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    Archived pages: 580 . Archive date: 2014-01.

  • Title: DODS Programmers Guide -- 0.1 Who is this Guide for?
    Descriptive info: Go backward to.. Preface.. Go up to.. Go forward to.. 0.. 2 Organization of this Document.. 1 Who is this Guide for?.. This guide is for people who wish to use the OPeNDAP software to write a new OPeNDAP data server, a new client, or a new client library.. Typically, this will only be those people who wish to serve data in a format that is not currently supported by the DODS team, or who have an existing application that uses an idiosyncratic or unusual API for data access.. Most people will be able to use one of the already written servers or client libraries.. See the..  ...   class library (API) that parallels the C spftware described here.. 1.. Also available are two tutorials,.. Writing an OPeNDAP Client.. and.. Writing an OPeNDAP Server.. , which descrie how to write a client or a server, respectively.. Because the type of information presented in a document like this depends to a large extent on the needs of its readers we welcome your feedback and comments.. In particular, if you have any questions about individual sections, email those questions and we'll send back an answer as well as including that information in the next version of this document.. Send queries to:.. support@unidata.. ucar.. edu.. Tom Sgouros, July 2, 2004..

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  • Title: DODS Programmers Guide -- 0.2 Organization of this Document
    Descriptive info: 3 Conventions.. This Guide is divided into five chapters.. Chapter.. provides background information on the organization of the toolkit software.. 2.. describes how to use the Network I/O classes to manage virtual connections.. 3.. discusses how to sub-class the toolkit C classes so that they are specialized for your specific use.. 4..  ...   the data server and the client-library for a new API.. 5.. describes how to link user programs with the new client-library implementation of an API.. Note that this is.. not.. a reference volume.. See.. The DODS Toolkit Reference.. for a concise listing of the member functions in each of the toolkit's classes..

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  • Title: DODS Programmers Guide -- 0.3 Conventions
    Descriptive info: 1 The DODS Client and Server Toolkit.. The typographic conventions.. shown.. in Table.. 3.. are followed in this guide and all the other DODS documentation.. Typographic Conventions.. Literal text.. Typed by the computer, or in a code listing.. User input.. Type this precisely as written.. Variables.. Used as a place holder for a user-specified  ...   in place.. Button Text.. Used to indicate text on a GUI button.. Menu Name.. This is the name of a GUI menu.. When referring to a button in a menu, we will often use the notation:.. Menu,Button.. For example,.. Options,Colors,Foreground.. would indicate the.. Foreground.. button in the.. Colors.. menu, selected under the.. Options.. menu..

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  • Title: DODS Programmers Guide -- 1 The DODS Client and Server Toolkit
    Descriptive info: Contents.. 1 The.. DAS.. DDS.. Objects.. The Distributed Oceanographic Data System (DODS) is a system used to facilitate access to scientific data on the internet.. Using DODS, you can turn any data analysis program that uses one of several data access APIs into a powerful, internet-friendly data browser.. For basic information about the structure of DODS and its components, please see.. The DODS Toolkit software consists of a collection of C classes used to build DODS data servers and clients:.. Type classes.. A complete implementation of the DODS data access protocol (DAP).. This consists of a set of virtual classes for different types of data, which must be sub-classed to use.. Data classes.. This is a set of classes, including both the Data Descriptor Structure (.. ) and the Data Attribute Structure (.. ), designed to contain information about a dataset's data.. Connect classes.. These classes are used by a DODS client to mimic a durable connection to a DODS  ...   variables, rooted at the class.. BaseType.. The following sections present an overview of these class hierarchies and describe, in general terms, how they are used to build the client-library and data server.. Section.. 1.. for information about the.. classes.. To use the type classes, they must be subclassed.. provides a detailed description of this process.. Detailed information about the.. themselves is available in the.. Descriptions of the classes and their member functions are in the reference material ,.. The DODS toolkit also contains two classes used by DODS clients to manage network connections between themselves and a server.. The.. Connect.. object manages a single connection with some DODS server, and the.. Connections.. object manages a group of.. objects.. The Connect class is meant to be subclassed when used by a real client library.. Object.. 2 The.. 2 The Type Hierarchy.. 2.. 1 Common Ancestor: BaseType.. 2 Simple Types.. 3 Vector Types: Array, List.. 4 Compound Types: Structure, Sequence, Function, Grid..

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  • Title: DODS Programmers Guide -- 1.1 The DAS and DDS Objects
    Descriptive info: The dataset attribute structure (DAS) and dataset descriptor structure (DDS) objects are used to store information about a data set's variables.. These objects are used on both the client and server sides, although there are class features that only pertain to one or another of the roles.. They can be thought of as metadata objects.. In this book, however, we will avoid the term.. metadata.. because often this is.. data.. to many users.. It might be said that neither the.. nor the.. contain actual science data -- the.. contains attribute information from the data set while the.. contains structural information about the data set and variables in the data set.. Since the boundary between data and metadata (or data attributes) is often a blurry one, this is not a distinction we will insist on.. To build both the.. , the server either reads information directly from the dataset or from DODS-specific.. ancillary data.. files,.. depending on the capabilities of the data access API used to access the data.. server.. filter.. programs.. do this and then transmit the resulting object to the client.. On the client side, the DODS client.. uses information in the DAS and DDS to satisfy API calls issued by the user program requesting information about variables, their type, shape, and attributes.. The  ...   it is possible to write the text representation for either object (using a text editor) and then use the parser to create the internal, C , object.. Furthermore, the text representation is a type of persistence and can be used to build a flexible object caching mechanism.. One possible use for this caching mechanism is to store the.. for a dataset and use the stored versions in place of opening the data set and reading information about it and its contents.. For large data sets with many variables this can result in a significant performance improvement, and several of the packaged DODS servers use it.. The caching mechanism may also be used on the server-side to store extra information about the data set--information that is not present in the data set proper, but which the data provider would like included when people access the data set via DODS.. In this scenario, the data server first integrates the data set file(s) using the API and builds the.. Once an initial version of the.. are resident in memory, the parser is used to read an external text file which contains additions to, or corrections of, the information extracted from the data file.. This information is the ancillary data introduced above.. Several of the DODS servers use this mechanism..

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  • Title: DODS Programmers Guide -- 1.1.1 The DAS Object
    Descriptive info: contains attribute information that is generally.. used by software when processing the variables; this object is specifically designed to hold all the information in a data set that has no where else to go.. Each variable can have an unlimited number of attributes.. There are also `global' attributes that apply to the dataset as a whole.. Each attribute is a set of three elements: the attribute name, type and value.. The supported types for an attribute are: Byte, Int32, Float64, String and URL, and vectors of these types.. These types have the same range of values as the corresponding types in the data access protocol.. A.. variable's attributes can be any qualities that are not part of the.. Thus, the type and shape of a variable are.. attributes; they are characteristics of the variable and are part of the.. (see Section.. ).. However, users may store any other information as attributes.. , including paragraphs of text, vectors of integers and floating point values, and so on.. There are some standard attributes required by the DODS standard.. The system.. could.. work without these attributes, but they are required anyway to make other aspects of the systems work better.. Many APIs support the concept of information in the data set that is not structured, and provide function calls to access that information.. object provides a place for all that information.. The data server can interrogate the data set and build the.. and the client-library can use the.. object to satisfy many of the API calls requesting information about the variables in a data set.. Figure.. is a diagram of the.. object.. consists of the following components:.. A list of attribute tables, each of which is a list of attributes for a particular data variable.. Since data  ...   An attribute table is a list of name-type-value triples used to describe a data variable.. Each attribute describes some aspect of the data variable.. The example list in figure.. shows what a table might look like for one of the data variables in figure.. (The types have been left out of the diagrams for clarity.. ).. An Attribute Table.. object contains member functions to add or retrieve.. AttrTable.. objects as well as individual attributes based on a variable name.. In addition, the.. object may be traversed using a.. Pix.. The value of an attribute can itself be another attribute table.. So, for example, an aggregate variable that contains other variables, such as a Structure, might have an attribute table for each of its member variables.. In figure.. , you can see illustrated the attributes of an aggregate data variable.. The first three attributes apply to the collection of data (it was taken with a CTD instrument, from the good ship R/V Endeavour, and so on), while the next three attributes reveal attributes of the constituent data variables.. Each of the values of these attributes is itself an attribute table, containing attribute data about that data type.. A Nested Attribute Table.. DODS attribute tables are modelled with the.. class.. Each.. object contains a doubly-linked list of attribute triples, of a structure named.. entry.. Each entry object contains a Name, Type and vector of values.. provides methods for reading, writing, and modifying the attribute table.. The DODS definition of dataset attributes contains a "Global" attribute.. This has nothing to do with the data structure of the.. object, but with its use in DODS.. Global attributes apply to all the variables in the dataset.. They can also be thought of as being attributes of the dataset itself..

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  • Title: DODS Programmers Guide -- 1.1.2 The DDS Object
    Descriptive info: is used to store information about the organization of the data set and its variables.. It contains information about the type and shape of variables.. While the.. is similar to the.. in that it is used to store information about the data set, it is used quite differently by both the client and server components of DODS.. is a stand-alone object and is used solely for the purpose of storing attributes of variables and the dataset.. , however, stores type information about a data set's variables by storing actual instances of those variables.. The DODS data access protocol variable objects have methods that can be used to read values from a data set or transfer the variable's value over the network.. This makes it convenient to use the.. object itself to hold data, and on the server side, the.. object is used by both the.. filter program and the data filter program.. (See.. for more information about the structure of the DODS server.. shows the structure of the.. class objects.. String.. object  ...   used to transmit the.. from server to client.. A parser and scanner to read a text.. and convert it into its C form.. This is used to read.. information from a disk, and also to receive it over the network.. A singly-linked list of parsed constraint expression clauses.. The clauses wait here to be evaluated.. A constraint expression parser and scanner to extract constraint expression clauses from the input constraint expression.. This component is responsible for creating the list of constraint expression clauses.. object provides methods to access and operate each of these components.. The two lists can be traversed with a g++.. is `lexically scoped' so that two.. Structure.. variables may have components with the same names; each component will be referred to using the.. name and the dot operator.. So for example, if a.. called.. ralph.. contains two structures,.. vitals.. new hip.. , and each structure contains a variable called.. age.. , you can differentiate the two by referring to one as.. vitals.. age.. and the other as.. new hip..

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  • Title: DODS Programmers Guide -- 1.2 The Type Hierarchy
    Descriptive info: Type Hierarchy.. is.. the set of classes that form the hierarchy used to build objects that contain data.. These classes comprise the data model for DODS.. They contain.. simple.. types such as integer and floating point values as well as.. compound.. types.. like structure and sequence.. Each type is embodied by a C class, and the classes are arranged in a class hierarchy, with a.. defining properties inherited by all the type classes.. This section contains a brief  ...   an application program.. For detailed descriptions of the characteristics of each type, including inheritance diagrams, please see the.. The DODS types can be divided into four categories:.. Simple Types.. Byte.. Int16.. UInt16.. Int32.. UInt32.. Float32.. Float64.. Str.. Url.. Vector Types.. List.. Array.. Compound Types.. Sequence.. Grid.. Unlike the other classes in the DODS toolkit, the type classes are abstract classes--in order to be used by a program, you must subclass the hierarchy and create concrete classes to instantiate..

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  • Title: DODS Programmers Guide -- 1.2.1 Common Ancestor: BaseType
    Descriptive info: The root of the type hierarchy is the abstract class.. This class, because it is abstract, is never instantiated itself.. is used as the base class for all of the different types of variables, and contains common member functions used by all the other type classes.. For simple variables such as.. , only the abstract virtual functions in.. need to be added to complete the class definition.. Compound types like.. usually require more.. A compound type contains one or more instances of.. , and requires methods to access, add and remove these member variables..

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  • Title: DODS Programmers Guide -- 1.2.2 Simple Types
    Descriptive info: The DODS simple data types consist of.. ,.. These data types match very closely the corresponding types in C or Fortran.. Note that--internally--each of these types uses either the C or C representation to hold the value of the object.. These abstract classes are direct descendents of.. which contain only their definitions for.. 's abstract member functions and a single constructor.. Variables which store bytes.. Equivalent to unsigned char on most UNIX workstations.. Variables which store integer values as 16-bit twos-complement signed integers.. Equivalent to.. int.. on most 16-bit  ...   integers.. on most 32-bit UNIX workstations.. A 32-bit unsigned integer value.. Variables which store floating point data.. Defined as the IEEE 32-bit floating point data type, equivalent to a.. float.. in ANSI C.. Defined as the IEEE 64-bit floating point data type, equivalent to a.. double.. Variables which store string information.. A DODS string is.. a sequence of characters referenced using a pointer (as it is in C), it is represented using a C object of the class.. Variables which store references to network resources.. This is a sub-class of..

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  • Title: DODS Programmers Guide -- 1.2.3 Vector Types: Array, List
    Descriptive info: The vector data types are.. is a simple ordering of elements of a single type.. An.. arranges the elements so that they can be easily accessed with one or more indices.. Instances of.. have different semantics than arrays in C.. They are multi-dimensional data structures which contain.. N.. instances of some data type.. Each instance in the array can be referred to by an integer index between.. 0 and N-1.. Multidimensional arrays are declared using C's syntax of a sequence of bracketed integer values:.. Int32 a[10][20].. declares an array of 10 arrays of 20 integers.. However, unlike C arrays, the.. class supports named dimensions.. In the preceding example, the array could have been declared:.. Int32 a[row = 10][col = 20].. where.. row.. col.. are the names of the first and second dimension, respectively.. You can use the..  ...   the array (10 by 20), but not the type of the elements (Int32).. The element type information is stored in the component variable which the instance of.. references.. When creating an array, the dimension sizes (and optionally their names) must be set.. Regardless of the shape of the array, it is always stored as a vector.. In order to access the element of a multidimensional array it is necessary to calculate the offset for a given element.. is an ordered collection of elements of unknown length.. This is in contrast to the.. , whose size is always known in advance.. When a List type is declared no size information is supplied, but in order to transmit a.. object the length of that list must be known.. Thus, internally the number of elements in the current value.. stored..

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