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<p>Library Version 18.1.40</p>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="rq_conf"></a>Queue and Recno access method specific configuration</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="rq_conf.html#am_conf_recno">Managing record-based
databases</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rq_conf.html#am_conf_extentsize">Selecting a Queue extent
size</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rq_conf.html#am_conf_re_source">Flat-text backing
files</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rq_conf.html#am_conf_renumber">Logically renumbering
records</a>
</span>
</dt>
</dl>
</div>
<p>
There are a series of configuration tasks which you can
perform when using the Queue and Recno access methods. They
are described in the following sections.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_conf_recno"></a>Managing record-based
databases</h3>
</div>
</div>
</div>
<p>
When using fixed- or variable-length record-based databases,
particularly with flat-text backing files, there are several
items that the user can control. The Recno access method can
be used to store either variable- or fixed-length data items.
By default, the Recno access method stores variable-length
data items. The Queue access method can only store
fixed-length data items.
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idm140654546148752"></a>Record Delimiters</h4>
</div>
</div>
</div>
<p>
When using the Recno access method to store
variable-length records, records read from any backing
source file are separated by a specific byte value which
marks the end of one record and the beginning of the next.
This delimiting value is ignored except when reading
records from a backing source file, that is, records may
be stored into the database that include the delimiter
byte. However, if such records are written out to the
backing source file and the backing source file is
subsequently read into a database, the records will be
split where delimiting bytes were found.
</p>
<p>
For example, UNIX text files can usually be interpreted
as a sequence of variable-length records separated by
ASCII newline characters. This byte value (ASCII 0x0a) is
the default delimiter. Applications may specify a
different delimiting byte using the <a href="../api_reference/C/dbset_re_delim.html" class="olink">DB->set_re_delim()</a>
method. If no backing source file is being used, there is
no reason to set the delimiting byte value.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idm140654545979136"></a>Record Length</h4>
</div>
</div>
</div>
<p>
When using the Recno or Queue access methods to store
fixed-length records, the record length must be specified.
Since the Queue access method always uses fixed-length
records, the user must always set the record length prior
to creating the database. Setting the record length is
what causes the Recno access method to store fixed-length,
not variable-length, records.
</p>
<p>
The length of the records is specified by calling the
<a href="../api_reference/C/dbset_re_len.html" class="olink">DB->set_re_len()</a> method. The default length of the records
is 0 bytes. Any record read from a backing source file or
otherwise stored in the database that is shorter than the
declared length will automatically be padded as described
for the <a href="../api_reference/C/dbset_re_pad.html" class="olink">DB->set_re_pad()</a> method. Any record stored that is
longer than the declared length results in an error. For
further information on backing source files, see <a class="xref" href="rq_conf.html#am_conf_re_source" title="Flat-text backing files">Flat-text backing
files</a>.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idm140654546399504"></a>Record Padding Byte Value</h4>
</div>
</div>
</div>
<p>
When storing fixed-length records in a Queue or Recno
database, a pad character may be specified by calling the
<a href="../api_reference/C/dbset_re_pad.html" class="olink">DB->set_re_pad()</a> method. Any record read from the backing
source file or otherwise stored in the database that is
shorter than the expected length will automatically be
padded with this byte value. If fixed-length records are
specified but no pad value is specified, a space character
(0x20 in the ASCII character set) will be used. For
further information on backing source files, see <a class="xref" href="rq_conf.html#am_conf_re_source" title="Flat-text backing files">Flat-text backing
files</a>.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_conf_extentsize"></a>Selecting a Queue extent
size</h3>
</div>
</div>
</div>
<p>
In Queue databases, records are allocated sequentially and
directly mapped to an offset within the file storage for the
database. As records are deleted from the Queue, pages will
become empty and will not be reused in normal queue
operations. To facilitate the reclamation of disk space a
Queue may be partitioned into extents. Each extent is kept in
a separate physical file.
</p>
<p>
Extent files are automatically created as needed and marked
for deletion when the head of the queue moves off the extent.
The extent will not be deleted until all processes close the
extent. In addition, Berkeley DB caches a small number of
extents that have been recently used; this may delay when an
extent will be deleted. The number of extents left open
depends on queue activity.
</p>
<p>
The extent size specifies the number of pages that make up
each extent. By default, if no extent size is specified, the
Queue resides in a single file and disk space is not
reclaimed. In choosing an extent size there is a tradeoff
between the amount of disk space used and the overhead of
creating and deleting files. If the extent size is too small,
the system will pay a performance penalty, creating and
deleting files frequently. In addition, if the active part of
the queue spans many files, all those files will need to be
open at the same time, consuming system and process file
resources.
</p>
<p>
You can set the Queue extent size using the
<a href="../api_reference/C/dbset_q_extentsize.html" class="olink">DB->set_q_extentsize()</a> method. You can see the current extent
size using the <a href="../api_reference/C/dbget_q_extentsize.html" class="olink">DB->get_q_extentsize()</a> method.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_conf_re_source"></a>Flat-text backing
files</h3>
</div>
</div>
</div>
<p>
It is possible to back any Recno database (either fixed or
variable length) with a flat-text source file. This provides
fast read (and potentially write) access to databases that are
normally created and stored as flat-text files. The backing
source file may be specified by calling the <a href="../api_reference/C/dbset_re_source.html" class="olink">DB->set_re_source()</a>
method.
</p>
<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 as described for the
<a href="../api_reference/C/dbset_re_delim.html" class="olink">DB->set_re_delim()</a> method. For example, standard UNIX byte
stream files can be interpreted as a sequence of variable
length records separated by ASCII newline characters. This is
the default.
</p>
<p>
When cached data would normally be written back to the
underlying database file (for example, when the <a href="../api_reference/C/dbclose.html" class="olink">DB->close()</a> or
<a href="../api_reference/C/dbsync.html" class="olink">DB->sync()</a> methods are called), the in-memory copy of the
database will be written back to the backing source
file.
</p>
<p>
The backing source file must already exist (but may be
zero-length) when <a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a> is called. By default, the backing
source file is read lazily, that is, 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_reference/C/dbopen.html" class="olink">DB->open()</a> call. This can be accomplished by calling
the <a href="../api_reference/C/dbset_flags.html" class="olink">DB->set_flags()</a> method with the <a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_SNAPSHOT" class="olink">DB_SNAPSHOT</a> flag.
</p>
<p>
Reading and writing the backing source file cannot 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 (a NULL was specified as the file argument
to <a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a>), <span class="bold"><strong>it is possible to lose the
contents of the backing source file if the system crashes
at the right instant</strong></span>. If a permanent file is
used to hold the database (a filename was specified as the
file argument to <a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a>), normal database recovery on that
file can be used to prevent information loss. It is still
possible that the contents of the backing source file itself
will be corrupted or lost if the system crashes.
</p>
<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
such as a text editor.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_conf_renumber"></a>Logically renumbering
records</h3>
</div>
</div>
</div>
<p>
Records stored in the Queue and Recno access methods are
accessed by logical record number. In all cases in Btree
databases, and optionally in Recno databases (see the
<a href="../api_reference/C/dbset_flags.html" class="olink">DB->set_flags()</a> method and the <a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_RENUMBER" class="olink">DB_RENUMBER</a> flag for more
information), record numbers are mutable. This means that the
record numbers may change as records are added to and deleted
from the database. The deletion of record number 4 causes any
records numbered 5 and higher to be renumbered downward by 1;
the addition of a new record after record number 4 causes any
records numbered 5 and higher to be renumbered upward by 1. In
all cases in Queue databases, and by default in Recno
databases, record numbers are not mutable, and the addition or
deletion of records to the database will not cause
already-existing record numbers to change. For this reason,
new records cannot be inserted between already-existing
records in databases with immutable record numbers.
</p>
<p>
Cursors pointing into a Btree database or a Recno database
with mutable record numbers maintain a reference to a specific
record, rather than a record number, that is, the record they
reference does not change as other records are added or
deleted. For example, if a database contains three records
with the record numbers 1, 2, and 3, and the data items "A",
"B", and "C", respectively, the deletion of record number 2
("B") will cause the record "C" to be renumbered downward to
record number 2. A cursor positioned at record number 3 ("C")
will be adjusted and continue to point to "C" after the
deletion. Similarly, a cursor previously referring to the now
deleted record number 2 will be positioned between the new
record numbers 1 and 2, and an insertion using that cursor
will appear between those records. In this manner records can
be added and deleted to a database without disrupting the
sequential traversal of the database by a cursor.
</p>
<p>
Only cursors created using a single <a href="../api_reference/C/db.html" class="olink">DB</a> handle can adjust
each other's position in this way, however. If multiple <a href="../api_reference/C/db.html" class="olink">DB</a>
handles have a renumbering Recno database open simultaneously
(as when multiple processes share a single database
environment), a record referred to by one cursor could change
underfoot if a cursor created using another <a href="../api_reference/C/db.html" class="olink">DB</a> handle
inserts or deletes records into the database. For this reason,
applications using Recno databases with mutable record numbers
will usually make all accesses to the database using a single
<a href="../api_reference/C/db.html" class="olink">DB</a> handle and cursors created from that handle, or will
otherwise single-thread access to the database, for example,
by using the Berkeley DB Concurrent Data Store product.
</p>
<p>
In any Queue or Recno databases, creating new records will
cause the creation of multiple records if the record number
being created is more than one greater than the largest record
currently in the database. For example, creating record number
28, when record 25 was previously the last record in the
database, will implicitly create records 26 and 27 as well as
28. All first, last, next and previous cursor operations will
automatically skip over these implicitly created records. So,
if record number 5 is the only record the application has
created, implicitly creating records 1 through 4, the <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a>
method with the <a href="../api_reference/C/dbcget.html#dbcget_DB_FIRST" class="olink">DB_FIRST</a> flag will return record number 5,
not record number 1. Attempts to explicitly retrieve
implicitly created records by their record number will result
in a special error return, <a class="link" href="program_errorret.html#program_errorret.DB_KEYEMPTY">DB_KEYEMPTY</a>.
</p>
<p>
In any Berkeley DB database, attempting to retrieve a
deleted record, using a cursor positioned on the record,
results in a special error return, <a class="link" href="program_errorret.html#program_errorret.DB_KEYEMPTY">DB_KEYEMPTY</a>.
In addition, when using Queue databases or Recno databases
with immutable record numbers, attempting to retrieve a
deleted record by its record number will also result in the
<a class="link" href="program_errorret.html#program_errorret.DB_KEYEMPTY">DB_KEYEMPTY</a> return.
</p>
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