Network Working Group R. Winter
Request for Comments: 219 CCA
NIC: 7549 3 September 1971
Category:
Updates: None
Obsoletes: None
User's View of the Datacomputer
MEMORANDUM
TO: Datacomputer Design File
FROM: R.A. Winter
SUBJECT: User's View of the Datacomputer
Date: September 3, 1971
________________________________________________________________________
Introduction
The datacomputer is a specialized node of the ARPA network that is
dedicated to the management of a large, shared database. By large we
mean several trillion bits of data, of which at least one trillion
are on-line. Shared may mean, for some files, shared by nearly all
the users in the ARPA network.
The name, datacomputer, derives from the idea that the system is
dedicated to data handling. Though the processor is capable of
general computation, it will not be used for that purpose. The
processor, like the mass storage device, is only a component of an
integrated system, which appears to the user as a black box.
There is one language for addressing the black box: data language.
This language defines everything it can do.
All the information presented in this memorandum is about the first
of a series of service offerings. We use the term access method to
refer collectively to a structure and the operations on it. Being
too modest to call the first one AM-1 (Access Method-1) we named it
DCAM-1 (Datacomputer Access Method-d). We expect subsequent DCAMs to
generalize DCAM-1. If the need arises, we will design parallel
services. All services will use the same data language.
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RFC 219 User's View of the Datacomputer September 1971
System Overview
The users of the datacomputer are programs running on other
computers, retrieving data from, and storing it in, the data base.
The environments, capabilities, and applications of these programs
vary widely; however, a chief design goal is to allow them to share
the data.
There is further variation among users in physical connection.
Remotely-located users' access is over a narrow link to the data-
computer's low-speed port. Local users are connected to the high-
speed port through a link 80 times wider.
Through its ports, the datacomputer accepts two kinds of input: data
and requests for services. Data is output through the ports as
requested.
In the data base, descriptions are stored separately from the data,
and data elements are named, typed and ordered according to them. A
measure of structure independence is obtained by writing access
requests in terms of the symbolic names of items in the data
description.
Directories are maintained by the system. A hierarchical naming
scheme is used, and access controls for privacy and data integrity
are provided.
Redundant copies of data and/or journals of changes are maintained by
the system and used to effect recovery under system control in case
of system error. These facilities can be operated under user control
if there is external error.
Since the datacomputer's only interface with the outside world is
through its ports, it sees the universe as a group of data streams.
Specifically, these are record streams, if one views all transactions
(in the data transfer protocol sense) as records. Associated with
each record stream is a data description, allowing the datacomputer
to parse the records into named, typed elements.
Thus all data elements--stream elements and data base--are named and
fully described. Data type conversion proceeds automatically, as a
function of old and new data types, and optional information supplied
by the user. Reconfiguration above the element level is a matter of
arrangement of elements in records; a full set of capabilities is
provided for this. In general, the using program is concerned with
the configuration of the stream records that comprise its interface
with the datacomputer. The internal configuration of data affects
the user only as it limits the data's accessibility or malleability.
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In fact, the user should not generally have to be aware of the
internal data configuration.
Although support on some level for all types of applications is
attempted, the first implementation gives particular attention to
large, simply-organized, shared files. Emphasis is placed on
allowing the user of such files to describe precisely what data is
really of interest to him, so that nothing but the desired
information is transmitted. This is crucial for avoiding overload of
the narrow link, and is accepted as a central design goal.
Data Base Organization
The database contains all information stored in the datacomputer. It
is a set of files, which are named, physically distinct, collections
of data.
The location of one file, the file directory, is known to the system.
It contains the names, locations, and certain attributes of all the
other files. Access to this file is restricted.
Internally, each file has its own organization, but each organization
is a particular application of a general model. The particular
application is defined by a file description associated with the
file.
In the general model, each file contains uniquely numbered records.
Each record contains named fields. A field of a certain name may
occur more than once in a given record, and a unique number is
associated with each occurrence. A field contains an elementary
piece of data, the value of the field.
The records are variable in format and size. Fields are variable in
length.
In addition to the records themselves; each file can contain an
index. The system maintains the index to the specifications of the
user. Conceptually, the index contains lists of pointers to records
having certain properties. A typical list might point to the records
containing the field STATE with the value MASSACHUSETTS.
The system supplies a unique, permanent, identifier for each record.
This identifier maps trivially into a location in the file, or at
worst, into a small region in which the record can be quickly
located. The identifier is used to pointers to the record, both from
the index and from other records.
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Besides the physical ordering, defined by record location, a logical
ordering will be maintained on request by the system. This can be
based on some simple function of record contents, such as the value
of certain fields. Alternatively, the user can compute the function,
and simply supply the result (for example, by saying "insert this
record after that one"). Retrieval from such ordered files can be
made either in physical or logical order.
In all such ordered files, if insertions are made, space must be
reserved for them and garbage collection must be done periodically.
A single field value is viewed as a homogeneous string of characters
or basic data units. It is described by giving the type (e.g.,
ASCII, BIT, binary integer, etc.) and the length is some unit
associated with the type. When the length of a field is constant
throughout the file, it is stored in the file description; otherwise
it appears with each occurrence of the field. The type of a field is
constant.
The information in the file description is sufficient to parse a
record into (field name, value) pairs. Also, given such a set of
pairs, and a file description, the system can produce a record
satisfying the description. Mapping in either direction, there is
only one possible result.
With a record, a file description, and a (field name, value) pair to
store in the record, there is also only one new record that can
result.
Thus a file description defines all the possible formats for a record
from a particular file.
Stream Organization
Streams are sequences of records passed from using programs to the
datacomputer or vice versa. The format of the records is defined as
in the file description. Thus streams have the same organization as
files, except they cannot be indexed. The operations defined on
streams are more limited than those defined on files, since the
records must be accessed in sequence.
There is no concept of permanent storage for streams. The records
move past the datacomputer one at a time, as though they were on a
conveyor belt.
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One record, the current record, is available to the datacomputer in
each stream. To begin formatting the subsequent record in an output
stream, the datacomputer transmits the current record. To access the
next record in an input stream, the datacomputer relinquishes access
to the current one.
Operations
When the user is interested in the contents of his whole file in
solving the problem at hand, the datacomputer's job is simple in
terms of information retrieval. There may be reformatting or
reordering, but location of the right data to operate on is trivial.
However, this will not be the standard usage of the datacomputer,
particularly for the remote user.
For most problems, the datacomputer expects to subset the file before
doing anything else. The larger the file compared to the subset, the
less acceptable it is to transact with the full file in order to form
the subset. And the datacomputer will have such enormous files that
using anything but a very small subset in one problem is most
unusual. Thus, subsetting without examining the entire file is a
fundamental requirement.
Normally, the subset will be considered formed when a list of the
relevant record id's or record addresses is known.
The index of the datacomputer file can be thought of as a collection
of primitive record id lists that the file designer expected to be
useful in forming interesting subsets. The values of all important
fields can be indexed. For example, every word in a field containing
a string of text might be indexed. In fact, an arbitrary function of
the contents of the record, or the relation of the record to other
records can be indexed.
The common logical operators (AND, OR and NOT) are defined for record
subsets. Arbitrarily complex expressions of them can be evaluated
with relatively little processor time or I/O. The ease of this
operation results from careful design of the index and strategies,
the most important of which is the parallel evaluation of the Boolean
functions on large groups of records. Certain statistical
operations--like counting the number of records satisfying a certain
Boolean condition--can be done directly on the index. This can be
used to derive question-answering strategies heuristically, or can be
the direct input to a statistical study.
Once the index has done all it can in subsetting, attention turns to
the records themselves. Certain conditions cannot be evaluated using
the index; an obvious case is the selection of records based on the
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value of an unindexed field. Also, certain data structures cannot be
explicitly represented in the file:record:field model. These must be
constructed by the user, out of groups of records linked by pointers,
or using other special mechanisms. The class of operations that is
useful in further record selection consists of field content testing,
pointer chasing, simple computation in the numerical and symbolic
senses, and various operations below the data element level, such as
pattern matching, string manipulation, etc. Such operations require
a control structure approaching that of the general purpose higher
level language. It is our intention to make all of this available,
though not with the goal of providing a computation facility, but
rather, a data management facility that is capable of using as much
knowledge as the programmer can supply.
A simple set of primitives is required for file maintenance in the
data structure we are talking about. The operations are:
1. add a field/record
2. delete a field/record
3. replace a field/record.
The difficult part, as in retrieval, is locating the element to be
operated on. Notice that individual record formats can be changed
at will: the set of possible formats is limited only by the file
description.
When record contents are changed, index entries that are a function
of them must be changed also. When the function determining what is
to be indexed is part of the file description, the maintenance of the
index is automatically performed by the system. Otherwise, this is
the responsibility of the user.
All fields in a record can be optional, variable length, allowed to
occur an arbitrary number of times (up to some fixed limit for each
field). Fields can be present and later be deleted from any record.
Fields can be added to the file description at any time. The only
reason for limiting the flexibility of a record format is to reduce
storage.
Applications
The system outlined here is intended to be suitable for many
applications; some examples are:
1. Storage and retrieval of dumps and other unstructured files. The
system will happily pack away your one enormous record, as quickly
and painlessly as possible.
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2. Applications that would normally be set up on tape: sequentially
accessed files that are copied over when they are changed. Most
record formats should be able to remain just as they are. If you
want to operate this way, the datacomputer imposes no overhead
(such as indexing) on you. The datacomputer willingly acts as
unsophisticated as a tape drive; it will pass your file, adding
and changing records as it copies them. It will pull off the
interesting ones, reconfigure if desired, and transmit them to
you. When you describe the data, you have solved the data sharing
problem for this application.
3. Simple-minded direct access applications. The great hairy index
structure neatly degenerates to imitate indexed sequential, simple
directly-addressed files, and other old standbys in the direct
access world.
4. Text/document retrieval. The indexing is made for this kind of
applications. In addition, documents can point to subdocuments,
related documents, etc.
5. Content-oriented, rapid retrieval applications are the specialty
of the house.
6. Large data bases used for statistical analysis or modeling such as
the census, the common social science data bases, etc.
7. Applications in which data element groups (such as records) are
related in a complex fashion, and the intelligence of the
datacomputer, which is close to the data and remote from the
computational facility, can be put to good use.
In all of these, an important consideration is size. We hope to
handle these applications properly on the datacomputer, even when the
files are of extraordinary size.
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Sandy Ginoza 9/2001. ]
[ Original has hand-written note in Postel's handwriting: ]
[ "Received 21 Sept 71". ]
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