<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns:MadCap="http://www.madcapsoftware.com/Schemas/MadCap.xsd" MadCap:lastBlockDepth="2" MadCap:lastHeight="120" MadCap:lastWidth="624" MadCap:disableMasterStylesheet="true" MadCap:tocPath="Basics Operations & Concepts" MadCap:InPreviewMode="false" MadCap:RuntimeFileType="Topic" MadCap:TargetType="WebHelp" MadCap:PathToHelpSystem="../../" MadCap:HelpSystemFileName="index.xml" MadCap:SearchType="Stem">
<head><title>ACID Properties And Transactions</title>
<script type="text/javascript">/* <![CDATA[ */
window.onload = function(){
var pathToFlash = $('html').attr('MadCap:PathToHelpSystem') + 'Content/Resources/Code/ZeroClipboard.swf';
ZeroClipboard.setMoviePath(pathToFlash);
function bindToClipBord(element,content){
var clip = new ZeroClipboard.Client();
clip.setText(content);
clip.glue(element);
};
if(location.protocol==='file:'){
$('.copylink-marker').remove();
} else{
$('.copylink-marker').each(function(){
var text = $(this).parent().parent().children('.prettyprint').html();
$(this).hover(function(){
bindToClipBord(this,text);
},
function(){});
});
}
prettyPrint();
};
/* ]]> */</script>
<link href="../SkinSupport/MadCap.css" rel="stylesheet" />
<link href="../Resources/Stylesheets/OnlineStyle.css" rel="stylesheet" />
<script src="../Resources/Code/prettify.js">
</script>
<script src="../Resources/Code/lang-vb.js">
</script>
<script src="../Resources/Code/jquery.min.js">
</script>
<script src="../Resources/Code/ZeroClipboard.js">
</script>
<script src="../SkinSupport/MadCapAll.js" type="text/javascript">
</script>
</head>
<body>
<p class="MCWebHelpFramesetLink" style="display: none;"><a href="../../index_CSH.html#basics/acid_and_transactions.htm" style="">Open topic with navigation</a>
</p>
<div class="MCBreadcrumbsBox"><span class="MCBreadcrumbsPrefix">You are here: </span><a class="MCBreadcrumbsLink" href="../basics.htm">Basics Operations & Concepts</a><span class="MCBreadcrumbsDivider"> > </span><span class="MCBreadcrumbs">ACID Properties And Transactions</span>
</div>
<p>
<script type="text/javascript">/*<![CDATA[*/document.write('<a href="' + location.href +'">');
document.write("Direct Link");
document.write('</a>');/*]]>*/</script>
</p>
<p>
</p>
<h1><a name="kanchor21"></a>ACID Properties And Transactions</h1>
<p>The <a href="http://en.wikipedia.org/wiki/ACID" target="_parent">ACID properties</a> are one of the oldest and most important concepts of database theory. It sets out the requirements for the database reliability:</p>
<ul>
<li value="1"><strong>Atomicity</strong>: This means that must follow an "all or nothing" rule. Each transaction is either successfully completed or in case of failure the state of the database isn't changed at all.<br /> For example in a
bank transfer transaction there are two steps: debit and credit. If the debit
operation was successful, but the credit failed, the whole transaction should
fail and the system should remain in the initial state.
</li>
<li value="2"><strong>Consistency</strong>: Consistency ensures that the database stays always in a consistent state. Each transaction takes the database from one consistent state to the next consistent state.</li>
<li value="3"><strong>Isolation</strong>: Isolation means that different operations cannot access modified data from another transaction that has not yet completed. There are different isolation-models. <a href="acid_and_transactions/isolation.htm" target="" title="" alt="" class="MCXref">See "Isolation"</a></li>
<li value="4"><strong>Durability</strong>: This just refers to the real goal of any data store. It just means that the data should be persistently stored. </li>
</ul>
<p>db4o fulfills the <a href="#" class="glossaryTerm_0">ACID</a> properties. Each <a href="object_container.htm">object container</a> has its own transaction. Each transaction is a unit of work and ensures the ACID properties. This means, that a db4o transaction is an atomic operation. Either all changes of the db4o transactions are committed and made persistent. Or in case of a failure or rollback no state is changed. The database is kept consistent even on application or database crashes. And the db4o transactions are isolated from each other. <a href="acid_and_transactions/db4o_tx_basics.htm" target="" title="" alt="" class="MCXref">See "db4o Transactions"</a></p>
<script type="text/javascript" src="../SkinSupport/MadCapBodyEnd.js">
</script>
</body>
</html>
