1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Berkeley DB Concepts</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Getting Started with Berkeley DB" /> 10 <link rel="up" href="introduction.html" title="Chapter��1.��Introduction to Berkeley DB" /> 11 <link rel="prev" href="introduction.html" title="Chapter��1.��Introduction to Berkeley DB" /> 12 <link rel="next" href="accessmethods.html" title="Access Methods" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Berkeley DB Concepts</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="introduction.html">Prev</a>��</td> 22 <th width="60%" align="center">Chapter��1.��Introduction to Berkeley DB </th> 23 <td width="20%" align="right">��<a accesskey="n" href="accessmethods.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="sect1" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title" style="clear: both"><a id="concepts"></a>Berkeley DB Concepts</h2> 33 </div> 34 </div> 35 </div> 36 <p> 37 Before continuing, it is useful to describe some of the larger concepts 38 that you will encounter when building a DB application. 39 </p> 40 <p> 41 Conceptually, DB databases contain <span class="emphasis"><em>records</em></span>. 42 Logically each record represents a single entry in the database. 43 Each such record contains two pieces of information: a key and a data. 44 This manual will on occasion describe a <span class="emphasis"><em>a record's 45 key</em></span> or a <span class="emphasis"><em>record's data</em></span> when it is 46 necessary to speak to one or the other portion of a database 47 record. 48 </p> 49 <p> 50 Because of the key/data pairing used for DB databases, they are 51 sometimes thought of as a two-column table. However, data (and 52 sometimes keys, depending on the access method) can hold arbitrarily 53 complex data. Frequently, C structures and other such mechanisms are 54 stored in the record. This effectively turns a 2-column table 55 into a table with <span class="emphasis"><em>n</em></span> columns, where 56 <span class="emphasis"><em>n-1</em></span> of those columns are provided by the structure's 57 fields. 58 </p> 59 <p> 60 Note that a DB database is very much like a table in a relational 61 database system in that most DB applications use more than one 62 database (just as most relational databases use more than one table). 63 </p> 64 <p> 65 Unlike relational systems, however, a DB database contains a single 66 collection of records organized according to a given access method 67 (BTree, Queue, Hash, and so forth). In a relational database system, 68 the underlying access method is generally hidden from you. 69 </p> 70 <p> 71 In any case, frequently DB 72 applications are designed so that a single database stores a specific 73 type of data (just as in a relational database system, a single table 74 holds entries containing a specific set of fields). Because most applications 75 are required to manage multiple kinds of data, a DB application will 76 often use multiple databases. 77 </p> 78 <p> 79 For example, consider an accounting application. This kind of an 80 application may manage data based on bank accounts, checking 81 accounts, stocks, bonds, loans, and so forth. An accounting application 82 will also have to manage information about people, banking institutions, 83 customer accounts, and so on. In a traditional relational database, all 84 of these different kinds of information would be stored and managed 85 using a (probably very) complex series of tables. In a DB 86 application, all of this information would instead be divided out and 87 managed using multiple databases. 88 </p> 89 <p> 90 DB applications can efficiently use multiple databases using an 91 optional mechanism called an <span class="emphasis"><em>environment</em></span>. 92 For more information, see <a class="xref" href="environments.html" title="Environments">Environments</a>. 93 </p> 94 <p> 95 You interact with most DB APIs using special structures that 96 contain pointers to functions. These callbacks are 97 called <span class="emphasis"><em>methods</em></span> because they look so much like a 98 method on a C++ class. The variable that you use to access these 99 methods is often referred to as a 100 <span class="emphasis"><em>handle</em></span>. For example, to use a database you will 101 obtain a handle to that database. 102 </p> 103 <p> 104 Retrieving a record from a database is sometimes called 105 <span class="emphasis"><em>getting the record</em></span> because the method that you use 106 to retrieve the records is called <code class="methodname">get()</code>. 107 Similarly, storing database records is sometimes called 108 <span class="emphasis"><em>putting the record</em></span> because you use the 109 <code class="methodname">put()</code> method to do this. 110 </p> 111 <p> 112 When you store, or put, a record to a database using its handle, the 113 record is stored according to whatever sort order is in use by the 114 database. Sorting is mostly performed based on the key, but sometimes 115 the data is considered too. If you put a record using a key that already 116 exists in the database, then the existing record is replaced with the 117 new data. However, if the database supports 118 duplicate records (that is, records with identical keys but 119 different data), then that new record is stored as a duplicate record and 120 any existing records are not overwritten. 121 </p> 122 <p> 123 If a database supports duplicate records, then you can use a database 124 handle to retrieve only the first record in a set of duplicate records. 125 </p> 126 <p> 127 In addition to using a database handle, you can also read and write data using a 128 special mechanism called a <span class="emphasis"><em>cursor</em></span>. Cursors are 129 essentially iterators that you can use to walk over the records in a 130 database. You can use cursors to iterate over a database from the first 131 record to the last, and from the last to the first. You can also use 132 cursors to seek to a record. In the event that a database supports 133 duplicate records, cursors are the only way you can access all the 134 records in a set of duplicates. 135 </p> 136 <p> 137 Finally, DB provides a special kind of a database called a 138 <span class="emphasis"><em>secondary database</em></span>. Secondary databases serve as an 139 index into normal databases (called primary database to distinguish them 140 from secondaries). Secondary databases are interesting because DB 141 records can hold complex data types, but seeking to a given record is 142 performed only based on that record's key. If you wanted to be able to 143 seek to a record based on some piece of information that is not the key, 144 then you enable this through the use of secondary databases. 145 </p> 146 </div> 147 <div class="navfooter"> 148 <hr /> 149 <table width="100%" summary="Navigation footer"> 150 <tr> 151 <td width="40%" align="left"><a accesskey="p" href="introduction.html">Prev</a>��</td> 152 <td width="20%" align="center"> 153 <a accesskey="u" href="introduction.html">Up</a> 154 </td> 155 <td width="40%" align="right">��<a accesskey="n" href="accessmethods.html">Next</a></td> 156 </tr> 157 <tr> 158 <td width="40%" align="left" valign="top">Chapter��1.��Introduction to Berkeley DB ��</td> 159 <td width="20%" align="center"> 160 <a accesskey="h" href="index.html">Home</a> 161 </td> 162 <td width="40%" align="right" valign="top">��Access Methods</td> 163 </tr> 164 </table> 165 </div> 166 </body> 167</html> 168