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<h2 class="title" style="clear: both"><a id="apireq"></a>Application Requirements</h2>
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<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 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 class="xref" href="usingtxns.html" title="Chapter 3. Transaction Basics">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 class="xref" href="filemanagement.html" title="Chapter 5. Managing DB Files">Managing DB Files</a> for more information.
</p>
</li>
<li>
<p>
<span>Transaction</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>Environment</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 <code class="methodname">commit()</code> method. To
complete a transaction and discard its work, you call its
<code class="methodname">abort()</code> 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 class="xref" href="autocommit.html" title="Auto Commit">Auto Commit</a>
for information on how to use auto commit.
</p>
</li>
<li>
<p>
Entity Store
</p>
<p>
If you are using the DPL, then you must
configure your entity stores for transactional
support before opening them (that is, before
obtaining a primary index from them for the first
time).
</p>
</li>
<li>
<p>
Database 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>
<p>
Note that if you are using the DPL, you do not
have to explicitly do anything to the underlying
databases unless you want to modify their default
behavior — such as the isolation level that they
use, for example.
</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 class="xref" href="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks">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 class="xref" href="txnconcurrency.html" title="Chapter 4. Concurrency">Concurrency</a>
for more information.
</p>
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