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  • Title: Writing an OPeNDAP Server -- 3 Writing your own OPeNDAP server
    Descriptive info: Go backward to.. 2.. 2 The JGOFS server.. Go up to.. Top.. Go forward to.. 4 The DAP Architecture.. 3 Writing your own OPeNDAP server.. If neither of the available user-configurable servers will work with your data, or if you wish to add some features they lack, you may write your own OPeNDAP server.. 3.. 1 Choose a language.. It is possible to take the.. DAP specification.. and implement a server which DAP-aware clients can use as a data source.. That is, it is possible to write a DAP server using just the protocol description.. For example, the data server hosted by the.. IRI/LDEO Climate Data Library.. at Columbia University is such a server.. However, server writers don't have to work from the DAP specification.. There are several toolkits available which make this process of writing your own server fairly simple.. The DODS/OPeNDAP project has both a C++ and a Java toolkit.. COLA provides a toolkit called Anagram that can be used to build servers that support server-side processing operations which can be used to generate temporary data sets.. 1.. The OPeNDAP project provides both a.. C++.. and a.. Java.. implementation of the DAP.. Each library includes both the classes that implement the various objects which comprise the DAP and support software that handles the mechanics of processing inbound requests and generating the correct responses.. To choose one of the toolkits, several factors should be weighed.. First, with which of the two programming languages are you most comfortable? Also to be considered are: What  ...   have none of the startup performance issues of CGI programs.. These servers are noticeably faster--about one order of magnitude--for very small requests.. In the future we plan to incorporate this software in our general distribution, contact.. technical support.. or the.. opendap-tech.. list for information/help.. 2 Server architecture.. The essence of the OPeNDAP server architecture is that a collection of programs are used to handle various requests made to the servers.. In addition to these `handler' and `service' programs there's also a dispatcher that interfaces to an http daemon.. The actual requests are made to the web daemon which then passes them along to the dispatcher.. The dispatcher examines the request and decides which handler or service program should process it and how that program should be passed parameters extracted from the request.. The C++ software works exactly as described above; the Java code is.. the same in principle but slightly different in practice, since it's based on servlets.. While the C++ toolkit uses an architecture based on CGI and the Java toolkit uses servlets, both share many characteristics.. If you understand how the servers are built, it will be easy to see how your own server can be implemented with minimal effort.. The Server Installation Guide's section on.. Server Architecture.. provides an excellent description of the CGI-based (C++) servers.. The Server Installation Guide contains a.. short how-to that covers setting up the Java software.. It also explains the software needed to run the servlet-based OPeNDAP servers.. James Gallagher jgallagher@gso.. uri.. edu , 2006-08-17, Revision: 14349..

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  • Title: Writing an OPeNDAP Server -- 4 The DAP Architecture
    Descriptive info: 5 Getting ready to write your components.. The DAP can be thought of as a layered protocol composed of MIME, HTTP, basic objects, and complex, presentation-style, responses.. 4.. 1 The DAP uses HTTP which in turn uses MIME.. Clients use HTTP when they make requests of DAP servers.. HTTP is a fairly straightforward protocol (.. for general information on HTTP see http://www.. w3.. org/Protocols/.. ,.. and for the HTTP/1.. 1 specification, see http://www.. org/Protocols/rfc2616/rfc2616.. html.. ).. It uses pseudo-MIME documents to encapsulate both the request sent from client to server and the response sent back.. This is important for the DAP because the DAP uses headers in both the request and response documents to transfer information.. However, for a programmer who intends to write a DAP server, exactly what gets written into those headers and how it gets written is not important.. Both the C++ and Java class libraries will handle these tasks for you (look at the.. DODSFilter class.. to see how).. It's important to know about, however, because if you decide not to use the libraries, or the parts that automate generating the correct MIME documents, then your server will have to generate the correct headers itself.. 2 The DAP defines three objects.. To transfer information from servers to clients, the DAP uses three objects.. Whenever a client asks a server for information, it does so by requesting one of these three objects (note: this is not strictly true, but the whole truth will be told in just a bit.. For now, assume it's true).. These are the Dataset Descriptor Structure (DDS), Dataset Attribute Structure (DAS), and Data object (DataDDS).. These are described in considerable detail in other documentation.. The Programmer's Guide contains a description of the.. DDS and DAS objects (see http://opendap.. org/api/pguide-html//pguide 6.. html).. These objects contain the name and types of the variables in a dataset, along with any attributes (name-value pairs) bound to the variables.. The DataDDS contains data values.. We have implemented the SDKs so that the DataDDS is a subclass of the DDS object that adds the capacity to store values with each variable.. COADS  ...   are some links to various datasets' ASCII, HTML and INFO responses:.. ASCII for the SST variable.. ASCII for wind speed and direction.. day num 3C170">ASCII for values within a date range.. The VERSION and HELP responses can be see by appending.. help.. or.. version.. to the end of the server's base URL.. For example:.. HELP: http://test.. opendap.. org/opendap/nph-dods/help.. VERSION: http://test.. org/opendap/nph-dods/version.. 4 Parts of the server you don't have to write.. You do not have to write handlers for the ASCII, HTML or INFO responses because the OPeNDAP server includes software that generates these using the DAS, DDS and DataDDS objects.. In addition, if you follow a simple rule about how you name the programs that generate the object responses, you'll be able to fit these within the existing dispatch software and can avoid writing that as well.. The rule is that the three objects are generated by programs named.. name.. das.. dds.. and.. dods.. (the last one generates the DataDDS object).. can be any text.. In practice, it should be short and describe the data with which it's designed to work.. Below is a snapshot of the directory which holds the programs that make up the OPeNDAP servers on my development computer.. The ASCII response is generated by the.. asciival.. program, The HTML and INFO responses are generated by the.. www int.. usage.. programs.. You can also see the dispatch program (.. nph-dods.. ) as well as the DAS, DDS and DataDDS handlers for the netCDF (nc), HDF (hdf), Matlab (mat), JGOFS (jg) and FreeForm (ff) servers.. [jimg@zanzibar etc]$ ls aclocal.. m4 ftp_dods_source.. html MIME/ asciival* handler_name.. pm nc_das* ChangeLog hdf_das@ nc_dds* check_perl.. sh* hdf_dds@ nc_dods* common_tests.. exp hdf_dods* nightly_dods_build.. conf config.. guess* HTML/ nightly_dods_build.. conf.. example config.. sub* HTTP/ nightly_dods_build.. sh* COPYRIGHT INSTALL-clients nph-dods* CVS/ INSTALL-matlab-client nph-dods.. in* cvsdate* installServers printenv* def* INSTALL-servers README deflate* install-sh* README-Matlab-GUI depend.. sh* jg_das* tar-builder.. pl* DODS_Cache.. pm jg_dds* test-dispatch.. sh* DODS_Dispatch.. pm jg_dods* ud_aclocal.. m4 dods.. ini jgofs_objects_readme* update-manifest.. pl* ff_das* localize.. sh* update-manifest.. pl~* ff_dds* LWP/ usage* ff_dods* Makefile.. common usage~* FilterDirHTML.. pm mat_das* usage-jg* ftp_dods_binary.. html mat_dds* www_int* ftp_dods_ml_gui.. html mat_dods* [jimg@zanzibar etc]$..

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  • Title: Writing an OPeNDAP Server -- 5 Getting ready to write your components
    Descriptive info: 6 Subclassing the data types.. The three object handlers are normally implemented in three separate programs.. Each program has a.. main().. function that looks like:.. #include iostream #include string #include "DDS.. h" #include "cgi_util.. h" #include "DODSFilter.. h" extern void read_descriptors(DDS dds, const string filename) throw (Error); int main(int argc, char *argv[]) { DDS dds; DODSFilter df(argc, argv); try { if (!df.. OK()) { df.. print_usage(); return 1; } if (df.. version()) { df.. send_version_info(); return 0; } // Read the netCDF file dataset descriptor in memory read_descriptors(dds, df.. get_dataset_name()); df.. read_ancillary_dds(dds); df.. send_dds(dds, true); } catch (Error e) { set_mime_text(cout, dods_error, df.. get_cgi_version()); e.. print(cout); return 1; } return 0; }.. Most of the software is boilerplate.. The first two lines of.. are shown here:.. DS dds; DODSFilter df(argc, argv);.. These declare an instance of DDS, to be used a little later as well as an instance of DODSFilter.. The latter is used to parse command line arguments fed to the program by the dispatch script.. By using this class in concert with the dispatch script, you can assume that the correct options and arguments will be passed into your program.. The instance of DODSFilter,.. df.. , contains accessors for all the switches that the dispatch script might use, so by passing.. argc.. argv.. to its constructor, you're sure to parse  ...   true);.. These lines are the heart of the handler.. Exactly what's going on here will be covered in more detail later.. However, each of the three object handlers contains similar code that builds the object to returned as the response and then passes that object to the.. DODSFilter::send dds.. send das.. send data.. method, depending on the type of object to be returned.. catch (Error e) { set_mime_text(cout, dods_error, df.. print(cout); return 1; } return 0;.. Rounding out the program is a catch block that picks up exceptions thrown by the any of DAP library code.. The DAP library throws two types of exceptions,.. Error.. InternalErr.. The latter is a subclass of.. , so catching just Error will get everything.. Note that you should also catch.. bad alloc.. exceptions at this level (the library does not) unless you catch them inside the function or method that builds the DDS, DAS or DataDDS.. If.. e.. is an.. , then when it prints, you'll see information about the file and line number where the problem was detected.. Regular.. objects print something that's more useful to users.. By calling the.. set mime text.. function (see the file cgi util.. cc) you're sure that the error message will be returned to the client in a form that both web browsers and more sophisticated clients can use..

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  • Title: Writing an OPeNDAP Server -- 6 Subclassing the data types
    Descriptive info: 7 Implementing the DDS object.. The DAP defines a data type hierarchy as the core of its data model.. This collection of data types includes scalar, vector and constructor types.. Most of the types are available in all modern programming languages with the exceptions being.. Url.. Sequence.. Grid.. In the DAP library, the class.. BaseType.. is the root of the data type tree.. 6.. 1 A quick review of the data types supported by the DAP.. The DAP supports the common scalar data types such as Byte, 16- and 32-bit signed and unsigned integers, and 32- and 64-bit floating point numbers.. The DAP also supports Strings and Urls as basic scalar types.. The DAP includes Arrays of unlimited size and dimensionality.. The DAP also supports three type-constructors:.. Structure.. A.. on the DAP mimics a struct in.. C.. is a table-like data structure inherited from the JGOFS data system.. It can be used to hold information that might be stored in relational databases or tables, either flat or hierarchical.. The JGOFS, FreeForm and HDF servers all use the.. data type.. Lastly, the.. data type is used to bind an array to a group of.. map vectors.. , single dimension arrays that provide non-integral values for the indices of the array.. The most typical use of a Grid is to provide latitude and longitude registration for some georeferenced array data (e.. g.. , a projected satellite image).. The DAP does not have a pointer data type, but in some cases the.. data type can be used as a pointer to variables between files.. More information about the.. DAP's data type hierarchy.. is given in the Programmer's Guide.. 2 The Matlab 5 data model and the DAP.. In the remainder of this tutorial the Matlab 5 OPeNDAP server will be used as an example.. It is a real, non-trivial, server that you can download and run, if you choose.. The Matlab 5 server makes a good example for this tutorial because the Matlab 5 data model is very limited.. Matlab 5 supports scalars, vectors and matrices.. For basic types it supports only floating point numbers and strings.. In the Matlab 5 server, we treat scalar strings as attributes and  ...   to a low-budget factory class ("low-budget" because it's not a class).. This function is used at various places in the DAP library when it need to create instances of.. Byte.. without knowing in advance the dynamic type of the object that actually will be created.. If all this sounds a little weird, just remember that your.. Int16.. ,.. classes -- whatever they may be called -- must all contain an implementation of this function and each should all return a pointer to an instance of the apropriate child class.. These functions will be used by the library to create instance of hte classes you have defined when writing your server.. In this case of the example Matlab server, it's an instance of the.. MATByte.. class.. If you look in the files for the Matlab server, you'll see that the function.. NewGrid.. returns a pointer to a new.. MATGrid.. , and so on.. Second, a constructor must be implemented and should take the name of the variable as its sole argument.. Third, your child classes should also define the.. ptr duplicate().. method.. This method returns a pointer to a new instance of an object in the same class.. Occasionally, in the DAP library, objects are declared with pointers specified as.. BaseType *.. If the.. new.. operator was used to copy such an object, the copied object would be an instance of BaseType (the static type of the object) not the type of the thing referenced (the dynamic type).. By using the.. method the DAP library is sure that when it copies an object, it's getting an instance of the subclass defined by your server.. Finally, each of the child classes must provide an implementation of the.. read.. This method is called by code in the DAP library to read values from the data set.. It will be explained in more detail when we get to building DataDDS responses.. For now, it's enough to know that if a particular server has no use for a given data type (it happens that the Matlab server will never need to create an instance of Byte, because Matlab 5 files can only store float64 matrices) this method should throw an InternalErr object..

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  • Title: Writing an OPeNDAP Server -- 7 Implementing the DDS object
    Descriptive info: 8 Implementing the DAS object.. Building a DDS object is the heart of writing your own OPeNDAP server.. This object will be used to generate the DDS response and it will be the basis of the DataDDS response.. You have to do two things to accomplish building the DDS.. First you must decide how the variables that make up your dataset can be represented using the data type hierarchy that is part of the DAP.. Once you have done this, you need to write code that can build an instance of DDS for your dataset.. In practice the hardest part of this choosing the appropriate data types for you data set; once you know how to map variables in your dataset to the DAP data types, writing code to build the DDS instance is easy.. Many data sets are actually a representative of large group.. In some cases there may be an API that can read the datasets and there may even be a formal data model.. In such a case you're best off using the API and writing general code to build the DDS while performing a depth-first scan of the variables in the dataset.. Here's how the Matlab server builds a DDS object:.. void read_descriptors(DDS dds_table, string filename) { MATFile *fp; Matrix *mp; // dataset name dds_table.. set_dataset_name(name_path(filename)); fp = matOpen(filename.. c_str(), "r"); if (fp == NULL) throw Error(string("Could not open the file: ") + filename); // Read all the matrices in file while ((mp = matGetNextMatrix(fp)) != NULL) { // String types are used as attributes if(mxIsNumeric(mp)) { if(mxIsComplex(mp)) { string Real = (string)mxGetName(mp) + "_Real"; // real part MakeMatrix( dds_table, Real, mxGetM(mp),mxGetN(mp)); string Imag = (string)mxGetName(mp) + "_Imaginary"; // imaginary part MakeMatrix( dds_table, Imag, mxGetM(mp),mxGetN(mp)); } else MakeMatrix( dds_table, (string)mxGetName(mp), mxGetM(mp), mxGetN(mp)); } mxFreeMatrix(mp); } matClose(fp);  ...   = NewFloat64(name); ar = NewArray(name); ar- add_var(bt); ar- append_dim(row,DimName+"_row"); ar- append_dim(column,DimName+"_column"); if (!dds_table) throw InternalErr(__FILE__, __LINE__, "NULL DDS object.. "); dds_table- add_var(ar); }.. This function has two main parts, the first, which is of less interest to this tutorial, checks to see if the matrix holds complex numbers and does some special stuff if it does.. The second part creates a new array of 64 bit floating point numbers.. Here's the code:.. BaseType *bt = NewFloat64(name); ar = NewArray(name); ar- add_var(bt); ar- append_dim(row, DimName+"_row"); ar- append_dim(column, DimName+"_column");.. The first line creates a new instance of the.. Float64.. data type and assigns it to a.. , the parent of all the data types.. There's no reason it couldn't be bound to an instance of.. , but in a server where there might be many types of arrays, it is easier to use a pointer to a more general object.. The second line creates a new Array instance and the third line binds the.. object to the new.. Array.. object, making the array an array of.. s.. The last two lines set the sizes of the Array's dimensions.. Because instances of Array occur frequently, it is a good idea to be familiar with the.. Vector.. classes (.. is the parent of both.. List.. Finally, the last line of the function,.. dds_table- add_var(ar);.. Adds variable.. ar.. to the DDS.. Note that our Matlab server supports only the data types that can appear in a Matlab 5 file.. This means that the only numeric data type supported is a matrix of 64 bit floating point numbers.. Strings, as mentioned earlier, are handled specially.. In most other servers, the code to build the variables and load them in a DDS object is more complex since it must handle mapping the dataset's different types to the DAP's..

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  • Title: Writing an OPeNDAP Server -- 8 Implementing the DAS object
    Descriptive info: 9 Implementing the DataDDS object.. To implement the DAS handler, start with the same boiler-plate code used to create a.. for the DDS handler and replace the call to the function that builds the DDS with one that builds a DAS.. An example of such a function from the Matlab server is shown below:.. void read_attributes(DAS das_table, string filename) { AttrTable *attr_table = das_table.. add_table("MAT_GLOBAL", new AttrTable); MATFile *fp = matOpen(filename.. c_str(), "r"); if (fp == NULL) throw Error(string("Could not open the file: ") + filename.. c_str()); // Read all the matrices in file Matrix *mp; while ((mp = matGetNextMatrix(fp)) != NULL) { // String types are used as attributes if(mxIsString(mp)) { // get size int X = mxGetN(mp); int Y = mxGetM(mp); char *str_rep = new char [X*Y+3]; // quote the string for parser *str_rep = '"'; mxGetString(mp, str_rep+1, X*Y+1); *(str_rep + X*Y + 1) = '"'; *(str_rep + X*Y + 2 )= '\0'; if (attr_table- append_attr(mxGetName(mp), "String",str_rep) == 0) { delete [] str_rep; mxFreeMatrix(mp); matClose(fp); throw Error(string("Couldn't output attribute: ") + mxGetName(mp)); } delete [] str_rep; } else { das_table.. add_table(mxGetName(mp), new AttrTable); } mxFreeMatrix(mp); } matClose(fp); return true; }.. In this example the server creates a single attribute container called MAT GLOBAL and loads all the data set attributes into it.. Most data set types (e.. , hdf) have both global attributes (those that apply to the entire data set) and attributes that only apply to a particular variable.. Such data sets have both a global attribute container.. and.. separate attribute containers for each variable.. Matlab has  ...   = matGetNextMatrix(fp)) != NULL) {.. Inside the.. while.. -loop that iterates over the data set's variables, we test for string variables.. The Matlab server assumes that all string variables in a data set are actually global attributes for the data set (and not `data' variables).. If a string variable is found, the code uses the variable's name and value as the attribute name and value (code that handles the case where a variable is not a string is explained further down).. Attributes are all string-valued in the DAP.. That is, even though the DAP supports the full range of scalar data types for attributes, the.. values.. are stored as strings.. The call to.. AttrTable::append attr.. adds the attribute tuple (Name, type and value) to the AttrTable instance.. Note that if.. fails, it returns zero and the code cleans up and throws an exception.. I elided that from this sniplet to focus the example.. // String types are used as attributes if(mxIsString(mp)) { // get size int X = mxGetN(mp); int Y = mxGetM(mp); char *str_rep = new char [X*Y+3]; // quote the string for parser *str_rep = '"'; mxGetString(mp,str_rep+1,X*Y+1); *(str_rep + X*Y + 1) = '"'; *(str_rep + X*Y + 2 )= '\0'; if (attr_table- append_attr(mxGetName(mp), "String", str_rep) == 0) {.. If the variable is not a string, then the Matlab server creates an empty attribute table for it.. This is to conform to the.. DAP 2.. 0 specification.. which states that all variable must have an attribute table, even if it is empty.. else { das_table.. add_table(mxGetName(mp), new AttrTable); }..

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  • Title: Writing an OPeNDAP Server -- 9 Implementing the DataDDS object
    Descriptive info: 10 Notes.. Building the DataDDS object handler follows the same pattern as before with the DDS and the DAS.. In fact, the DDS handler can be modified and used as the DataDDS handler by simply changing the call to.. DODSFilter::send dds().. to a call to.. send data().. However, before the.. method will work, we must return to the data type child classes and add more functionality.. In the data type child classes you created we must now implement the method.. read().. This method will be called by the DAP software that sends data values.. To understand how the.. method will be used, it's instructive to look at the code that calls it.. In the DAP data type classes, each of the scalar, vector and constructor types has a method called.. serialize().. Below is shown.. Byte's.. version of this method (Look at the code for.. if you'd like to see how.. is used and pay particular attention to.. DDS::send data().. You don't have to write your own version of.. , this is.. shown here to provide you with some background information about the role of the.. method in building the DataDDS response.. bool Byte::serialize(const string dataset, DDS dds, XDR *sink, bool ce_eval) { if (!read_p()) read(dataset); // read() throws Error and InternalErr if (ce_eval !dds.. eval_selection(dataset)) return true; if (!xdr_char(sink, (char *) _buf)) throw Error( "Network I/O Error.. Could not send byte data.. \n\ This may be due to a bug in DODS, on the server or a\n\ problem with the network connection.. "); return true; }.. method is broken into three parts:.. Read the data for this variable if that has not already been done.. We'll see later that when.. successfully completes, it sets a boolean that the DAP library tests with the.. read p().. returns.. true.. then data for this variable has already been read.. Next the constraint expression (CE) is evaluated.. The constraint expression was passed to the server as part of the DataDDS request.. Parsing the CE and ensuring it's evaluated correctly is handled for you by the DAP library.. Finally, if the CE evaluates to.. , then we serialize the binary data that was read into this instance with the.. call at the beginning of this method.. CE evaluation actually happens in two phases.. In the first phase, the.. expression is parsed.. During this process, variables that are `projected' are marked as such and a linked list of `selection' nodes is built.. method is called only for variables that are part of the projections (that is, that are to be sent back to the client).. The second phase of CE evaluation happens inside.. when the.. DDS::ce eval.. method is used to evaluate the clauses.. If all the clauses evaluate to true, then the current variable is sent.. Here's the.. method for the Matlab server's.. class (.. MATArray.. This is by far the most complex looking piece of code in this tutorial, but it's really not very complicated once broken down.. bool MATArray::read(const string dataset) { if (read_p()) // Nothing to do return false; MATFile *fp = matOpen(dataset.. c_str(), "r"); if (fp == NULL) throw Error(string("Could not open the file: ") + dataset.. c_str()); Array::Dim_Iter p = dim_begin(); int start = dimension_start(p,true); int stride = dimension_stride(p, true); int stop = dimension_stop(p, true); p++; int start_2 = dimension_start(p,true); int stride_2 = dimension_stride(p, true); int stop_2 = dimension_stop(p, true); // get real part of the complex matrix double *DataPtr; Matrix *mp; size_t pos; if ((pos = name().. find("_Real")) != name().. npos) { string Rname = name().. substr(0,pos); mp = matGetMatrix(fp,Rname.. data()); DataPtr = mxGetPr(mp); } else{ // get Img part of the complex matrix if ((pos = name().. find("_Imaginary")) != name().. npos) { string Iname = name().. substr(0,pos); mp = matGetMatrix(fp,Iname.. data()); DataPtr = mxGetPi(mp); } else{ mp = matGetMatrix(fp,name().. data()); DataPtr = mxGetPr(mp); // get the matrix structure } } if (DataPtr == NULL) throw Error(string("Error reading matrix")); if(start+stop+stride == 0){ //default rows start = 0; stride = 1; stop = mxGetM(mp)-1; } if(start_2+stop_2+stride_2 == 0){  ...   real part of the complex matrix double *DataPtr; Matrix *mp; size_t pos; if ((pos = name().. substr(0,pos); mp = matGetMatrix(fp, Rname.. substr(0,pos); mp = matGetMatrix(fp, Iname.. data()); DataPtr = mxGetPr(mp); // get the matrix structure } } if (DataPtr == NULL) throw Error(string("Error reading matrix"));.. Once the data have been read from the data set we need to check for sub-sampling that may have been specified by the client and passed to the server via the CE.. The CE was automatically parsed by the boilerplate code, but we need to explicitly look at the values because data set types are fairly idiosyncratic about how they use this information.. While the Matlab server reads the entire array from the dataset and then applies the sub-sampling information,.. many.. data set types provide ways to subsample variables through their own API (e.. , HDF, NetCDF,.. ).. In such cases, you'd read the CE information, then use it to read the data values.. The most important point is that.. you don't always have to read the entire variable from the data when using the DAP.. In fact, most servers don't, they make sure to use the data set's underlying API in the most efficient way possible, something that the DAP was designed to make possible.. Since the Matlab server supports only Matlab 5 and since Matlab 5 supported only two dimensional matrices, we grab a pointer to the first and second dimensions using the.. Array::first dim().. Array::next dim().. methods.. Array::dimension start().. dimension stride().. dimension stop().. methods are used to the read the start and stop indices and the sub-sampling stride for the dimension referenced by the.. Array::Dim Iter.. p.. 5.. Array::Dim_Iter p = first_dim(); int start_1 = dimension_start(p,true); int stride_1 = dimension_stride(p, true); int stop_1 = dimension_stop(p, true); p++; // increment iterator to get the second dim.. int start_2 = dimension_start(p,true); int stride_2 = dimension_stride(p, true); int stop_2 = dimension_stop(p, true); if(start_1 + stop_1 + stride_1 == 0){ //default rows start = 0; stride = 1; stop = mxGetM(mp)-1; } if(start_2 + stop_2 + stride_2 == 0){ //default columns start_2 = 0; stride_2 = 1; stop_2 = mxGetN(mp)-1; }.. Using the information from the CE, the array values are sub-sampled and copied to new storage.. Again, this step is generally not necessary when it's possible to subsample variables in the data set using an API, et cetera.. int Len = (((stop_1 -start_1)/stride_1)+1)*(((stop_2-start_2)/stride_2)+1); int Tcount = 0; dods_float64 *BufFlt64 = new dods_float64[Len]; for (int row = start_1; row = stop_1; row += stride_1) { for(int column = start_2; column = stop_2; column+=stride_2) { *(BufFlt64+Tcount) = (dods_float64) *(DataPtr+row+column*mxGetM(mp)); Tcount++; } }.. The values, now read and sub-sampled are copied into the DAP variable object.. The DAP methods enforce a strict policy that all memory allocated outside of the library must be deleted outside the library, and vice versa.. So values sorted in the.. BufFlt64.. array are copied to new storage allocated inside the.. object.. This code deletes the.. array.. Also important is the call to.. set read p().. with the value.. this sets the.. read p.. property.. so that, should this function be run again while building this particular response, it will know the data have already been read.. val2buf((void *)BufFlt64); delete [] BufFlt64; set_read_p(true);.. The remaining software frees resources allocated via the Matlab data access API.. As was explained earlier, the.. return value is a hold over from an earlier version of the DAP library.. mxFreeMatrix(mp); matClose(fp); return false;.. To build a.. function that will return the DataDDS, copy the one for the the DDS but change the call for.. to.. DODSFilter::send data().. This will work because the code leading up to the.. call builds the DDS object, then the.. call will arrange to build the DataDDS response using the DDS.. During this process it will parse and evaluate the CE and call the.. methods for the variables in the DDS.. Thus the DDS will contain variables loaded with values which can then be used to create the information in the DataDDS object/response..

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  • Title: Writing an OPeNDAP Server -- 10 Notes
    Descriptive info: References.. Here's a collection of information that might be important to specific servers but is hard to fit into a general tutorial.. It's possible to replace the three separate programs, one for the DAS, DDS and DataDDS, with a single program.. Clearly there all share close to 90 of the same code.. You can tell which object is being requested by either using the old Unix trick of making the programs symbolic links to a single executable and then checking to see which name was used to invoke it, or by modifying the dispatch script slightly to pass that information along with the other parameters.. Similarly, you can subclass the DAS and DDS objects adding methods to them to build up the attributes and variables instead of writing functions to do that task.. For complex data sets, it's likely that you'll need to create some new fields in the data type classes you specialized.. If you're writing a server that will principally be for internal use, or you expect many many small accesses, you  ...   contain many files.. The first is to set up a `file server' a kind of catalog of URLs that is itself a OPeNDAP data set.. The second is called the Aggregation Server (AS).. The AS can automatically aggregate discrete datasets, accessed as either as files (in some cases) or URLs to produce a single data set.. See the.. and/or contact.. User Support.. for help with this.. When you're returning error messages, be sure to trim file pathnames so that the real path to data is hidden.. If you have a data set and its API requires you to store a lot of state information that's not specific to a given variable or data type, consider creating a mixin class and making your data type children inherit from it in addition to the DAP classes.. Because the `virtual contractors' are used to create instances of the data type classes, you can be sure that in your server, all instances of Byte, etc.. , contain the mixin.. Use the.. dynamic cast.. operator to access the mixin's methods..

    Original link path: /api/ws-html/writing_server_10.html
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  • Title: Writing an OPeNDAP Server -- References
    Descriptive info: Footnotes.. Scott Meyers.. Effective C++: 50 Specific Ways to Improve Your Programs and Designs.. Addison-Wesley, 1992.. [2].. Bjarne Stroustrup.. The C++ Programming Language.. Addison-Wesley, Reading, Ma.. , 1991..

    Original link path: /api/ws-html/writing_server_11.html
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  • Title: Writing an OPeNDAP Server -- Footnotes
    Descriptive info: (1).. For information about Anagram or the GDS and FDS servers, see the COLA web site.. (2).. This is the oft discussed phenomenon of `slicing,' see Meyers.. , Stroustrup.. , et c.. , for a complete explaination.. (3).. By convention, global attribute containers which hold values read from the data set has the suffix GLOBAL; the first part of the container's name is either read from the dataset or  ...   a variable is read while evaluating the constraint expression.. So it can be the case that the values for a variable are read before the.. method calls.. (5).. The Matlab server included in the 3.. 4 source code distribution uses the deprecated.. Pix.. (pseduo-index) interators.. This example uses the STL-based iterators.. (6).. stands for.. Predicate.. ; maybe not the best naming convention given the prevalence of pointers in C software..

    Original link path: /api/ws-html/writing_server_12.html
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  • Title: Overview
    Descriptive info: Overview.. Package.. Class.. Use.. Tree.. Deprecated.. Index.. Help.. PREV NEXT.. FRAMES.. NO FRAMES.. All Classes.. Packages.. opendap.. dap.. This package contains the OPeNDAP core classes, known as the Data Access Protocol (DAP).. functions.. parser.. This package contains Bison 2.. 4.. 2 generated DAP parser classes.. Server..  ...   contains classes used to test the proper function of opendap.. XMLparser.. util.. This package contains utility classes used by several OPeNDAP packages.. geturl.. This package contains classes to implement Geturl, the sample OPeNDAP DAP2 client.. gui.. This package contains the GUI status window class used by Geturl..

    Original link path: /api/javaDocs/overview-summary.html
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