Protocol Support


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# DAP4 Introduction {#dap4_introduction}

Beginning with netCDF version 4.5.0, optional support is provided for
accessing data from servers supporting the DAP4 protocol.

DAP4 support is enabled if the _--enable-dap_ option
is used with './configure'. If DAP4 support is enabled, then
a usable version of _libcurl_ must be specified
using the _LDFLAGS_ environment variable (similar to the way
that the _HDF5_ libraries are referenced).
Refer to the installation manual for details.
By default DAP4 support is enabled if _libcurl_ is found.
DAP4 support can be disabled using the _--disable-dap_.

DAP4 uses a data model that is, by design, similar to,
but -- for historical reasons -- not identical with,
the netCDF-Enhanced (aka netcdf-4) model.
Generically, the DAP4 data model is encoded in XML document
called a DMR.
For detailed information about the DAP4 DMR, refer to
the DAP4 specification Volume 1:
http://docs.opendap.org/index.php/DAP4:_Specification_Volume_1

# Accessing Data Using the DAP4 Prototocol {#dap4_accessing_data}

In order to access a DAP4 data source through the netCDF API, the
file name normally used is replaced with a URL with a specific
format. The URL is composed of three parts.
-  URL - this is a standard form URL with specific markers to indicate that
   it refers to a DAP4 encoded dataset. The markers are of the form
   "dap4", "mode=dap4", or "/thredds/dap4". The following
   examples show how they are specified. Note that the "/thredds/dap4"
   case will only work when accessing a Thredds-based server.
   +  [dap4]http://remotetest.unidata.ucar.edu/d4ts/test.01
   +  [mode=dap4]http://remotetest.unidata.ucar.edu/d4ts/test.01
   +  http://remotetest.unidata.ucar.edu/d4ts/test.01#dap4
   +  http://remotetest.unidata.ucar.edu/d4ts/test.01#mode=dap4
   +  http://thredds.ucar.edu/thredds/dap4/...

-  Constraints - these are suffixed to the URL and take the form
    “?dap4.ce=\<expression\>”. The form of the constraint expression
    is somewhat complicated, and the specification should be consulted.

- Client parameters - these may be specified in either of
   two ways.  The older, deprecated form prefixes text to the
   front of the url and is of the the general form [\<name>]
   or [\<name>=value].  Examples include [show=fetch] or [noprefetch].
   The newer, preferred form prefixes the
   parameters to the end of the url using the semi-standard '#'
   format: e.g. http://....#show=fetch&noprefetch.

It is possible to see what the translation does to a particular DAP4
data source in two steps. First, one can examine the DMR
source through a web browser and then second, one can examine
the translation using the "ncdump -h" command to see the
corresponding netCDF-4 representation.

For example, if a web browser is given the following (fictional) URL,
it will return the DMR for the specified dataset (in XML format).
\code
     http://remotetest.unidata.ucar.edu/d4ts/test.01.dmr.xml#dap4
\endcode

By using the following ncdump command, it is possible to see the
equivalent netCDF-4 translation.

\code
     ncdump -h '[dap4]http://remotetest.unidata.ucar.edu/d4ts/test.01'
\endcode

# Defined Client Parameters {#dap4_defined_params}

Currently, a limited set of client parameters is
recognized. Parameters not listed here are
ignored, but no error is signalled.

Parameter Name Legal Values Semantics
- "log" | "log=<file>" - Turn on logging and send the log output to
  the specified file. If no file is specified, then output is sent to standard
  error.
- "show=fetch" - This parameter causes the netCDF code to log a copy
  of the complete url for every HTTP get request. If logging is
  enabled, then this can be helpful in checking to see the access
  behavior of the netCDF code.
- "translate=nc4" - This parameter causes the netCDF code to look
  for specially named elements in the DMR XML in order to
  achieve a better translation of the DAP4 meta-data to NetCDF enhanced
  metadata.
- "opaquesize=<integer>" - This parameter causes the netCDF code to
  convert DAP4 variable size OPAQUE objects to netcdf-4 fixed size
  objects and forces all of them to be of the size specified.
- "fillmismatch" - Unfortunately, a number of servers sometimes
  fail to make sure that the type of the "_FillValue" attribute of a variable
  is the same as the type of the containing variable. Setting this tag
  caused the netcdf translation to attempt to fix this mismatch. If not set,
  then an error will occur.

# Notes on Debugging DAP4 Access {#dap4_debug}

The DAP4 support has a logging facility.
Turning on this logging can
sometimes give important information. Logging can be enabled by
using the client parameter "log" or "log=filename",
where the first case will send log output to standard error and the
second will send log output to the specified file.

Users should also be aware that if one is
accessing data over an NFS mount, one may see some .nfsxxxxx files;
those can be ignored.

## HTTP Configuration. {#dap4_http2_config}

Limited support for configuring the http connection is provided via
parameters in the “.daprc” configuration file (aka ".dodsrc").
The relevant .daprc file is
located by first looking in the current working directory, and if not
found, then looking in the directory specified by the “$HOME”
environment variable.

Entries in the .daprc file are of the form:
````
     ['['<url>']']<key>=<value>
````

That is, it consists of a key name and value pair and optionally
preceded by a url enclosed in square brackets.

For given KEY and URL strings, the value chosen is as follows:

If URL is null, then look for the .daprc entry that has no url prefix
and whose key is same as the KEY for which we are looking.

If the URL is not null, then look for all the .daprc entries that
have a url, URL1, say, and for which URL1 has the same host and port
as URL. All parts of the url's except host and port are ignored.
For example, if URL = http//x.y/a, then it will match
entries of the form
_[http//x.y/a]KEY=VALUE_ or _[http//x.y/b]KEY=VALUE_.
It will not match an entry of the form _[http//x.y:8080]KEY=VALUE
because the second has a port number (8080) different than the URL.
Finally from the set so constructed, choose the first matching entry.

Currently, the supported set of keys (with descriptions) are as
follows.

-# HTTP.VERBOSE
        Type: boolean ("1"/"0")
        Description: Produce verbose output, especially using SSL.
        Related CURL Flags: CURLOPT_VERBOSE

-# HTTP.DEFLATE
        Type: boolean ("1"/"0")
        Description: Allow use of compression by the server.
        Related CURL Flags: CURLOPT_ENCODING

-# HTTP.COOKIEJAR
        Type: String representing file path
        Description: Specify the name of file into which to store cookies. Defaults to in-memory storage.
        Related CURL Flags:CURLOPT_COOKIEJAR

-# HTTP.CREDENTIALS.USER
        Type: String representing user name
        Description: Specify the user name for Digest and Basic authentication.
        Related CURL Flags:

-# HTTP.CREDENTIALS.PASSWORD
        Type: String representing password
        Type: boolean ("1"/"0")
        Description: Specify the password for Digest and Basic authentication.
        Related CURL Flags:

-# HTTP.SSL.CERTIFICATE
        Type: String representing file path
        Description: Path to a file containing a PEM cerficate.
        Related CURL Flags: CURLOPT_CERT

-# HTTP.SSL.KEY
        Type: String representing file path
        Description: Same as HTTP.SSL.CERTIFICATE, and should usually have the same value.
        Related CURL Flags: CURLOPT_SSLKEY

-# HTTP.SSL.KEYPASSWORD
        Type: String representing password
        Description: Password for accessing the HTTP.SSL.KEY/HTTP.SSL.CERTIFICATE
        Related CURL Flags: CURLOPT_KEYPASSWORD

-# HTTP.SSL.CAPATH
        Type: String representing directory
        Description: Path to a directory containing trusted certificates for validating server certificates.
        Related CURL Flags: CURLOPT_CAPATH

-# HTTP.SSL.VALIDATE
        Type: boolean ("1"/"0")
        Description: Cause the client to verify the server's presented certificate.
        Related CURL Flags: CURLOPT_SSL_VERIFYPEER, CURLOPT_SSL_VERIFYHOST

-# HTTP.TIMEOUT
        Type: String ("dddddd")
        Description: Specify the maximum time in seconds that you allow the http transfer operation to take.
        Related CURL Flags: CURLOPT_TIMEOUT, CURLOPT_NOSIGNAL, CURLOPT_CONNECTIONTIMEOUT

-# HTTP.PROXY_SERVER
        Type: String representing url to access the proxy: (e.g.http://[username:password@]host[:port])
        Description: Specify the needed information for accessing a proxy.
        Related CURL Flags: CURLOPT_PROXY, CURLOPT_PROXYHOST, CURLOPT_PROXYUSERPWD

-# HTTP.READ.BUFFERSIZE
        Type: String ("dddddd")
        Description: Specify the the internal buffer size for curl reads.
        Related CURL Flags: CURLOPT_BUFFERSIZE, CURL_MAX_WRITE_SIZE (16kB),
                            CURL_MAX_READ_SIZE (512kB).

-# HTTP.KEEPALIVE
        Type: String ("on|n/m")
        Description: Specify that TCP KEEPALIVE should be enabled and that the associated idle wait time is n and that the associated repeat interval is m. If the value is of the form is the string "on", then turn on keepalive, but do not set idle or interval.
        Related CURL Flags: CURLOPT_TCP_KEEPALIVE, CURLOPT_TCP_KEEPIDLE,
                            CURLOPT_TCP_KEEPINTVL.

-# HTTP.CONNECTIONTIMEOUT
        Type: String ("dddddd")
        Description: Specify the maximum time in seconds that you allow the http connection to complete.
        Related CURL Flags: CURLOPT_TIMEOUT, CURLOPT_NOSIGNAL

The related curl flags line indicates the curl flags modified by this
key. See the libcurl documentation of the _curl_easy_setopt()_ function
for more detail (http://curl.haxx.se/libcurl/c/curl_easy_setopt.html).

For servers that require client authentication, as a rule, the following
.daprc entries should be specified.
````
HTTP.SSL.VALIDATE
HTTP.COOKIEJAR
HTTP.SSL.CERTIFICATE
HTTP.SSL.KEY
HTTP.SSL.CAPATH
````

Additionally, the _HTTP.SSL.CERTIFICATE_ and _HTTP.SSL.KEY_
entries should have same value, which is the file path for a
certificate. The HTTP.SSL.CAPATH entry should
be the path to a directory containing validation "certificates".

# Point of Contact {#dap4_poc}

__Author__: Dennis Heimbigner<br>
__Email__: dmh at ucar dot edu<br>
__Initial Version__: 6/5/2017<br>
__Last Revised__: 11/7/2018

*/