data_open.so

berkeley db 4.6.21的源码。berkeley db是一个简单的数据库管理系统

m4_comment([$Id: data_open.so,v 1.13 2004/04/28 13:15:29 bostic Exp $])

m4_ref_title(m4_tam Applications,
    Opening the databases,, transapp/env_open, transapp/put)

m4_p([dnl
Next, we open three databases ("color" and "fruit" and "cats"), in the
database environment.  Again, our m4_ref(Db) database handles are
declared to be free-threaded using the m4_ref(DB_THREAD) flag, and so
may be used by any number of threads we subsequently create.])

include(ref/transapp/data_open.cs)

m4_p([dnl
After opening the database, we can use the m4_ref(db_stat) utility to
display information about a database we have created:])

m4_indent([dnl
prompt__GT__ db_stat -h TXNAPP -d color
53162   Btree magic number.
8       Btree version number.
Flags:
2       Minimum keys per-page.
8192    Underlying database page size.
1       Number of levels in the tree.
0       Number of unique keys in the tree.
0       Number of data items in the tree.
0       Number of tree internal pages.
0       Number of bytes free in tree internal pages (0% ff).
1       Number of tree leaf pages.
8166    Number of bytes free in tree leaf pages (0.% ff).
0       Number of tree duplicate pages.
0       Number of bytes free in tree duplicate pages (0% ff).
0       Number of tree overflow pages.
0       Number of bytes free in tree overflow pages (0% ff).
0       Number of pages on the free list.])

m4_p([dnl
The database open must be enclosed within a transaction in order to be
recoverable.  The transaction will ensure that created files are
re-created in recovered environments (or do not appear at all).
Additional database operations or operations on other databases can be
included in the same transaction, of course.  In the simple case, where
the open is the only operation in the transaction, an application can
set the m4_ref(DB_AUTO_COMMIT) flag instead of creating and managing
its own transaction handle.  The m4_ref(DB_AUTO_COMMIT) flag will
internally wrap the operation in a transaction, simplifying application
code.])

m4_p([dnl
The previous example is the simplest case of transaction protection for
database open.  Obviously, additional database operations can be done
in the scope of the same transaction.  For example, an application
maintaining a list of the databases in a database environment in a
well-known file might include an update of the list in the same
transaction in which the database is created.  Or, an application might
create both a primary and secondary database in a single transaction.])

m4_p([dnl
m4_ref(Db) handles that will later be used for transactionally protected
database operations must be opened within a transaction.  Specifying a
transaction handle to database operations using m4_ref(Db) handles not
opened within a transaction will return an error.  Similarly, not
specifying a transaction handle to database operations that will modify
the database, using handles that were opened within a transaction, will
also return an error.])

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