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Network Working Group                                        G. H. MealyRequest for Comments: 195                                           HARVNIC 7140                                                   16 July, 1971Categories:     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]