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<p>
This manual is meant as an introduction to the Berkeley DB library.
Consequently, it describes how to build a very simple, single-threaded
application. Consequently, this manual omits a great many powerful
aspects of the DB database engine that are not required by simple
applications. One of these is important enough that it warrants a brief
overview here: environments.
</p>
<p>
While environments are frequently not used by applications running in
embedded environments where every byte counts, they will be used by
virtually any other DB application requiring anything other than
the bare minimum functionality. An <span class="emphasis"><em>environment</em></span> is
essentially an encapsulation of one or more databases. Essentially, you
open an environment and then you open databases in that environment.
When you do so, the databases are created/located in a location relative
to the environment's home directory.
</p>
<p>
Environments offer a great many features that a stand-alone DB
database cannot offer:
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<p>
Multi-database files.
</p>
<p>
It is possible in DB to contain multiple databases in a
single physical file on disk. This is desirable for those
application that open more than a few handful of databases.
However, in order to have more than one database contained in
a single physical file, your application
<span class="emphasis"><em>must</em></span> use an environment.
</p>
</li>
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<p>
Multi-thread and multi-process support
</p>
<p>
When you use an environment, resources such as the in-memory
cache and locks can be shared by all of the databases opened in the
environment. The environment allows you to enable
subsystems that are designed to allow multiple threads and/or
processes to access DB databases. For example, you use an
environment to enable the concurrent data store (CDS), the
locking subsystem, and/or the shared memory buffer pool.
</p>
</li>
<li>
<p>
Transactional processing
</p>
<p>
DB offers a transactional subsystem that allows for full
ACID-protection of your database writes. You use environments to
enable the transactional subsystem, and then subsequently to obtain
transaction IDs.
</p>
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<li>
<p>
High availability (replication) support
</p>
<p>
DB offers a replication subsystem that enables
single-master database replication with multiple read-only
copies of the replicated data. You use environments to enable
and then manage this subsystem.
</p>
</li>
<li>
<p>
Logging subsystem
</p>
<p>
DB offers write-ahead logging for applications that want to
obtain a high-degree of recoverability in the face of an
application or system crash. Once enabled, the logging subsystem
allows the application to perform two kinds of recovery
("normal" and "catastrophic") through the use of the information
contained in the log files.
</p>
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<p>
For more information on these topics, see the
<i class="citetitle">Berkeley DB Getting Started with Transaction Processing</i> guide and the
<i class="citetitle">Berkeley DB Getting Started with Replicated Applications</i> guide.
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