Data computers-data descriptions and access language

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.

      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.

   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.

   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.


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