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<div class="sect1" lang="en" xml:lang="en">
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<h2 class="title" style="clear: both"><a id="intro_dbis"></a>What is Berkeley DB?</h2>
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<dl>
<dt>
<span class="sect2">
<a href="intro_dbis.html#idm140654545012672">Data Access Services</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="intro_dbis.html#idm140654545166048">Data management services</a>
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<dt>
<span class="sect2">
<a href="intro_dbis.html#idm140654545389776">Design</a>
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<p>
So far, we have discussed database systems in general
terms. It is time now to consider Berkeley DB in particular
and see how it fits into the framework we have introduced. The
key question is, what kinds of applications should use
Berkeley DB? </p>
<p>
Berkeley DB is an embedded database library
that provides scalable, high-performance,
transaction-protected data management services to
applications. Berkeley DB provides a simple function-call API
for data access and management.
</p>
<p>
Berkeley DB is distributed under both Open Source and commercial
licenses. By "Open Source" we mean a license that conforms to
the <a class="ulink" href="http://opensource.org/osd.html" target="_top"> Open Source
Definition</a>. This license guarantees Berkeley DB is
freely available for use and redistribution in other Open
Source applications. In addition, Oracle Corporation sells commercial
licenses allowing the redistribution of Berkeley DB in
proprietary applications. In all cases the complete source
code for Berkeley DB is freely available for download and use.
</p>
<p>
Berkeley DB is "embedded" because it links directly into
the application. It runs in the same address space as the
application. As a result, no inter-process communication,
either over the network or between processes on the same
machine, is required for database operations. Berkeley DB
provides a simple function-call API for a number of
programming languages, including C, C++, Java, Perl, Tcl,
Python, and PHP. All database operations happen inside the
library. Multiple processes, or multiple threads in a single
process, can all use the database at the same time as each
uses the Berkeley DB library. Low-level services like locking,
transaction logging, shared buffer management, memory
management, and so on are all handled transparently by the
library.
</p>
<p>
The Berkeley DB library is extremely portable. It runs
under almost all UNIX and Linux variants, Windows, and a
number of embedded real-time operating systems. It runs on
both 32-bit and 64-bit systems. It has been deployed on
high-end Internet servers, desktop machines, and on palmtop
computers, set-top boxes, in network switches, and elsewhere.
Once Berkeley DB is linked into the application, the end user
generally does not know that there is a database present at
all.
</p>
<p>
Berkeley DB is scalable in a number of respects. The
database library itself is quite compact (under 300 kilobytes
of text space on common architectures), which means it is
small enough to run in tightly constrained embedded systems,
but yet it can take advantage of gigabytes of memory and
terabytes of disk if you are using hardware that has those
resources.
</p>
<p>
Each of Berkeley DB's database files can contain up to 256
terabytes of data, assuming the underlying filesystem is
capable of supporting files of that size. Note that Berkeley
DB applications often use multiple database files. This means
that the amount of data your Berkeley DB application can
manage is really limited only by the constraints imposed by
your operating system, filesystem, and physical hardware.
</p>
<p>
Berkeley DB also supports high concurrency, allowing
thousands of users to operate on the same database files at
the same time.
</p>
<p>
Berkeley DB generally outperforms relational and
object-oriented database systems in embedded applications for
a couple of reasons. First, because the library runs in the
same address space, no inter-process communication is required
for database operations. The cost of communicating between
processes on a single machine, or among machines on a network,
is much higher than the cost of making a function call.
Second, because Berkeley DB uses a simple function-call
interface for all operations, there is no query language to
parse, and no execution plan to produce.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm140654545012672"></a>Data Access Services</h3>
</div>
</div>
</div>
<p>
Berkeley DB applications can choose the storage
structure that best suits the application. Berkeley DB
supports hash tables, Btrees, simple record-number-based
storage, and persistent queues. Programmers can create
tables using any of these storage structures, and can mix
operations on different kinds of tables in a single
application.
</p>
<p>
Hash tables are generally good for very large databases
that need predictable search and update times for
random-access records. Hash tables allow users to ask,
"Does this key exist?" or to fetch a record with a known
key. Hash tables do not allow users to ask for records
with keys that are close to a known key.
</p>
<p>
Btrees are better for range-based searches, as when the
application needs to find all records with keys between
some starting and ending value. Btrees also do a better
job of exploiting <span class="emphasis"><em>locality of
reference</em></span>. If the application is likely to
touch keys near each other at the same time, the Btrees
work well. The tree structure keeps keys that are close
together near one another in storage, so fetching nearby
values usually does not require a disk access.
</p>
<p>
Record-number-based storage is natural for applications
that need to store and fetch records, but that do not have
a simple way to generate keys of their own. In a record
number table, the record number is the key for the record.
Berkeley DB will generate these record numbers
automatically.
</p>
<p>
Queues are well-suited for applications that create
records, and then must deal with those records in creation
order. A good example is on-line purchasing systems.
Orders can enter the system at any time, but should
generally be filled in the order in which they were
placed.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm140654545166048"></a>Data management services</h3>
</div>
</div>
</div>
<p>
Berkeley DB offers important data management services,
including concurrency, transactions, and recovery. All of
these services work on all of the storage structures.
</p>
<p>
Many users can work on the same database concurrently.
Berkeley DB handles locking transparently, ensuring that
two users working on the same record do not interfere with
one another.
</p>
<p>
The library provides strict ACID transaction semantics,
by default. However, applications are allowed to relax the
isolation guarantees the database system makes.
</p>
<p>
Multiple operations can be grouped into a single
transaction, and can be committed or rolled back
atomically. Berkeley DB uses a technique called
<span class="emphasis"><em>two-phase locking</em></span> to be sure that
concurrent transactions are isolated from one another, and
a technique called <span class="emphasis"><em>write-ahead
logging</em></span> to guarantee that committed changes
survive application, system, or hardware failures.
</p>
<p>
When an application starts up, it can ask Berkeley DB
to run recovery. Recovery restores the database to a clean
state, with all committed changes present, even after a
crash. The database is guaranteed to be consistent and all
committed changes are guaranteed to be present when
recovery completes.
</p>
<p>
An application can specify, when it starts up, which
data management services it will use. Some applications
need fast, single-user, non-transactional Btree data
storage. In that case, the application can disable the
locking and transaction systems, and will not incur the
overhead of locking or logging. If an application needs to
support multiple concurrent users, but does not need
transactions, it can turn on locking without transactions.
Applications that need concurrent, transaction-protected
database access can enable all of the subsystems.
</p>
<p>
In all these cases, the application uses the same
function-call API to fetch and update records.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm140654545389776"></a>Design</h3>
</div>
</div>
</div>
<p>
Berkeley DB was designed to provide industrial-strength
database services to application developers, without
requiring them to become database experts. It is a classic
C-library style <span class="emphasis"><em>toolkit</em></span>, providing a
broad base of functionality to application writers.
Berkeley DB was designed by programmers, for programmers:
its modular design surfaces simple, orthogonal interfaces
to core services, and it provides mechanism (for example,
good thread support) without imposing policy (for example,
the use of threads is not required). Just as importantly,
Berkeley DB allows developers to balance performance
against the need for crash recovery and concurrent use. An
application can use the storage structure that provides
the fastest access to its data and can request only the
degree of logging and locking that it needs.
</p>
<p>
Because of the tool-based approach and separate
interfaces for each Berkeley DB subsystem, you can support
a complete transaction environment for other system
operations. Berkeley DB even allows you to wrap
transactions around the standard UNIX file read and write
operations! Further, Berkeley DB was designed to interact
correctly with the native system's toolset, a feature no
other database package offers. For example, on UNIX
systems Berkeley DB supports hot backups (database backups
while the database is in use), using standard UNIX system
utilities, for example, dump, tar, cpio, pax or even cp.
On other systems which do not support filesystems with
read isolation, Berkeley DB provides a tool for safely
copying files.
</p>
<p>
Finally, because scripting language interfaces are
available for Berkeley DB (notably Tcl and Perl),
application writers can build incredibly powerful database
engines with little effort. You can build
transaction-protected database applications using your
favorite scripting languages, an increasingly important
feature in a world using CGI scripts to deliver HTML.
</p>
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