UserGuideOPeNDAPMessages - Revision history

TomSgouros: expunged aliases

2009-10-01T17:36:59Z

expunged aliases

← Older revision		Revision as of 17:36, 1 October 2009
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	\|-		\|-
	\| \|\|"global-attr"		\| \|\|"global-attr"
	\|-
	~~\| \|\| "alias"~~
	\|-		\|-
	\| "global-attr" \|\| <font color="green">Global</font> "variable" {"attr-list"}		\| "global-attr" \|\| <font color="green">Global</font> "variable" {"attr-list"}
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	\|-		\|-
	\| \|\| <font color="green">Url</font>		\| \|\| <font color="green">Url</font>
	\|-
	~~\|"alias" \|\| <font color="green">Alias</font> "alias-name variable;"~~
	\|-		\|-
	\|"variable" \|\| user-chosen variable name		\|"variable" \|\| user-chosen variable name
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	\|-		\|-
	\|"container" \|\| user-chosen container name		\|"container" \|\| user-chosen container name
	\|-
	~~\| "alias-name" \|\| user-chosen alias name~~
	\|-		\|-
	\|}		\|}
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	}		}
	}		}
	~~Alias Matilda Bill.Matilda;~~
	Global {		Global {
	String Name "FamilyData";		String Name "FamilyData";
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	}		}
	}		}
	~~Alias T CTD.Temp;~~
	~~Alias S CTD.Salt;~~
	}		}
	}		}
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	color="green">FNO_Global</font> attributes, as a convenience to those		color="green">FNO_Global</font> attributes, as a convenience to those
	users.		users.

	~~Similar conveniences can be provided using the Alias feature. In the~~
	~~example above, the temperature variable~~
	~~can be referred to as <font color="green">FNO_Global.T</font> if~~
	~~desired. That is, a global alias can provide a client with a known~~
	~~attribute name to query for some property, even if that attribute name~~
	~~is not an integral part of the dataset.~~

	Using global attributes, a dataset or catalog administrator can create		Using global attributes, a dataset or catalog administrator can create
	a layer of ~~aliases and~~ attributes to make OPeNDAP datasets conform to		a layer of attributes to make OPeNDAP datasets conform to
	several different dataset naming standards. This becomes significant		several different dataset naming standards. This becomes significant
	when trying to compile an OPeNDAP dataset database.		when trying to compile an OPeNDAP dataset database.

TomSgouros: removed reference to aliases

2009-10-01T17:34:09Z

removed reference to aliases

← Older revision		Revision as of 17:34, 1 October 2009
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	accuracy of that measurement.		accuracy of that measurement.

	~~====Aliases====~~

	~~Building on the previous example, it might be true that it would be~~
	~~convenient to refer to Matilda without prefixing every reference with~~
	~~<font color="green">Bill</font>. In this case, we can define an ''alias'' attribute as follows:~~

	~~<pre>~~
	~~Attributes {~~
	~~Bill {~~
	~~String LastName "Evans";~~
	~~Byte Age 53;~~
	~~String DaughterName "Matilda";~~
	~~Matilda {~~
	~~String LastName "Fink";~~
	~~Byte Age 26;~~
	}
	}
	~~Alias Matilda Bill.Matilda;~~
	}
	~~</pre>~~

	~~An Example of Attribute Alias~~



	By defining an equivalence between the alias <font color="green">Matilda</font> and the original attribute <font color="green">Bill.Matilda</font>, the string <font color="green">Matilda.Age</font> can be used with or without the prefix <font color="green">Bill</font>. In either case, the attribute value will be 26.

	====Global Attributes====		====Global Attributes====

TomSgouros: /* THREDDS */

2009-09-30T19:14:56Z

THREDDS

← Older revision		Revision as of 19:14, 30 September 2009
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	===THREDDS===		===THREDDS===

	Some OPeNDAP servers can make sensible replies to requests for THREDDS		Some OPeNDAP servers (including [[Hyrax]], the server supplied by the OPeNDAP group) can make sensible replies to requests for THREDDS
	catalog information. This can serve to "advertise" a		catalog information. This can serve to "advertise" a
	server's data by having it appear in catalogs accumulated by THREDDS		server's data by having it appear in catalogs accumulated by THREDDS

TomSgouros: /* DDX */

2009-09-30T19:13:52Z

DDX

← Older revision		Revision as of 19:13, 30 September 2009
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	[[#Constraint_Expressions\|constraint expression]]. Like the DDS, the		[[#Constraint_Expressions\|constraint expression]]. Like the DDS, the
	DDX will describe only the data actually returned.		DDX will describe only the data actually returned.

			The DDX and the DataDDX (containing the dataBLOB) are the nucleus of what will become version 4 of the DAP.

	===THREDDS===		===THREDDS===

TomSgouros: /* The OPeNDAP Messages */

2009-09-30T02:44:48Z

The OPeNDAP Messages

← Older revision		Revision as of 02:44, 30 September 2009
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	data and other services. The queries come in the form of specially		data and other services. The queries come in the form of specially
	formed URLs and the replies consist of MIME documents whose		formed URLs and the replies consist of MIME documents whose
	contents are described in the sections below.		contents are described in the sections below. (Technically speaking, the response document headers aren't exactly right, but they function in the same way.)

	All the requests start with a root URL, and they all are in the form		All the requests start with a root URL, and they all are in the form

TomSgouros: /* Operators, Special Functions, and Data Types */

2009-09-30T02:39:03Z

Operators, Special Functions, and Data Types

← Older revision		Revision as of 02:39, 30 September 2009
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	each data type. These operators are listed in the table below.		each data type. These operators are listed in the table below.

	~~Except for the * operation defined on the URL data type, all~~ the		All the
	operators defined for the scalar base types are boolean operators		operators defined for the scalar base types are boolean operators
	whose result depends on the specified comparison between its		whose result depends on the specified comparison between its
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	\| String \|\| = != ~=		\| String \|\| = != ~=
	\|-		\|-
	\| URL \|\| *		\| URL \|\| = != ~=
	\|-		\|-
	\| '''Compound Types'''		\| '''Compound Types'''
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	<font color='green'>s/time</font> and		<font color='green'>s/time</font> and
	<font color='green'>s/temperature</font>.		<font color='green'>s/temperature</font>.

	~~A special dereferencing~~
	~~<font color='green'>*</font> operator is defined for a URL. This is~~
	~~roughly analogous to the pointer-dereference operator of ANSI C. That~~
	~~is, if the variable <font color='green'>my-url</font> is defined as a~~
	~~URL data type, then <font color='green'>my-url</font> indicates the~~
	~~string spelling out the URL, and <font color='green'>*my-url</font>~~
	~~indicates the actual data indicated by the URL.~~

	The ~= operator returns true when the character string on the left of		The ~= operator returns true when the character string on the left of

TomSgouros: /* Using URLs in a Constraint Expression */

2009-09-30T02:38:09Z

Using URLs in a Constraint Expression

← Older revision		Revision as of 02:38, 30 September 2009
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	returns a boolean value.</blockquote>		returns a boolean value.</blockquote>

	~~===Using URLs in a Constraint Expression===~~

	~~The OPeNDAP data access protocol defines a special data type to handle~~
	~~distributed data: URL. This is a scalar data type, much like the~~
	~~String type, intended to hold one OPeNDAP URL. It generally points at~~
	~~some remote dataset or data value. Using this data type, a constraint~~
	~~expression may make the data returned from one OPeNDAP data server~~
	~~dependent on data held at an entirely different site.~~

	~~In order to accommodate this data type, OPeNDAP defines a special~~
	~~"dereference" operator *. Similar to its function with pointers in C,~~
	~~applying this operator to a URL returns the data specified by that~~
	~~URL. The URL data type itself contains only a character string. It~~
	~~must be dereferenced to produce a reference to the data named by the~~
	~~URL.~~

	~~The following example will return all the stations containing oxygen~~
	~~values greater than fifteen:~~

	~~<pre>~~
	~~?station&station.cast.O2>15.0~~
	~~</pre>~~

	~~Similarly, the following constraint expression will yield all the~~
	~~stations in the dataset whose value is greater than that of the oxygen~~
	~~value indicated by the URL:~~

	~~<pre>~~
	~~?station&station.cast.O2>*http://ocean.edu/etc/nc/data?O2MAX~~
	~~</pre>~~

	~~Finally, suppose that the dataset itself contained a variable of type~~
	~~URL, and that this URL contained the address of oxygen data stored at~~
	~~some other site. The data descriptor for the dataset might look like~~
	~~the following:~~

	~~<pre>~~
	~~Dataset {~~
	~~Sequence{~~
	.
	.
	.
	~~URL O2cal;~~
	.
	.
	.
	~~} station;~~
	~~} arabian-sea;~~
	~~</pre>~~

	~~We can now write the previous constraint as:~~

	~~<pre>~~
	~~?station&station.cast.O2>*O2cal~~
	~~</pre>~~

	~~URLs stored in remote datasets may also be used in the projection~~
	~~clause of the constraint expression. Imagine a dataset that consists~~
	~~only of a list of URLs for each square degree of latitude and~~
	~~longitude. A user could query this dataset for the actual list of~~
	~~URLs, or, by using the * operator, could construct a constraint~~
	~~expression that would return the actual data indicated by the URLs in~~
	~~the target dataset.~~

TomSgouros: /* Constraint Expression Functions */

2009-09-30T01:58:54Z

Constraint Expression Functions

← Older revision		Revision as of 01:58, 30 September 2009
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	For example:		For example:

	[http://test.opendap.org/dap/data/nc/sst.mnmean.nc.gz.asc?version()]		http://test.opendap.org/dap/data/nc/sst.mnmean.nc.gz.asc?version()

	And:		And:

	[http://test.opendap.org/dap/data/nc/sst.mnmean.nc.gz.asc?geogrid()]		http://test.opendap.org/dap/data/nc/sst.mnmean.nc.gz.asc?geogrid()

	<blockquote>		<blockquote>

TomSgouros: /* Constraint Expression Functions */

2009-09-30T01:58:21Z

Constraint Expression Functions

← Older revision		Revision as of 01:58, 30 September 2009
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	===Constraint Expression Functions===		===Constraint Expression Functions===

	~~!! selection functions vs. projection functions~~


	An OPeNDAP data server may define its own set of functions that may be		An OPeNDAP data server may define its own set of functions that may be
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	it must either return a boolean value, or be part of a test that		it must either return a boolean value, or be part of a test that
	returns a boolean value.</blockquote>		returns a boolean value.</blockquote>



	===Using URLs in a Constraint Expression===		===Using URLs in a Constraint Expression===

TomSgouros: /* Operators, Special Functions, and Data Types */

2009-09-30T01:57:47Z

Operators, Special Functions, and Data Types

← Older revision		Revision as of 01:57, 30 September 2009
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	! Class !! Operators		! Class !! Operators
	\|-		\|-
	\| Simple Types		\| '''Simple Types'''
	\|-		\|-
	\| Byte, Int, UInt, Float* \|\| < > = != <= >=		\| Byte, Int, UInt, Float* \|\| < > = != <= >=
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	\| URL \|\| *		\| URL \|\| *
	\|-		\|-
	\| Compound Types		\| '''Compound Types'''
	\|-		\|-
	\| Array\|\| [start:stop] [start:stride:stop]		\| Array\|\| [start:stop] [start:stride:stop]