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Berkeley DB Reference Guide:
  Access Methods

### 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 DB->open 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 Database environment introduction.

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