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<! "@(#)re_source.so 10.1 (Sleepycat) 10/26/98">
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<title>Berkeley DB Reference Guide: Access Methods</title>
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<h3>Berkeley DB Reference Guide: Access Methods</h3>
<p>
<h1 align=center>Record backing file (re_source, DB_SNAPSHOT)</h1>
<p>
It is possible to back any Recno database (either fixed or variable length)
with a flat-text source file. The reason for this is to provide fast read
(and potentially write) access to databases that are normally created and
stored as flat-text files. The backing source file may be declared as
part of the <a href="../../api_c/Db/open.html">db_open</a> call to open the database, specifically by
setting the <a href="../../api_c/DbInfo/info.html#re_source">re_source</a> element of the DB_INFO structure.
<p>
The backing source file will be read to initialize the database. In the
case of variable length records, the records are assumed to be separated
by the <a href="../../api_c/DbInfo/info.html#re_delim">re_delim</a> element of the DB_INFO structure. For
example, standard UNIX byte stream files can be interpreted as a sequence
of variable length records separated by ASCII newline characters.
<p>
In addition, when cached data would normally be written back to the
underlying database file (e.g., <a href="../../api_c/Db/close.html">DB->close</a> or <a href="../../api_c/Db/sync.html">DB->sync</a>
functions are called), the in-memory copy of the database will be written
back to the backing source file.
<p>
By default, the backing source file is read lazily, i.e., records are not
read from the backing source file until they are requested by the
application. If multiple processes (not threads) are accessing a recno
database concurrently and either inserting or deleting records, the
backing source file must be read in its entirety before more than a single
process accesses the database, and only that process should specify the
backing source file as part of the <a href="../../api_c/Db/open.html">db_open</a> call. This can be
accomplished by setting the <a href="../../api_c/DbInfo/info.html#DB_SNAPSHOT">DB_SNAPSHOT</a> flag in the
DB_INFO structure.
<p>
Reading and writing the backing source file <b>cannot</b> be
transactionally protected because it involves filesystem operations that
are not part of the Berkeley DB transaction methodology. For this reason, if a
temporary database is used to hold the records, i.e., a NULL was specified
as the file argument to <a href="../../api_c/Db/open.html">db_open</a>, <b>it is possible to lose
the contents of the backing source file</b>, e.g., if the system crashes at
the right instant. If a permanent file is used to hold the database, i.e.,
a file name was specified as the file argument to <a href="../../api_c/Db/open.html">db_open</a>, normal
database recovery on that file can be used to prevent information loss,
although it is still possible that the contents of the backing source file
itself will be lost if the system crashes.
<p>
The backing source file must already exist (but may be zero-length) when
<a href="../../api_c/Db/open.html">db_open</a> is called.
<p>
For all of the above reasons, the backing source file is generally used to
specify databases that are read-only for Berkeley DB applications, and that
are either generated on the fly by software tools, or modified using a
different mechanism, e.g., a text editor.
<p>
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