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<h2 class="title"><a id="introduction"></a>Chapter 1. Introduction</h2>
</div>
</div>
<div></div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="introduction.html#txnintro">Transaction Benefits</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="introduction.html#sysfailure">A Note on System Failure</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="introduction.html#apireq">Application Requirements</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="introduction.html#multithread-intro">Multi-threaded
and Multi-process
Applications</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="recovery-intro.html">Recoverability</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="perftune-intro.html">Performance Tuning</a>
</span>
</dt>
</dl>
</div>
<p>
This book provides a thorough introduction and discussion on transactions as
used with Berkeley DB (DB). It begins by offering a general overview to
transactions, the guarantees they provide, and the general application
infrastructure required to obtain full transactional protection for your
data.
</p>
<p>
This book also provides detailed examples on how to write a
transactional application. Both single threaded and multi-threaded <span>(as well as multi-process
applications)</span> are discussed. A detailed description of various
backup and recovery strategies is included in this manual, as is a
discussion on performance considerations for your transactional application.
</p>
<p>
You should understand the concepts from the
<span>
<i class="citetitle">Getting Started with Berkeley DB</i>
</span>
guide before reading this book.
</p>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="txnintro"></a>Transaction Benefits</h2>
</div>
</div>
<div></div>
</div>
<p>
Transactions offer your application's data protection from
application or system failures. That is, DB transactions offer
your application full ACID support:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<span class="bold"><b>A</b></span>tomicity
</p>
<p>
Multiple database operations are treated as a single unit of
work. Once committed, all write operations performed under
the protection of the transaction are saved to your databases.
Further, in the event that you abort a transaction, all write
operations performed during the transaction are discarded.
In this event, your database is left in the state it was in
before the transaction began, regardless of the number or
type of write operations you may have performed during the
course of the transaction.
</p>
<p>
Note that DB transactions can span one or more
database handles.
</p>
</li>
<li>
<p>
<span class="bold"><b>C</b></span>onsistency
</p>
<p>
Your databases will never see a partially completed
transaction. This is true even if your application fails while there are
in-progress transactions. If the application or system fails,
then either all of the database changes appear when the
application next runs, or none of them appear.
</p>
<p>
In other words, whatever consistency requirements your application has will never be violated by DB.
If, for example, your application requires every record to include an employee ID, and your
code faithfully adds that ID to its database records, then DB will never
violate that consistency requirement. The ID will remain in the database records until such a time as your
application chooses to delete it.
</p>
</li>
<li>
<p>
<span class="bold"><b>I</b></span>solation
</p>
<p>
While a transaction is in progress, your databases will appear
to the transaction as if there are no other operations
occurring outside of the transaction. That is, operations
wrapped inside a transaction will always have a clean and
consistent view of your databases. They never have to see
updates currently in progress under the protection of another transaction.
Note, however, that isolation guarantees can be
relaxed from the default setting. See
<a href="isolation.html">Isolation</a>
for more information.
</p>
</li>
<li>
<p>
<span class="bold"><b>D</b></span>urability
</p>
<p>
Once committed to your databases, your modifications will
persist even in the event of an application or system failure.
Note that like isolation, your durability guarantee can be
relaxed. See <a href="usingtxns.html#nodurabletxn">Non-Durable Transactions</a>
for more information.
</p>
</li>
</ul>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="sysfailure"></a>A Note on System Failure</h3>
</div>
</div>
<div></div>
</div>
<p>
From time to time this manual mentions that transactions protect your data against 'system or application
failure.' This is
true up to a certain extent. However, not all failures are created equal and no data protection
mechanism can protect you against every conceivable way a computing system can find to die.
</p>
<p>
Generally, when this book talks about protection against failures, it means that
transactions offer protection against
the likeliest culprits for system and application crashes. So long as your data modifications have been
committed to disk, those modifications should persist even if your application or OS subsequently fails.
And, even if the application or OS fails in the middle of a transaction commit (or abort), the data on disk
should be either in a consistent state, or there should be enough data available to bring
your databases into a consistent state (via a recovery procedure, for example). You may, however,
lose whatever data you were committing at the
time of the failure, but your databases will be otherwise unaffected.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Be aware that many disks have a disk write cache and on
some systems it is enabled by default. This means that
a transaction can have committed, and to your
application the data may appear to reside on disk, but
the data may in fact reside only in the write cache at
that time. This means that if the disk write cache is
enabled and there is no battery backup for it, data can
be lost after an OS crash even when maximum durability
mode is in use. For maximum durability, disable the
disk write cache or use a disk write cache with a
battery backup.
</p>
</div>
<p>
Of course, if your <span class="emphasis"><em>disk</em></span> fails, then the transactional benefits described in this book
are only as good as the backups you have taken.
<span>
By spreading your data and log files across separate disks,
you can minimize the risk of data loss due to a disk failure, but even in this case it is possible to
conjure a scenario where even this protection is insufficient (a fire in the machine room, for example) and
you must go to your backups for protection.
</span>
</p>
<p>
Finally, by following the programming examples shown in this book, you can write your code so as to protect
your data in the event that your code crashes. However, no programming API can protect you against logic
failures in your own code; transactions cannot protect you from simply writing the wrong thing to your
databases.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="apireq"></a>Application Requirements</h3>
</div>
</div>
<div></div>
</div>
<p>
In order to use transactions, your application has certain
requirements beyond what is required of non-transactional protected
applications. They are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Environments.
</p>
<p>
Environments are optional for non-transactional
applications, but they are required for transactional
applications.
</p>
<p>
Environments are optional for non-transactional
applications that use the base API, but they are required for transactional
applications. (Of course, applications that use the
DPL always require the DPL.)
</p>
<p>
Environment usage is described in detail in
<a href="usingtxns.html">Transaction Basics</a>.
</p>
</li>
<li>
<p>
Transaction subsystem.
</p>
<p>
In order to use transactions, you must explicitly
enable the transactional subsystem for your
application, and this must be done at the time that
your environment is first created.
</p>
</li>
<li>
<p>
Logging subsystem.
</p>
<p>
The logging subsystem is required for recovery purposes, but
its usage also means your application may require a
little more administrative effort than it does when logging
is not in use. See <a href="filemanagement.html">Managing DB Files</a> for more information.
</p>
</li>
<li>
<p>
<span>DB_TXN</span>
handles.
</p>
<p>
In order to obtain the atomicity guarantee offered by
the transactional subsystem (that is, combine multiple
operations in a single unit of work), your application must use
transaction handles. These handles are obtained from your
<span>DB_ENV</span>
objects. They should normally be short-lived, and their usage is
reasonably simple. To complete a transaction and save
the work it performed, you
call its <tt class="methodname">commit()</tt> method. To
complete a transaction and discard its work, you call its
<tt class="methodname">abort()</tt> method.
</p>
<p>
In addition, it is possible to use auto commit if you want
to transactional protect a single write operation. Auto
commit allows a transaction to be used without
obtaining an explicit transaction handle. See
<a href="autocommit.html">Auto Commit</a>
for information on how to use auto commit.
</p>
</li>
<li>
<p>
<span>Database</span>
open requirements.
</p>
<p>
<span>In addition to using
environments and initializing the
correct subsystems, your</span>
application must transaction protect the database
opens<span>,
and any secondary index associations,</span>
if subsequent operations on the databases are to be transaction
protected. The database open and secondary index
association are commonly transaction protected using
auto commit.
</p>
</li>
<li>
<p>
Deadlock detection.
</p>
<p>
Typically transactional applications use multiple
threads of control when accessing the database. Any
time multiple threads are used on a single resource,
the potential for lock contention arises. In turn, lock
contention can lead to deadlocks. See
<a href="blocking_deadlocks.html">Locks, Blocks, and Deadlocks</a>
for more information.
</p>
<p>
Therefore, transactional applications must frequently
include code for detecting and responding to deadlocks.
Note that this requirement is not
<span class="emphasis"><em>specific</em></span> to transactions
– you can certainly write concurrent
non-transactional DB applications. Further, not
every transactional application uses concurrency and
so not every transactional application must
manage deadlocks. Still, deadlock management is so
frequently a characteristic of transactional
applications that we discuss it in this
book. See <a href="txnconcurrency.html">Concurrency</a>
for more information.
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="multithread-intro"></a>Multi-threaded
<span>and Multi-process</span>
Applications</h3>
</div>
</div>
<div></div>
</div>
<p>
DB is designed to support multi-threaded <span>and
multi-process</span> applications, but their usage means
you must pay careful attention to issues of concurrency.
Transactions help your application's concurrency by providing various levels of
isolation for your threads of control. In addition, DB
provides mechanisms that allow you to detect and respond to
deadlocks (but strictly speaking, this is not limited to just
transactional applications).
</p>
<p>
<span class="emphasis"><em>Isolation</em></span> means that database modifications made by
one transaction will not normally be seen by readers from another
transaction until the first commits its changes. Different threads
use different transaction handles, so
this mechanism is normally used to provide isolation between
database operations performed by different threads.
</p>
<p>
Note that DB supports different isolation levels. For example,
you can configure your application to see uncommitted reads, which means
that one transaction can see data that has been modified but not yet
committed by another transaction. Doing this might mean your
transaction reads data "dirtied" by another transaction,
but which subsequently might change before that
other transaction commits its changes.
On the other hand, lowering your isolation
requirements means that your application can experience
improved throughput due to reduced lock contention.
</p>
<p>
For more information on concurrency, on managing isolation
levels, and on deadlock detection, see <a href="txnconcurrency.html">Concurrency</a>.
</p>
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