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<title>Berkeley DB Reference Guide: Opening multiple databases in a single file</title>
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<td><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></td>
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Opening multiple databases in a single file
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.
To open or create a file that will include more than a single database,
specify a database name when calling the <a href="../../api_c/db_open.html">DB-&gt;open</a> method.
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
databases. Queue 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.
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.
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.
Configuring databases sharing a file
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.
Caching databases sharing a file
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.
Locking in databases based on sharing a file
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.
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.
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 href="../../ref/env/intro.html">Database
environment introduction.</a>
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