bigpic.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: bigpic.so,v 8.32 2003/11/27 18:25:57 bostic Exp $])

m4_ref_title(Architecture, The big picture,, java/faq, arch/progmodel)

m4_p([dnl
The previous chapters in this Reference Guide have described
applications that use the m4_db access methods for fast data storage
and retrieval.  The applications described in the following chapters
are similar in nature to the access method applications, but they are
also threaded and/or recoverable in the face of application or system
failure.])

m4_p([dnl
Application code that uses only the m4_db access methods might appear
as follows:])

include(ref/arch/bigpic1.cs)

m4_p([dnl
The underlying m4_db architecture that supports this is])

m4_picgif(center, smallpic, small)

m4_p([dnl
As you can see from this diagram, the application makes calls into the
access methods, and the access methods use the underlying shared memory
buffer cache to hold recently used file pages in main memory.])

m4_p([dnl
When applications require recoverability, their calls to the Access
Methods must be wrapped in calls to the transaction subsystem.  The
application must inform m4_db where to begin and end transactions, and
must be prepared for the possibility that an operation may fail at any
particular time, causing the transaction to abort.])

m4_p([dnl
An example of transaction-protected code might appear as follows:])

include(ref/arch/bigpic2.cs)

m4_p([dnl
In this example, the same operation is being done as before; however,
it is wrapped in transaction calls.  The transaction is started with
m4_ref(txn_begin) and finished with m4_ref(txn_commit).  If the
operation fails due to a deadlock, the transaction is aborted using
m4_ref(txn_abort), after which the operation may be retried.])

m4_p([dnl
There are actually five major subsystems in m4_db, as follows:])

m4_tagbegin
m4_tag(Access Methods, [dnl
The access methods subsystem provides general-purpose support for
creating and accessing database files formatted as Btrees, Hashed files,
and Fixed- and Variable-length records.  These modules are useful in
the absence of transactions for applications that need fast formatted
file support.  See m4_ref(dbh_open) and m4_ref(dbh_cursor) for more
information.  These functions were already discussed in detail in the
previous chapters.])

m4_tag(Memory Pool, [dnl
The Memory Pool subsystem is the general-purpose shared memory buffer pool
used by m4_db.  This is the shared memory cache that allows multiple
processes and threads within processes to share access to databases.  This
module is useful outside of the m4_db package for processes that require
portable, page-oriented, cached, shared file access.])

m4_tag(Transaction, [dnl
The Transaction subsystem allows a group of database changes to be
treated as an atomic unit so that either all of the changes are done,
or none of the changes are done.  The transaction subsystem implements
the m4_db transaction model.  This module is useful outside of the m4_db
package for processes that want to transaction-protect their own data
modifications.])

m4_tag(Locking, [dnl
The Locking subsystem is the general-purpose lock manager used by m4_db.
This module is useful outside of the m4_db package for processes that
require a portable, fast, configurable lock manager.])

m4_tag(Logging, [dnl
The Logging subsystem is the write-ahead logging used to support the
m4_db transaction model.  It is largely specific to the m4_db package,
and unlikely to be useful elsewhere except as a supporting module for
the m4_db transaction subsystem.])

m4_tagend

m4_p([dnl
Here is a more complete picture of the m4_db library:])

m4_picgif(center, bigpic, large)

m4_p([dnl
In this model, the application makes calls to the access methods and to
the Transaction subsystem.  The access methods and Transaction subsystems
in turn make calls into the Memory Pool, Locking and Logging subsystems
on behalf of the application.])

m4_p([dnl
The underlying subsystems can be used independently by applications.
For example, the Memory Pool subsystem can be used apart from the rest
of m4_db by applications simply wanting a shared memory buffer pool, or
the Locking subsystem may be called directly by applications that are
doing their own locking outside of m4_db.  However, this usage is not
common, and most applications will either use only the access methods
subsystem, or the access methods subsystem wrapped in calls to the m4_db
transaction interfaces.])

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