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<div>
<h2 class="title"><a id="arch"></a>Chapter 8. Berkeley DB Architecture </h2>
</div>
</div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="arch.html#arch_bigpic">The big picture</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="arch_progmodel.html">Programming model</a>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="arch_bigpic"></a>The big picture</h2>
</div>
</div>
</div>
<p>
The previous chapters in this Reference Guide have described
applications that use the Berkeley 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.
</p>
<p>
Application code that uses only the Berkeley DB access
methods might appear as follows:
</p>
<pre class="programlisting">switch (ret = dbp->/put(dbp, NULL, &key, &data, 0)) {
case 0:
printf("db: %s: key stored.\n", (char *)key.data);
break;
default:
dbp->/err(dbp, ret, "dbp->/put");
exit (1);
}</pre>
<p>
The underlying Berkeley DB architecture that supports this
is
</p>
<div class="mediaobject">
<img src="arch_smallpic.gif" />
</div>
<p>
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.
</p>
<p>
When applications require recoverability, their calls to the
Access Methods must be wrapped in calls to the transaction
subsystem. The application must inform Berkeley 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.
</p>
<p>
An example of transaction-protected code might appear as
follows:
</p>
<pre class="programlisting">for (fail = 0;;) {
/* Begin the transaction. */
if ((ret = dbenv->/txn_begin(dbenv, NULL, &tid, 0)) != 0) {
dbenv->/err(dbenv, ret, "dbenv->/txn_begin");
exit (1);
}
/* Store the key. */
switch (ret = dbp->/put(dbp, tid, &key, &data, 0)) {
case 0:
/* Success: commit the change. */
printf("db: %s: key stored.\n", (char *)key.data);
if ((ret = tid->/commit(tid, 0)) != 0) {
dbenv->/err(dbenv, ret, "DB_TXN->/commit");
exit (1);
}
return (0);
case DB_LOCK_DEADLOCK:
default:
/* Failure: retry the operation. */
if ((t_ret = tid->/abort(tid)) != 0) {
dbenv->/err(dbenv, t_ret, "DB_TXN->/abort");
exit (1);
}
if (fail++ == MAXIMUM_RETRY)
return (ret);
continue;
}
}</pre>
<p>
In this example, the same operation is being done as before;
however, it is wrapped in transaction calls. The transaction
is started with <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> and finished with <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a>. If
the operation fails due to a deadlock, the transaction is
aborted using <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a>, after which the operation may be
retried.
</p>
<p>
There are actually five major subsystems in Berkeley DB, as
follows:
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">Access Methods</span>
</dt>
<dd>
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 <a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a>
and <a href="../api_reference/C/dbcursor.html" class="olink">DB->cursor()</a> for more information. These functions
were already discussed in detail in the previous
chapters.
</dd>
<dt>
<span class="term">Memory Pool</span>
</dt>
<dd>
The Memory Pool subsystem is the general-purpose
shared memory buffer pool used by Berkeley 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
Berkeley DB package for processes that require
portable, page-oriented, cached, shared file
access.
</dd>
<dt>
<span class="term">Transaction</span>
</dt>
<dd>
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 Berkeley DB transaction model. This
module is useful outside of the Berkeley DB package
for processes that want to transaction-protect their
own data modifications.
</dd>
<dt>
<span class="term">Locking</span>
</dt>
<dd>
The Locking subsystem is the general-purpose
lock manager used by Berkeley DB. This module is
useful outside of the Berkeley DB package for
processes that require a portable, fast, configurable
lock manager.
</dd>
<dt>
<span class="term">Logging</span>
</dt>
<dd>
The Logging subsystem is the write-ahead logging
used to support the Berkeley DB transaction model. It
is largely specific to the Berkeley DB package, and
unlikely to be useful elsewhere except as a supporting
module for the Berkeley DB transaction
subsystem.
</dd>
</dl>
</div>
<p>
Here is a more complete picture of the Berkeley DB
library:
</p>
<div class="mediaobject">
<img src="arch_bigpic.gif" />
</div>
<p>
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.
</p>
<p>
The underlying subsystems can be used independently by
applications. For example, the Memory Pool subsystem can be
used apart from the rest of Berkeley 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 Berkeley 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 Berkeley DB transaction interfaces.
</p>
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