notxn.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: notxn.so,v 10.15 2001/05/22 19:39:31 bostic Exp $])

m4_ref_title(Locking Subsystem,
    Locking without transactions,
    @locking without transactions, lock/page, lock/twopl)

m4_p([dnl
If an application runs with locking specified, but not transactions (for
example, m4_ref(dbenv_open) is called with m4_ref(DB_INIT_LOCK) or
m4_ref(DB_INIT_CDB) specified, but not m4_ref(DB_INIT_TXN)), locks are
normally acquired during each m4_db operation and released before the
operation returns to the caller.  The only exception is in the case of
cursor operations.  Cursors identify a particular position in a file.
For this reason, cursors must retain read locks across cursor calls to
make sure that the position is uniquely identifiable during a subsequent
cursor call, and so that an operation using m4_ref(DB_CURRENT) will
always refer to the same record as a previous cursor call.  These cursor
locks cannot be released until the cursor is either repositioned and a
new cursor lock established (for example, using the m4_ref(DB_NEXT)
or m4_ref(DB_SET) flags), or the cursor is closed.  As a result,
application writers are encouraged to close cursors as soon as
possible.])

m4_p([dnl
It is important to realize that concurrent applications that use locking
must ensure that two concurrent threads do not block each other.
However, because Btree and Hash access method page splits can occur at
any time, there is virtually no way to guarantee that an application
that writes the database cannot deadlock.  Applications running without
the protection of transactions may deadlock, and can leave the database
in an inconsistent state when they do so.  Applications that need
concurrent access, but not transactions, are more safely implemented
using the m4_cam Product.])

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