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<title>Berkeley DB Reference Guide: The big picture</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++">
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<table width="100%"><tr valign=top>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Architecture</dl></h3></td>
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<td align=right><a href="../../ref/am_misc/faq.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../reftoc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/arch/progmodel.html"><img src="../../images/next.gif" alt="Next"></a>
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<h1 align=center>The big picture</h1>
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<p>The previous chapters in this Reference Guide have described
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applications that use the Berkeley DB access methods for fast data storage
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and retrieval. The applications described in the following chapters
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are similar in nature to the access method applications, but they are
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also threaded and/or recoverable in the face of application or system
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<p>Application code that uses only the Berkeley DB access methods might appear
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<p><blockquote><pre>switch (ret = dbp->put(dbp, NULL, &key, &data, 0)) {
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printf("db: %s: key stored.\n", (char *)key.data);
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dbp->err(dbp, ret, "dbp->put");
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<p>The underlying Berkeley DB architecture that supports this is
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<p align=center><img src="smallpic.gif" alt="small">
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<p>As you can see from this diagram, the application makes calls into the
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access methods, and the access methods use the underlying shared memory
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buffer cache to hold recently used file pages in main memory.
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<p>When applications require recoverability, their calls to the Access
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Methods must be wrapped in calls to the transaction subsystem. The
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application must inform Berkeley DB where to begin and end transactions, and
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must be prepared for the possibility that an operation may fail at any
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particular time, causing the transaction to abort.
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<p>An example of transaction-protected code might appear as follows:
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<p><blockquote><pre>for (fail = 0;;) {
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/* Begin the transaction. */
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if ((ret = txn_begin(dbenv, NULL, &tid, 0)) != 0) {
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dbenv->err(dbenv, ret, "txn_begin");
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switch (ret = dbp->put(dbp, tid, &key, &data, 0)) {
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/* Success: commit the change. */
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printf("db: %s: key stored.\n", (char *)key.data);
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if ((ret = txn_commit(tid, 0)) != 0) {
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dbenv->err(dbenv, ret, "txn_commit");
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case DB_LOCK_DEADLOCK:
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/* Failure: retry the operation. */
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if ((t_ret = txn_abort(tid)) != 0) {
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dbenv->err(dbenv, t_ret, "txn_begin");
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if (++fail == MAXIMUM_RETRY)
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<p>In this example, the same operation is being done as before; however,
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it is wrapped in transaction calls. The transaction is started with
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<a href="../../api_c/txn_begin.html">txn_begin</a> and finished with <a href="../../api_c/txn_commit.html">txn_commit</a>. If the
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operation fails due to a deadlock, the transaction is aborted using
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<a href="../../api_c/txn_abort.html">txn_abort</a>, after which the operation may be retried.
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<p>There are actually five major subsystems in Berkeley DB, as follows:
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<p><dt>Access Methods<dd>The access methods subsystem provides general-purpose support for
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creating and accessing database files formatted as Btrees, Hashed files,
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and Fixed- and Variable-length records. These modules are useful in
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the absence of transactions for applications that need fast formatted
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file support. See <a href="../../api_c/db_open.html">DB->open</a> and <a href="../../api_c/db_cursor.html">DB->cursor</a> for more
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information. These functions were already discussed in detail in the
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<p><dt>Memory Pool<dd>The Memory Pool subsystem is the general-purpose shared memory buffer pool
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used by Berkeley DB. This is the shared memory cache that allows multiple
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processes and threads within processes to share access to databases. This
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module is useful outside of the Berkeley DB package for processes that require
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portable, page-oriented, cached, shared file access.
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<p><dt>Transaction<dd>The Transaction subsystem allows a group of database changes to be
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treated as an atomic unit so that either all of the changes are done,
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or none of the changes are done. The transaction subsystem implements
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the Berkeley DB transaction model. This module is useful outside of the Berkeley DB
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package for processes that want to transaction-protect their own data
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<p><dt>Locking<dd>The Locking subsystem is the general-purpose lock manager used by Berkeley DB.
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This module is useful outside of the Berkeley DB package for processes that
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require a portable, fast, configurable lock manager.
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<p><dt>Logging<dd>The Logging subsystem is the write-ahead logging used to support the
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Berkeley DB transaction model. It is largely specific to the Berkeley DB package,
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and unlikely to be useful elsewhere except as a supporting module for
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the Berkeley DB transaction subsystem.
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<p>Here is a more complete picture of the Berkeley DB library:
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<p align=center><img src="bigpic.gif" alt="large">
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<p>In this model, the application makes calls to the access methods and to
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the Transaction subsystem. The access methods and Transaction subsystems
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in turn make calls into the Memory Pool, Locking and Logging subsystems
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on behalf of the application.
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<p>The underlying subsystems can be used independently by applications.
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For example, the Memory Pool subsystem can be used apart from the rest
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of Berkeley DB by applications simply wanting a shared memory buffer pool, or
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the Locking subsystem may be called directly by applications that are
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doing their own locking outside of Berkeley DB. However, this usage is not
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common, and most applications will either use only the access methods
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subsystem, or the access methods subsystem wrapped in calls to the Berkeley DB
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transaction interfaces.
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