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rfc:rfc195

Network Working Group G. H. Mealy Request for Comments: 195 HARV NIC 7140 16 July, 1971 Categories: D.4, D.7

      Data Computers -- Data Descriptions and Access Language
 According to the minutes of the NWG meeting in May (RFC 164), it
 appears that a unified approach to Network data management is being
 proposed to CCA.  The purpose of this paper is to discuss some of the
 problems involved and to suggest possible avenues of approach toward
 their resolution.  Parenthetically, I believe that a non-unified
 approach leads to even worse problems.
 My main remarks are predicated on a few assumptions and their
 consequences.  Since some or all may turn out to be wrong, it seems
 appropriate to state them explicitly.  The steps in the arguments
 leading from the assumptions to their consequences may appear to be
 (and in fact may be) less than obvious.  They are all of a piece,
 however, and revolve around the necessity for doing business with a
 number of dissimilar HOST systems while attempting to make it
 unnecessary for an individual user or user program to know the
 details of data file organization and representation.  Given this as
 an objective, I believe that the arguments are quite direct.
 Assumptions
 ------------
    1. We face the usual set of naming, cataloging, protection,
       backup, etc. problems.
       (I say this only to dismiss the subject as far as the following
       is concerned.  In my estimation, these problems and feasible
       solutions are reasonably well understood; our real problem in
       this area is in reaching agreement on specifics while leaving
       sufficient ratholes for future expansion).
    2. Files stored will contain arbitrarily complex data objects.
    3. The organization of any file (that is, the way its structure is
       mapped into physical storage by the data computer) will
       normally be unknown by the user.

Healy [Page 1] RFC 195 Data Computers July 1971

    4. Data items in files may be stored in arbitrary representations
       (e.g., those of the originating user's HOST rather than that of
       the data computer or other "standard" representation).
    5. Access to a file will normally be to some subset of it. (I.e.,
       the unit for transmission will usually be part of a file rather
       than the whole file, and access will not necessarily be
       sequential).
 Consequences
 ------------
    1. A method of data description significantly more powerful than
       now commonly available (as with COBOL or PL/I) is required.
       The descriptions must be stored with the files.  Data item
       representations and storage organizations must be describable.
    2. The data computer must offer a "data reconfiguration service",
       based on use of the data descriptions.
    3. A representation and organization-independent level of
       discourse must be made available for controlling access.
 Data Description
 -----------------
 As it happens, the descriptive facilities in ELl (References 1 and 2)
 are almost adequate as they stand.  ELl is an extensible language --
 the compiler and interpreter for ELl are principal components of a
 system implemented on the PDP-lO at Harvard -- which allows the
 definition of arbitrary data structures in terms of a few primitive
 data types (BOOL, CHAR, INT, REAL, SYMBOL, MODE, FORM, and ROUTINE).
 These data types are of the sort I called "generic" in Reference 3.
 To the EL1 implementation on the PDP-10, say, we would have to add
 methods to describe a specific representation of INT, etc. and
 primitive routines to convert between specific representations.
 In the ECL system (in which EL1 is embedded), there is no rigid
 distinction between compile time and run time.  In particular, if the
 arguments and free variables of a routine are evaluable at compile
 time, then the routine is evaluated and the value replaces the call.
 More generally, arbitrarily large amounts of a routine being compiled
 may collapse into values.  As far as the data computer is concerned,
 this offers the possibility of producing tailor-made data
 reconfiguration programs, taking maximum advantage of the data
 descriptions at compile time rather than using a strictly
 interpretative mode of operation.

Healy [Page 2] RFC 195 Data Computers July 1971

 Access Language
 ---------------
 Here, I am on less firm ground.  I will suggest, however, that some
 of the ideas of Sattley, et al (Reference 4) deserve consideration.
 I will quote from the Reference:
 "... Our proposal is a language for describing the transferable
 features of files, in which conventional programming languages (e.g.,
 FORTRAN, ALGOL, etc.,) can be embedded, and from which the
 information necessary to optimize the use of secondary storage can be
 easily abstracted.  This language defines our abstract model of
 secondary storage in the same way that FORTRAN defined an abstract
 machine.  This language should have (at least) the following
 features:
    1. File declarations name the file and the elements in the file,
       and specify the range of forms that the elements can take.
       Each file has precisely one named element.  Each file includes
       the (maximum) size (in number of elements) of the file.
    2. Subsets of files can be created by means of grouping
       declarations.  Such subsets can be nested.
    3. Subsets of files can be named by means of naming declarations.
       Such declarations can also name individual elements of the
       file.  Some form of implicit naming, allowing language
       constructs such as GET ANOTHER TRIPLE, is included.
    4. Members of a set (i.e., elements in a subset or file, subsets
       in a containing subset or file) can be ordered by order
       declarations.  Some form of arbitrary but fixed ordering,
       allowing language constructs such as GET NEXT TRIPLE, is
       included.
    5. The portions of a file transacted with at a point of access is
       declared.  The size of this portion can be expressed in
       absolute or relative terms.
    6. At each point of access to secondary storage, an environment is
       described (or referenced) which contains those declarations of
       types (l)-(5) necessary to define the transaction with
       secondary.

Healy [Page 3] RFC 195 Data Computers July 1971

 A language with the above features makes no mention of hardware
 devices, but it provides the programmer with the means of defining
 the algorithm-dependent features of his files so that those files
 might be transferred efficiently from machine to machine".
 In the Sattley, et al study, the notion was that a compiler would
 take the source program and actually compile the hardware-dependent
 file accessing code.  In our environment, we are concerned with
 control commands to the data computer (e.g., GET NEXT WALDO) and
 supplying the data computer with enough information to define a
 WALDO.  The basic functions would seem to be the same, in either
 case, albeit implemented rather differently.

References

 1.  Wegbreit, B. The Treatment of Data Types in EL1.  Technical
     Report, Division of Engineering and Applied Physics, Harvard
     University, Cambridge, Massachusetts, May 1971.
 2.  Wegbreit, B. The ECL Programming System.  Technical Report,
     Division of Engineering and Applied Physics, Harvard University,
     Cambridge, Massachusetts, April 1971.
 3.  Mealy, G. H. Another Look at Data.  AFIPS Conference Proceedings,
     vol. 31, 1967 Fall Joint Computer Conference
 4.  Sattley, K., Millstein, R. and Warshall, S. On Program
     Transferability.  Report CA-7011-2411, Massachusetts Computer
     Associates, Wakefield, Massachusetts, Movember 1970.
     [ This RFC was put into machine readable form for entry ]
     [ into the online RFC archives by Larry Masinter 10/99 ]

Healy [Page 4]

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