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106 lines
3.9 KiB
TeX
106 lines
3.9 KiB
TeX
\chapter{Object store}
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\label{chap-object-store}
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The \emph{object store} is a system-wide database containing
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any kind of objects. Each object is a \emph{capability}.
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An object in the store can optionally be associated with a certain
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number of \emph{attributes}. An attribute is a \emph{pair} consisting
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of the \emph{attribute name} and the \emph{attribute value}. The
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attribute name is a symbol in the \texttt{keyword} package. The
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attribute value can be any object.
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\begin{tabularx}{\linewidth}%
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{|>{\setlength\hsize{.2\hsize}}X|%
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>{\setlength\hsize{.8\hsize}}X|}
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\hline
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Keyword & Possible values\\
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\hline\hline
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\textbf{category} &
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The nature of the object such
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as \textbf{movie}, \textbf{music}, \textbf{article}, \textbf{book}, \textbf{user
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manual}, \textbf{dictionary}, \textbf{course}, \textbf{lecture},
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\textbf{recipe}, \textbf{program}, \textbf{bank statement},
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\textbf{email}. These would be chosen from an
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editable set that is defined per user.\\
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\hline
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\textbf{name} &
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A string that is displayed with the object, such as "A Dramatic
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Turn of Events", "Three seasons", "Alternative energy".\\
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\hline
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\textbf{author} &
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An object identifying a person, an organization, a company,
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etc. \\
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\hline
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\textbf{genre} &
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\textbf{progressive
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metal}, \textbf{science}, \textbf{algorithms}, \textbf{garbage
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collection}, \textbf{game}, \textbf{programming language
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implementation}, \textbf{operating system}. These would be
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chosen from an editable set that is defined per user.\\
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\hline
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\textbf{format} &
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This attribute can be used to identify the file type of documents such
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as \textbf{PDF}, \textbf{ogg/vorbis}, \textbf{MPEG4}, \textbf{PNG}, in
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which case the attribute can be assigned automatically, but also to
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identify the source format of files in a directory containing
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things like articles or user manuals, for
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example \textbf{LaTeX}, \textbf{Texinfo}, \textbf{HTML}. These would
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be chosen from an editable set that is defined per user. \\
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\hline
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\textbf{date of creation} &
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A date interval.\\
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\hline
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\textbf{composer} &
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An object representing a person. On a compilation album there
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can be more than one attribute of this kind. \\
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\hline
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\textbf{language} &
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An object representing a natural language such
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as \textbf{English}, \textbf{Vietnamese}, or a programming languages
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such as \textbf{Lisp}, \textbf{Python}. These would
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be chosen from an editable set that is defined per user. If
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appropriate, a document can have several of these attributes, for
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instance if some program uses multiple programming languages, or
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if a document is written using several languages, such as a
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dictionary. \\
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\hline
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\textbf{duration} &
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An object representing a duration. \\
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\hline
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\textbf{source control} &
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\textbf{GIT}, \textbf{SVN}, \textbf{CVS}, \textbf{darcs}, etc. These
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would be chosen from an editable set that is defined per user.\\
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\hline
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\end{tabularx}
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In a typical operating system installation, there are many fairly
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large objects such as movies, music files, pictures, etc. The amount
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of data associated with such an object that would be stored in the
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object store is typically very small compared to the object itself.
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Even a fairly modest text file probably has $10^4 - 10^5$ characters
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in it, whereas the meta-data probably takes up no more than $10^2 -
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10^3$ bytes. It is therefore likely that the entire object store will
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fit in main memory. Scanning the entire object store would then take
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at most a few second of CPU time. For better performance, one or more
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\emph{indexes} could be created. The objects could for instance be
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divided by \emph{category}.
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Searching the object store amounts to defining a \emph{filter},
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i.e. a function that, given a set of keyword/value pairs, returns
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\emph{true} if and only if the corresponding object should be included
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in the search result. The result is returned to the user in the form
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of a \emph{directory object} which is a list of \emph{object entries}
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where each entry contains the object itself and the attributes of the
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object from the store, if any.
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