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399 lines
15 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Chapter 10. Secondary Databases</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB Java Edition" />
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<link rel="up" href="baseapi.html" title="Part II. Programming with the Base API" />
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<link rel="prev" href="cursorUsage.html" title="Cursor Example" />
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<link rel="next" href="keyCreator.html" title="Implementing Key Creators" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.2.7.5</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Chapter 10. Secondary Databases</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="cursorUsage.html">Prev</a> </td>
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<th width="60%" align="center">Part II. Programming with the Base API</th>
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<td width="20%" align="right"> <a accesskey="n" href="keyCreator.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="chapter" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title"><a id="indexes"></a>Chapter 10. Secondary Databases</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="indexes.html#DbAssociate">Opening and Closing Secondary Databases</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="keyCreator.html">Implementing Key
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<span>Creators</span>
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="secondaryProps.html">Secondary Database Properties</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="readSecondary.html">Reading Secondary Databases</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="secondaryDelete.html">Deleting Secondary Database Records</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="secondaryCursor.html">
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<span>Using Secondary Cursors</span>
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="joins.html">Database Joins</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="joins.html#joinUsage">Using Join Cursors</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="joins.html#joinconfig">JoinCursor Properties</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="indexusage.html">Secondary Database Example</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="indexusage.html#secondaryMyDbEnv">Opening Secondary Databases with
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MyDbEnv</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="indexusage.html#exampleReadSecondaries">Using Secondary Databases with ExampleInventoryRead</a>
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</span>
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</dt>
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</dl>
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</dd>
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</dl>
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</div>
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<p>
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Usually you find database records by means of the record's key. However,
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the key that you use for your record will not always contain the
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information required to provide you with rapid access to the data that you
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want to retrieve. For example, suppose your
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<code class="classname">Database</code>
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contains records related to users. The key might be a string that is some
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unique identifier for the person, such as a user ID. Each record's data,
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however, would likely contain a complex object containing details about
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people such as names, addresses, phone numbers, and so forth.
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While your application may frequently want to query a person by user
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ID (that is, by the information stored in the key), it may also on occasion
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want to locate people by, say, their name.
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</p>
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<p>
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Rather than iterate through all of the records in your database, examining
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each in turn for a given person's name, you create indexes based on names
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and then just search that index for the name that you want. You can do this
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using secondary databases. In JE, the
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<code class="classname">Database</code>
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that contains your data is called a
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<span class="emphasis"><em>primary database</em></span>. A database that provides an
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alternative set of keys to access that data is called a <span class="emphasis"><em>secondary
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database</em></span><span>,
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and these are managed using <code class="classname">SecondaryDatabase</code>
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class objects.</span> In a secondary database, the keys are your alternative
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(or secondary) index, and the data corresponds to a primary record's key.
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</p>
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<p>
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You create a secondary database by using a <code class="classname">SecondaryConfig</code>
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class object to identify an implementation of a
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<code class="classname">SecondaryKeyCreator</code>
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class object that is used to create keys based on data found in the primary
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database. You then pass this <code class="classname">SecondaryConfig</code>
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object to the <code class="classname">SecondaryDatabase</code> constructor.
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</p>
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<p>
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Once opened, JE manages secondary databases for you. Adding or deleting
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records in your primary database causes JE to update the secondary as
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necessary. Further, changing a record's data in the primary database may cause
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JE to modify a record in the secondary, depending on whether the change
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forces a modification of a key in the secondary database.
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</p>
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<p>
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Note that you can not write directly to a secondary database.
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<span>While methods
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exist on <code class="classname">SecondaryDatabase</code> and
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<code class="classname">SecondaryCursor</code> that appear to allow this, they in
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fact always throw
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<code class="classname">UnsupportedOperationException</code>.</span>
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To change the data referenced by a
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<code class="classname">SecondaryDatabase</code>
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record, modify the primary database instead. The exception to this rule is
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that delete operations are allowed on the
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<span><code class="classname">SecondaryDatabase</code> object.</span>
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See <a class="xref" href="secondaryDelete.html" title="Deleting Secondary Database Records">Deleting Secondary Database Records</a> for more
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information.
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</p>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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Secondary database records are updated/created by JE
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only if the
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<span><code class="methodname">SecondaryKeyCreator.createSecondaryKey()</code> method</span>
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returns
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<span><code class="literal">true</code>.</span>
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If
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<code class="literal">false</code>
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is returned, then JE will not add the key to the secondary database, and
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in the event of a record update it will remove any existing key.
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</p>
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<p>
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See <a class="xref" href="keyCreator.html" title="Implementing Key Creators">Implementing Key
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<span>Creators</span>
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</a> for more
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<span>information on this interface and method.</span>
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</p>
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</div>
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<p>
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When you read a record from a secondary database, JE automatically
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returns
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<span>the key and data</span>
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from the corresponding record in the primary database.
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<span>
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</span>
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</p>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="DbAssociate"></a>Opening and Closing Secondary Databases</h2>
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</div>
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</div>
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</div>
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<p>
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You manage secondary database opens and closes using the
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<span>
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<code class="methodname">Environment.openSecondaryDatabase()</code> method.
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</span>
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Just as is the case with primary databases, you must provide
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<code class="methodname">Environment.openSecondaryDatabase()</code>
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with the database's
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name and, optionally, other properties such as whether duplicate
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records are allowed, or whether the secondary database can be created on
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open. In addition, you must also provide:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>A handle to the primary database that this secondary database is
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indexing. Note that this means that secondary databases are maintained
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only for the specified <code class="classname">Database</code> handle. If you
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open the same <code class="classname">Database</code> multiple times for write
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(such as might occur when opening a database for read-only and read-write in the same application),
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then you should open the <code class="classname">SecondaryDatabase</code> for
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each such <code class="classname">Database</code> handle.</p>
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</li>
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<li>
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<p>A <code class="classname">SecondaryConfig</code> object that provides
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properties specific to a secondary database. The most important of
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these is used to identify the key creator for the database. The key
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creator is responsible for generating keys for the secondary database.
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See <a class="xref" href="secondaryProps.html" title="Secondary Database Properties">Secondary Database Properties</a> for details.</p>
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</li>
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</ul>
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</div>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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Primary databases <span class="emphasis"><em>must not</em></span> support duplicate records.
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Secondary records point to primary records using the primary key, so that key must be unique.
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</p>
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</div>
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<p>So to open (create) a secondary database, you:</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>Open your primary database.</p>
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</li>
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<li>
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<p>Instantiate your key creator.</p>
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</li>
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<li>
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<p>Instantiate your <code class="classname">SecondaryConfig</code> object.</p>
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</li>
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<li>
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<p>Set your key creator object on your <code class="classname">SecondaryConfig</code>
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object.</p>
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</li>
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<li>
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<p>Open your secondary database, specifying your primary database
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and your <code class="classname">SecondaryConfig</code> at that time.</p>
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</li>
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</ol>
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</div>
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<p>For example:</p>
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<a id="je_index1"></a>
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<pre class="programlisting">package je.gettingStarted;
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import com.sleepycat.bind.tuple.TupleBinding;
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import com.sleepycat.je.Database;
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import com.sleepycat.je.DatabaseConfig;
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import com.sleepycat.je.DatabaseException;
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import com.sleepycat.je.Environment;
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import com.sleepycat.je.SecondaryDatabase;
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import com.sleepycat.je.SecondaryConfig;
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import java.io.File;
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...
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DatabaseConfig myDbConfig = new DatabaseConfig();
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SecondaryConfig mySecConfig = new SecondaryConfig();
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myDbConfig.setAllowCreate(true);
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mySecConfig.setAllowCreate(true);
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// Duplicates are frequently required for secondary databases.
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mySecConfig.setSortedDuplicates(true);
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// Open the primary
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Environment myEnv = null;
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Database myDb = null;
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SecondaryDatabase mySecDb = null;
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try {
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String dbName = "myPrimaryDatabase";
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myEnv = new Environment(new File("/tmp/JEENV"), null);
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myDb = myEnv.openDatabase(null, dbName, myDbConfig);
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// A fake tuple binding that is not actually implemented anywhere
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// in this manual. The tuple binding is dependent on the data in use.
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// Tuple bindings are described earlier in this manual.
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TupleBinding myTupleBinding = new MyTupleBinding();
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// Open the secondary.
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// Key creators are described in the next section.
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FullNameKeyCreator keyCreator =
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new FullNameKeyCreator(myTupleBinding);
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// Get a secondary object and set the key creator on it.
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mySecConfig.setKeyCreator(keyCreator);
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// Perform the actual open
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String secDbName = "mySecondaryDatabase";
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mySecDb = myEnv.openSecondaryDatabase(null, secDbName, myDb,
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mySecConfig);
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} catch (DatabaseException de) {
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// Exception handling goes here ...
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}</pre>
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<p>To close a secondary database, call its close() method. Note that
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for best results, you should close all the secondary databases associated
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with a primary database before closing the primary.</p>
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<p>For example:</p>
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<a id="je_index2"></a>
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<pre class="programlisting">try {
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if (mySecDb != null) {
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mySecDb.close();
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}
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if (myDb != null) {
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myDb.close();
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}
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if (myEnv != null) {
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myEnv.close();
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}
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} catch (DatabaseException dbe) {
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// Exception handling goes here
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}</pre>
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</div>
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</div>
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<div class="navfooter">
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||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="cursorUsage.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="baseapi.html">Up</a>
|
||
</td>
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||
<td width="40%" align="right"> <a accesskey="n" href="keyCreator.html">Next</a></td>
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||
</tr>
|
||
<tr>
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||
<td width="40%" align="left" valign="top">Cursor Example </td>
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||
<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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||
</td>
|
||
<td width="40%" align="right" valign="top"> Implementing Key
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||
<span>Creators</span>
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||
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||
</td>
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||
</tr>
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||
</table>
|
||
</div>
|
||
</body>
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||
</html>
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