Sindbad~EG File Manager
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<h2 class="title" style="clear: both"><a id="am_opensub"></a>Opening multiple databases in a
single file</h2>
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<dt>
<span class="sect2">
<a href="am_opensub.html#idm140654540976064">Configuring databases sharing a file</a>
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</dt>
<dt>
<span class="sect2">
<a href="am_opensub.html#idm140654546210656">Caching databases sharing a file</a>
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<span class="sect2">
<a href="am_opensub.html#idm140654546233792">Locking in databases based on sharing a file</a>
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<p>
Applications may create multiple databases within a single
physical file. This is useful when the databases are both
numerous and reasonably small, in order to avoid creating a
large number of underlying files, or when it is desirable to
include secondary index databases in the same file as the
primary index database. Putting multiple databases in a single
physical file is an administrative convenience and unlikely to
affect database performance.
</p>
<p>
To open or create a file that will include more than a
single database, specify a database name when calling the
<a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a> method.
</p>
<p>
Physical files do not need to be comprised of a single type
of database, and databases in a file may be of any mixture of
types, except for Queue and Heap databases. Queue and Heap
databases must be created one per file and cannot share a file
with any other database type. There is no limit on the number
of databases that may be created in a single file other than
the standard Berkeley DB file size and disk space
limitations.
</p>
<p>
It is an error to attempt to open a second database in a
file that was not initially created using a database name,
that is, the file must initially be specified as capable of
containing multiple databases for a second database to be
created in it.
</p>
<p>
It is not an error to open a file that contains multiple
databases without specifying a database name, however the
database type should be specified as DB_UNKNOWN and the
database must be opened read-only. The handle that is returned
from such a call is a handle on a database whose key values
are the names of the databases stored in the database file and
whose data values are opaque objects. No keys or data values
may be modified or stored using this database handle.
</p>
<div class="sect2" lang="en" xml:lang="en">
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<h3 class="title"><a id="idm140654540976064"></a>Configuring databases sharing a file</h3>
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<p>
There are four pieces of configuration information which
must be specified consistently for all databases in a
file, rather than differing on a per-database basis. They
are: byte order, checksum and encryption behavior, and
page size. When creating additional databases in a file,
any of these configuration values specified must be
consistent with the existing databases in the file or an
error will be returned.
</p>
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<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm140654546210656"></a>Caching databases sharing a file</h3>
</div>
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<p>
When storing multiple databases in a single physical
file rather than in separate files, if any of the
databases in a file is opened for update, all of the
databases in the file must share a memory pool. In other
words, they must be opened in the same database
environment. This is so per-physical-file information
common between the two databases is updated
correctly.
</p>
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<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm140654546233792"></a>Locking in databases based on sharing a file</h3>
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<p>
If databases are in separate files (and access to each
separate database is single-threaded), there is no reason
to perform any locking of any kind, and the two databases
may be read and written simultaneously. Further, there
would be no requirement to create a shared database
environment in which to open those two databases.
</p>
<p>
However, since multiple databases in a file exist in a
single physical file, opening two databases in the same
file simultaneously requires locking be enabled, unless
all of the databases are read-only. As the locks for the
two databases can only conflict during page allocation,
this additional locking is unlikely to affect performance.
The exception is when Berkeley DB Concurrent Data Store is
configured; a single lock is used for all databases in the
file when Berkeley DB Concurrent Data Store is configured,
and a write to one database will block all accesses to all
databases.
</p>
<p>
In summary, programmers writing applications that open
multiple databases in a single file will almost certainly
need to create a shared database environment in the
application as well. For more information on database
environments, see <a class="xref" href="env.html#env_intro" title="Database environment introduction">Database environment introduction</a>
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