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<th width="60%" align="center">Chapter 8. Database Records</th>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="usingDbt"></a>Reading and Writing Database Records</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="usingDbt.html#databaseWrite">Writing Records to the Database</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="usingDbt.html#databaseRead">Getting Records from the Database</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="usingDbt.html#recordDelete">Deleting Records</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="usingDbt.html#datapersist">Data Persistence</a>
</span>
</dt>
</dl>
</div>
<p>
When reading and writing database records, be aware that there are some
slight differences in behavior depending on whether your database supports duplicate
records. Two or more database records are considered to be duplicates of
one another if they share the same key. The collection of records
sharing the same key are called a <span class="emphasis"><em>duplicates set.</em></span>
<span>
In DB, a given key is stored only once for a single duplicates set.
</span>
</p>
<p>
By default, DB databases do
not support duplicate records. Where duplicate records are supported,
cursors (see below) are <span>typically</span> used
to access all of the records in the duplicates set.
</p>
<p>
DB provides two basic mechanisms for the storage and retrieval of database
key/data pairs:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
The
<code class="methodname">Database.put()</code>
and
<code class="methodname">Database.get()</code>
methods provide the easiest access for all non-duplicate records in the database.
These methods are described in this section.
</p>
</li>
<li>
<p>Cursors provide several methods for putting and getting database
records. Cursors and their database access methods are described in
<a class="xref" href="Cursors.html" title="Chapter 9. Using Cursors">Using Cursors</a>.</p>
</li>
</ul>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="databaseWrite"></a>Writing Records to the Database</h3>
</div>
</div>
</div>
<p>
Records are stored in the database using whatever organization is
required by the access method that you have selected. In some cases (such as
BTree), records are stored in a sort order that you may want to define
(see <a class="xref" href="btree.html#comparators" title="Setting Comparison Functions">Setting Comparison Functions</a> for more information).
</p>
<p>
In any case, the mechanics of putting and getting database records do not
change once you have selected your access method, configured your
sorting routines (if any), and opened your database. From your
code's perspective, a simple database put and get is largely the
same no matter what access method you are using.
</p>
<p>You can use the following methods to put database records:</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="methodname">Database.put()</code>
</p>
<p>
Puts a database record into the database. If your database does not
support duplicate records, and if the provided key already exists in
the database, then the currently existing record is replaced with
the new data.
</p>
</li>
<li>
<p>
<code class="methodname">Database.putNoOverwrite()</code>
</p>
<p>
Disallows overwriting (replacing) an existing record in the
database. If the provided key already exists in the database,
then this method returns
<code class="literal">OperationStatus.KEYEXIST</code> even if
the database supports duplicates.
</p>
</li>
<li>
<p>
<code class="methodname">Database.putNoDupData()</code>
</p>
<p>
Puts a database record into the database. If the provided key
and data already exists in the database (that is, if you are
attempting to put a record that compares equally to an existing
record), then this returns
<code class="literal">OperationStatus.KEYEXIST</code>.
</p>
</li>
</ul>
</div>
<p>
When you put database records, you provide both the key and the data as
<code class="classname">DatabaseEntry</code> objects. This means you must
convert your key and data into a Java <code class="literal">byte</code> array. For
example:
</p>
<a id="java_dbt3"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.Database;
...
// Database opens omitted for clarity.
// Databases must NOT be opened read-only.
String aKey = "myFirstKey";
String aData = "myFirstData";
try {
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));
myDatabase.put(null, theKey, theData);
} catch (Exception e) {
// Exception handling goes here
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="databaseRead"></a>Getting Records from the Database</h3>
</div>
</div>
</div>
<p>
The <code class="classname">Database</code> class provides several
methods that you can use to retrieve database records. Note that if your
database supports duplicate records, then these methods will only ever
return the first record in a duplicate set. For this reason, if your
database supports duplicates, you should use a cursor to retrieve
records from it. Cursors are described in <a class="xref" href="Cursors.html" title="Chapter 9. Using Cursors">Using Cursors</a>.
</p>
<p>
You can use either of the following methods to retrieve records from the database:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="methodname">Database.get()</code>
</p>
<p>Retrieves the record whose key matches the key provided to the
method. If no records exists that uses the provided key, then
<code class="literal">OperationStatus.NOTFOUND</code> is returned.</p>
</li>
<li>
<p>
<code class="methodname">Database.getSearchBoth()</code>
</p>
<p>Retrieve the record whose key matches both the key and the data
provided to the method. If no record exists that uses the provided
key and data, then <code class="literal">OperationStatus.NOTFOUND</code> is
returned.</p>
</li>
</ul>
</div>
<p>Both the key and data for a database record are returned as
byte arrays in <code class="classname">DatabaseEntry</code> objects. These objects are
passed as parameter values to the <code class="methodname">Database.get()</code> method.
</p>
<p>In order to retrieve your data once <code class="classname">Database.get()</code>
has completed, you must retrieve the <code class="literal">byte</code> array stored
in the <code class="classname">DatabaseEntry</code> and then convert that
<code class="literal">byte</code> array back to the
appropriate datatype. For example:</p>
<a id="java_dbt4"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.Database;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
...
Database myDatabase = null;
// Database opens omitted for clarity.
// Database may be opened read-only.
String aKey = "myFirstKey";
try {
// Create a pair of DatabaseEntry objects. theKey
// is used to perform the search. theData is used
// to store the data returned by the get() operation.
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
DatabaseEntry theData = new DatabaseEntry();
// Perform the get.
if (myDatabase.get(null, theKey, theData, LockMode.DEFAULT) ==
OperationStatus.SUCCESS) {
// Recreate the data String.
byte[] retData = theData.getData();
String foundData = new String(retData, "UTF-8");
System.out.println("For key: '" + aKey + "' found data: '" +
foundData + "'.");
} else {
System.out.println("No record found for key '" + aKey + "'.");
}
} catch (Exception e) {
// Exception handling goes here
}</pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="recordDelete"></a>Deleting Records</h3>
</div>
</div>
</div>
<p>
You can use the
<code class="methodname">Database.delete()</code>
method to delete a record from the database. If your database supports
duplicate records, then all records associated with the provided key are
deleted. To delete just one record from a list of duplicates, use a
cursor. Cursors are described in <a class="xref" href="Cursors.html" title="Chapter 9. Using Cursors">Using Cursors</a>.
</p>
<p>
You can also delete every record in the database by using
<code class="methodname">Environment.truncateDatabase().</code>
</p>
<p>For example:</p>
<a id="java_dbt5"></a>
<pre class="programlisting">package db.GettingStarted;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.Database;
...
Database myDatabase = null;
// Database opens omitted for clarity.
// Database can NOT be opened read-only.
try {
String aKey = "myFirstKey";
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
// Perform the deletion. All records that use this key are
// deleted.
myDatabase.delete(null, theKey);
} catch (Exception e) {
// Exception handling goes here
}</pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="datapersist"></a>Data Persistence</h3>
</div>
</div>
</div>
<p>
When you perform a database modification, your modification is made
in the in-memory cache. This means that your data modifications
are not necessarily flushed to disk, and so your data may not appear
in the database after an application restart.
</p>
<p>
Note that as a normal part of closing a database, its cache is
written to disk. However, in the event of an application or system
failure, there is no guarantee that your databases will close
cleanly. In this event, it is possible for you to lose data. Under
extremely rare circumstances, it is also possible for you to
experience database corruption.
</p>
<p>
Therefore, if you care if your data is durable across system
failures, and to guard against the rare possibility of
database corruption, you should use transactions to protect your
database modifications. Every time you commit a transaction, DB
ensures that the data will not be lost due to application or
system failure. Transaction usage is described in the
<span>
<em class="citetitle">Berkeley DB Getting Started with Transaction Processing</em> guide.
</span>
</p>
<p>
If you do not want to use transactions, then the assumption is that
your data is of a nature that it need not exist the next time your
application starts. You may want this if, for example, you are using
DB to cache data relevant only to the current application
runtime.
</p>
<p>
If, however, you are not using transactions for some reason and you
still want some guarantee that your database modifications are
persistent, then you should periodically
<span>run environment syncs.</span>
Syncs cause any dirty entries in the in-memory cache and the
operating system's file cache to be written to disk. As
such, they are quite expensive and you should use them sparingly.
</p>
<p>
Remember that by default a sync is performed any time a non-transactional
database is closed cleanly. (You can override this behavior by
specifying
<code class="literal">true</code>
on the call to
<span><code class="methodname">Database.close()</code>.)</span>
That said, you can manually run a sync by calling
<code class="methodname">Database.sync().</code>
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
If your application or system crashes and you are not using
transactions, then you should either discard and recreate your
databases, or verify them. You can verify a database using
<span>Database.verify().</span>
If your databases do not verify cleanly, use the
<span class="command"><strong>db_dump</strong></span> command to salvage as much of the
database as is possible. Use either the <code class="literal">-R</code> or
<code class="literal">-r</code> command line options to control how
aggressive <span class="command"><strong>db_dump</strong></span> should be when salvaging
your databases.
</p>
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