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+RFC 242
+NIC 7672
+Categories: D.4, D.7
+
+
+               DATA DESCRIPTIVE LANGUAGE FOR SHARED DATA
+
+
+                                L. Haibt
+                               A. Mullery
+
+
+                    Thomas J. Watson Research Center
+                         Yorktown Heights, N.Y.
+
+
+                             July 19, 1971
+
+
+Introduction
+
+    A primary consequence of the use of networks of computers is the
+demand for more efficient shared use of data.
+
+    Many of the impedements to easy shared data follow from the many
+diverse ways of representing and making reference to the same data.
+Almost all of these problems have been known before data was shared
+through computer networks, but the network facility has simply
+emphasized the problems.
+
+    For convenience of discussion, representation differences will be
+classified in three categories. The first category is one of very local
+representation - the bit patterns for the character set, for fixed point
+and floating point numbers. These differences are usually imposed by
+differences in CPU's and storage devices. Translations from one
+representation to another at another at this level can usually be made a
+unit at a time (e.g. computer word by computer word) with the most
+serious problems occurring when there are some values in one
+representation scheme which have no corresponding meaning in the other
+representation scheme, as, for expamble, when trying to translate
+eight-bit bytes to six-bit bytes.
+
+    A second category of differences has to do with the representation
+of collections of data, e.g., their size, ordering and location.
+
+
+
+
+
+
+
+                                                                [Page 1]
+
+A third category of representation differences which is a little
+difficult to characterize has to do with all the more complex structures
+that data collections may have - for example, files with indexes, fields
+with internal pointers and cross references, and collections of files
+such as partitioned data sets and generation data sets in OS 360.
+
+    The approach to coping with these problems within our project of
+Network/440 has been to work on the development of a descriptive
+language which would permit the specification of those aspects of data
+representation which would be subject to transformation in moving data
+about in a network. Then, the network data managment system would be
+able to refer to the descriptions as needed in the data management
+function. For example, to a large extent, one could supply two
+descriptions to the data manager, one wich indicates how data is now
+represented, and one which indicates how a copy of it should look, and
+the data managment systems could invoke the necessary transformations to
+make the proper copy.
+
+    This approach to specifying data transformation contrasts somewhat
+with systems, such as the RAND Form Machine, which provide a formalism
+for specifying the particular translation alogrithms for changing form
+one form to another. the descriptor-to descriptor approach seems to
+simplyfy the programming burden when creating new field formats. Neither
+method of specifying translations precludes the use of a Network
+Standard Reprsentation.
+
+Structure
+
+    The descriptive language assumes that data may have an inherent
+structure independent of other groupings, such as name groupings,
+locking groupings, etc., imposed on it. A data structure description
+consists of groupings of established data value type codes. The list of
+established data value types should be sufficient, through appropriate
+groupings, to describe any hierarchical structure of data.
+
+    The data type identifies how the data value is to be interpreted. A
+list of data type codes is given below. This list must be able identify
+each data type that may exist in a data set in any machine in the
+network. However, for data sets that contain only data types of the
+machine on which it is stored, it is not necessary that a different code
+be defined for different forms of any single type that may exist among
+different machines. The data type specified in the description along
+with the identification of the machine at which the data is stored is
+sufficient to completely describe all such forms of the data types. A
+tentative list of machine dependent type codes, compiled by
+
+
+
+
+
+
+                                                                [Page 2]
+
+G. Howe and T. Kibler is as follows:
+
+          F    floating point
+
+          I    fixed
+
+          D    double precision floating point
+
+          C    character string
+
+          X    complex
+
+          P    packed decimal
+
+          L    logical
+
+
+    It is desirable to be able to construct data sets that contain
+either data types not allowable at the machine at which the data set
+is stored or, possibly, even types that say not exist at any machine
+in the network. For example, one may wish to store eight bit data on a
+six bit machine.  This may, in principle at least, be done by defining
+a logical data set containing eight bit bytes in terms of a real data
+set containing six bit characters. For this, however, data value type
+descriptors have to be defined that are machine independent. The basic
+machine independent data type is as follows:
+
+          B    bit.
+
+It is not clear at this time that any others are necessary since others
+can be built from this one. For convenience, other standard machine
+independent data types may later be defined.
+
+    Two other machine independent types are useful in describing
+structures. These are:
+
+          Z    null
+
+          O    omit.
+
+the null type indicates that there is no data corresponding to this
+item: however, the item should be counted as existing in the structure.
+The omit type indicates just the opposite: there is data that should not
+be counted as an item, it should be ignored.
+
+
+
+
+
+
+
+                                                                [Page 3]
+
+A grouping of data values is described by the list of elements of
+the grouping enclosed in parentheses. An element of grouping may be
+either a data value as described by one of the data value type codes, or
+a grouping. The list consists of these elements, separated by commas and
+indicates that the elements appear in the grouping in the order
+indicated. For example, the description:
+
+          ((C,C),(F,F,I))
+
+
+describes a data collection consisting of two subgroupings, the first
+subgrouping consisting of two data values of type 'C', and the second
+subgrouping consisting of two data values of type 'F' followed by a data
+value of type 'I'. the structure of this data collection is thus a three
+level tree which may be shown in two dimensions as follows:
+
+          ( )
+           |
+          ( )---( )
+           |     |
+           C-C   F-F-I
+
+Properties
+
+
+    Other properties of data beyond that of the structures and
+composition of the data set have also to be described.  These may be
+assigned to items of the data collection, where an item may be defined
+as an individual data value, or a grouping of these, by modifying the
+item description with the specification of the preperties that apply to
+it. The notation that will be used will be an infix notation of the
+form:
+
+         operand operator |[extent]|
+
+where the operator indicates the property type, the operand the property
+value and the optional extent the numer of items to which the property
+applies. Normally the property is assumed to apply to just the following
+item in the description. If the property is to apply to more than just
+the following item description, this is indicated by specifying a number
+as the extent, this number indicating how many of the following item
+definitions at the same level the property is to apply to.
+
+    Type - The structure description of the data set is a constitutional
+or syntactic description of the data set. In some cases it is necessary
+to give a discription of the use or meaning of an element. For example,
+in some complex data structures, the linkages of the structures may be
+represented as data values in the data set. Thus, though the more
+
+
+
+                                                                [Page 4]
+
+complex data structure is represented in a hierarchical form and, as a
+result, is in a form describable by the above notation, the data values
+that represent the linkages, and their meaning, must somehow he
+represented in the data description in order for the complex data
+structure to be truly described. As another example, one may wish
+ascribe to some level of the data structure the type 'record' so that
+the data set can be used by some system which uses the concept 'record'
+in accessing the data.
+
+    What an initial set of such types should be has not been deicded.
+
+    Names - Items of a data structure can be given names by modifying
+the items description with a notation of the form:
+
+          name n |[extent]|.
+
+Depending on the context of its use, the name can refer to the
+description itself or to the data pertaining to the named part of the
+description. The name is assumed to be unique only within the scope of
+extent of the next higher encompassing name unless otherwise indicated
+by giving another encompassing name as the scope. This may be the name
+of the whole data set or description, for example. The scope of a name
+is specified by preceding the inner name by the outer name or names,
+separated by dots. The name:
+
+          A,BETA
+
+indicates that the scope of the name BETA is A.
+
+    The name applies to just the following item in the description
+unless otherwise indicated by including the extent parameter, For
+exampel, the description:
+
+          (An(C,C), (Bn[ 2 ]F,Cn[ 2 ]F,I))
+
+indicates that name A is given to the item that contains two data values
+of type 'C', the name B to two data values, both of type 'F', and the
+name C to the last two data values, one of type 'F', and the other of
+type 'I'. Notice that with this notation, extents can overlap. For
+example, in the above description, the extent of name B overlapped that
+of name C.
+
+    In a description, the same name can be applied to more than one item
+definition either by use of the extent parameter, or by actually
+specifying the name at each item to be included in the extent of the
+name. If a name is multiply-applied within the same scope, then the name
+is assumed to apply to the aggregate of the items to which it has been
+given. Thus is possible to apply names to aggregates of items that are
+
+
+
+                                                                [Page 5]
+
+not necessarily sequential.
+
+    Lock - During the course of processing data, it may be necessary to
+lock out use of some portion of data to other users. Seqmentation of a
+data set into units for locking purposes may be indicated by the
+notation:
+
+          k|[extent]|.
+
+Whether or not the data is locked and the type of lock applied (for
+example, write protect or read/write protect) is specified at the time
+the data is used.
+
+    Authorization - Authorization for a user to access data may be
+governed by some access code assigned to the data.  This access code can
+be specified in the description by modifying the desired elements of the
+description with an indication of the code. The notation is:
+
+          code a |[extent ]|
+
+Control.
+
+    Two modifiers are provided which govern the existence of items in
+the definition. The first is the repetition modifier:
+
+          factor r |[extent ]|.
+
+This causes the following item definition or item definitions (if the
+extent indicates more than one) to be repeated. Thus the description
+
+          (3rC)
+
+is equivalent to the description
+
+          (C,C,C).
+
+The other control modifier is the condition modifier:
+
+          condition c |[extent ]|.
+
+If the condition specified is not true, then the following item
+definition is ignored. The condition is specified by a Boolean
+expression.
+
+    Since several modifiers may apply to an item definition, there is a
+problem concerning the relationship among them. For example, if a
+repetition modifier and a conditional modifier apply to an item, does
+the condition apply to all the repeated items, or only to the first,
+
+
+
+                                                                [Page 6]
+
+assuming the extent of the condition modifier is one? The effect of
+multiple modifiers is dependent on the order in which they are
+evaluated. Two possible conventions come to mind. One says that
+repetitions are expanded first, then properties applied, and finally
+conditions applied to the resulting expanded item definitions,
+independent of the order in which the modifiers were specified in the
+description. Thus the description
+
+          (A=3c [ 4 ]4rF,I)
+
+is equivalent to
+
+          (4rA=3c[ 4 ]F,T),
+
+and if the condition is true, is equivalent to
+
+          (F,F,F,F,I),
+
+or, if the condition is not true, is equivalent to
+
+          (T).
+
+The other convention is that the modifiers are evaluated in the order in
+which they appear in the description, perhaps.  in reverse order - the
+modifier immediately preceding an item definition is evaluated first,
+then the one next preceding, etc. This gives more flexibility of meaning
+to the mulitple modifiers. For example, the descriptions
+
+          (A=3c3rC)
+
+and
+
+          (3rA=3cC)
+
+are not equivalent.  In the first, only the first of the three
+repetitions is affected by the condition whereas in the second, ll three
+repetitions are affected. Since this second convention is more flexible,
+it shall be the one assumed. This convention allows, for example, the
+repetition modifier to the applied to a named item as shown:
+
+          (3rAnC).
+
+The name A applies to the three items (in effect, the name A is applied
+three times). This facility allows a name to be applied to a vertical
+column in a two dimensioned array by, for example, the description:
+
+          (3r[ 3 ]C,AnC,C)
+
+
+
+
+                                                                [Page 7]
+
+which given the name A to the second column of the 3x3 array.
+
+
+Reference
+
+    Named descriptions, or parts of descriptions, that have already been
+defined may be inserted into a description using the notation:
+
+          $ specification.
+
+Is a description, the reference is used as an item definition of a
+string fo item definitions. The item definitions used are those defined
+by the name given. Names that apply to the named item or items as a
+whole in the description in which it is defined are ignored by the
+description at which is referred. However, names that apply to parts of
+the named item are carried over to the description at which it is
+referred. For example the description
+
+          (An(F,F),I,$A)
+
+is equivalent to
+
+          (An(F,F),T,(F,F)).
+
+Notice that the name "A" was not carried over to where the description
+was referenced since it applied to the referred-to item or items as a
+whole.
+
+    Parts of a data set or description must be able to be specified for
+use in a reference. This specification is in terms of the structure of
+the data set or description. The specification has the form of a data
+set name, or description name, followed by modifiers which particularize
+to specifications, to the part desired. The four types of modifiers are
+for going down a level, going up a level, going frontwards along a
+level, and going backwards.
+
+    Down - To go down a level from that previously specified, the
+modifier has the following form:
+
+         . item
+
+or
+
+         . (item |,extent| |,=value|).
+
+
+Having gone down a level, the item indicates which particular item at
+this level is the first (or only one) desired. This may be a number or a
+
+
+
+                                                                [Page 8]
+
+name. If more than one are desired, then the extent indicates how many
+items. (* as extent means all remaining items at that level, !  means
+the first item that meets the conditions that may he get on it or
+subitems in following modifiers.) The items selected may he conditioned
+by their contents. If a value is given, then only those items with the
+value indicated are selected. For example,
+
+          A.1.1
+
+specifies the first field of the first record of data set A,
+
+          A.(1*).1
+
+specified the first field of all the records of A,
+
+          A.1.(1,2)
+
+specifies the first two fields of the first record of data
+
+set A,
+
+          A.(1,*).(1,="768174")
+
+specified only the first fields of all the records of A that
+
+have value "768174", and
+
+          A.(1,!)-(1,="768174")
+
+finds the first field that has value "768174".
+
+    Up- To go up a level from that previously specified,
+
+the modifier has the following form:
+
+          ' item
+
+or
+
+          ' (item |,extent| |,=value|).
+
+Going up a level specifies the item up one level that contains the item
+previously specified. The item indicates which particular item at this
+level is desired where the containing item is considered the first. For
+example,
+
+          A.(1,!).(1,=768174")'1
+
+
+
+
+                                                                [Page 9]
+
+specifies the first record whose first field has value "768174".
+
+    Forward. - To go forward on the same level as that previously
+specified, the modifier is as follows:
+
+          + item
+
+or
+
+          + (item |,extent| |,=value|).
+
+
+This modifier is useful when an item following the one which has a
+certain value in a field is desired. It may also be used when the data
+set name is really a pointer, into the data set, which has beet set
+previously. Pointers may or may net be described in a section elsewhere.
+Backward. - To go backward on the same level as that previously
+specified, the modifier has the following form:
+
+          - item
+
+or
+
+          - (item |,extent| |,=value|).
+
+An example of the use of this modifier is when an item preceding the one
+which has a certain value in a field is desired. This might be
+specified:
+
+          A.(1,!).(2,="768174")'1-1
+
+
+
+
+       [ This RFC was put into machine readable form for entry ]
+       [ into the online RFC archives Gottfried Janik   9/97   ]
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                               [Page 10]
+