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<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>