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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-C7FB54C4-43C9-562A-A18C-2818AF624774" xml:lang="en"><title>Permanent
File Store</title><shortdesc>A permanent file store implements most of the operations defined
by the store abstract framework. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Streams in a permanent file store can be:</p>
<ul>
<li id="GUID-5D731E0A-C4B7-56CE-AC27-25118A58506A"><p>overwritten</p> </li>
<li id="GUID-13FF892F-8241-59A6-A39F-D0F2705F79A0"><p>replaced</p> </li>
<li id="GUID-A8E9E699-6017-58FA-BC54-155B7838A172"><p>deleted</p> </li>
<li id="GUID-95C70244-4BE3-5E20-83FA-1B1DCFB03F49"><p>created in advance of
being written to.</p> </li>
</ul>
<p>A permanent file store is useful for the type of application which treats
the data in the store as its prime copy. Typically, the object network comprising
the application’s data is structured so that sections of data can be loaded
in, and, if changed, written back to the store. Memory is used as a workplace
for changing data. Such an application never replaces the entire store. This
approach requires a sophisticated structure and greater sophistication within
the application.</p>
<p>Typically, a permanent file store is used by an application, such as a
database, which maintains all its data in a file, but occasionally edits some
entries in that database.</p>
<p> In this paradigm:</p>
<ul>
<li id="GUID-56E7EE36-B74F-57B4-9CD6-FCA87D987B89"><p>the database file is
initially created</p> </li>
<li id="GUID-4BA53AA2-0304-581A-AD48-2411C9986F69"><p>when an entry is created,
it as added to the database</p> </li>
<li id="GUID-70E72107-4ADE-5BF0-8199-C7B904CA72EB"><p>the program maintains
an internal, non-persistent, copy of the indexes in the database file</p> </li>
<li id="GUID-EB9EAF48-6D8A-5D6F-BC63-C2036FAC61D4"><p>when an entry is selected
for display or editing, its data is retrieved from the database; all such
entries’ data are maintained in an internal, non-persistent, form</p> </li>
<li id="GUID-C455382A-F50F-5C92-B59B-A5E5FC966C93"><p>when an entry is edited
and saved, just that single entry is replaced in the database</p> </li>
<li id="GUID-3BE61AA6-BA20-577B-88AD-8798A12B9F52"><p>when the application
exits, there is nothing to do to the database, since there should be no data
that was not already saved (except, perhaps, the current entry if it was being
edited)</p> </li>
</ul>
<p>Such an application never replaces the entire store. The database file
itself is permanent; it is not erased when an entry is saved. Because of this,
much more care is needed when maintaining this type of store. All links within
the database must be maintained without corruption, and no stream subnetworks
in the database should become inaccessible, otherwise space is wasted. The
order in which streams are written to a permanent file store is not important
as streams can be changed and rewritten. </p>
<p>Overwriting an existing stream does not change its length. However, attempting
to write beyond the end of the stream fails and results in a leave. Replacing
a stream can result in a stream which is longer or shorter than the original.</p>
<p>Note the difference between a persistent store (one that can be closed
and re-opened), and a permanent file store (it is not replaced wholesale;
rather, its individual entries are edited, inserted and deleted, but the store
itself is permanent). A permanent file store is, of course, persistent.</p>
<p>When the content of a permanent file store is naturally represented by
a relational database, it is usually much more convenient to manipulate it
using the relational database manager.</p>
<p>Permanent file stores are encapsulated by the <codeph>CPermanentFileStore</codeph> class.</p>
</conbody><related-links>
<link href="GUID-C0414BDC-06FD-5E3C-93F7-DD3467CAA75E.dita"><linktext>DBMS</linktext>
</link>
</related-links></concept>