<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
<!-- This component and the accompanying materials are made available under the terms of the License 
"Eclipse Public License v1.0" which accompanies this distribution, 
and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
<!-- Initial Contributors:
    Nokia Corporation - initial contribution.
Contributors: 
-->
<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-71F33868-0C59-5CE8-95BD-304066DB8946" xml:lang="en"><title>Change
notification</title><shortdesc>Describes change notification and when change notifications are
required. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Clients can be notified of changes to a database through an <codeph>RDbNotifier</codeph> object.
This class is particularly useful for shared databases.</p>
<p>A client makes an asynchronous request for notification, and this request
completes when:</p>
<ul>
<li id="GUID-F1437C46-26A1-5332-A001-D1A59D1D7D1B"><p>a transaction is committed</p> </li>
<li id="GUID-CAC96C67-DA03-502C-A096-A1F0B95E1967"><p>a transaction is rolled
back</p> </li>
<li id="GUID-5A06F0C0-B151-56B5-92A6-C9CE2E88C0CE"><p>the database has been
recovered</p> </li>
<li id="GUID-C30258A7-BFC9-59A5-B12A-CDF6B0145EEA"><p>the database is closed</p> </li>
<li id="GUID-7A2EC591-EBDD-57F1-9644-E33C1DD27C41"><p>all read-locks are removed</p> </li>
</ul>
<p>Any number of notifiers can be opened by a client on the same database
and they all act identically.</p>
<p>The lifetime of a notifier object is independent of the database that it
is observing. For example, a client can close a database <i>before</i> closing
a notifier which is observing that database. In this case, the notifier continues
to report all activity on the database caused by other clients. Once the last
client has closed the database, the notifier reports the status <codeph>RDbNotifier::EClose</codeph>;
any further requests for change notification complete immediately with the
same status, i.e. <codeph>RDbNotifier::EClose</codeph>.</p>
<p>Notifiers cannot be opened on client-side databases.</p>
<p>Only one request can be outstanding on a single notifier at any one time.
If further database events occur while a client is handling the completion
of a previous database event, the notifier remembers the most significant
event; this is determined as:</p>
<ul>
<li id="GUID-AD0E2488-553B-59C3-97FE-756D60A00BEB"><p>a database recovery
event (<codeph>RDbNotifier::ERecover</codeph>) is more important than a transaction
rollback event (<codeph>RDbNotifier::ERollback</codeph>)</p> </li>
<li id="GUID-62700E30-4911-50A5-B8B8-B1F0431D0EF3"><p>a transaction rollback
event (<codeph>RDbNotifier::ERollback</codeph>)is more important than a transaction
committed event (<codeph>RDbNotifier::ECommit</codeph>)</p> </li>
</ul>
<p>Read-locks removed events (<codeph>RDbNotifier::EUnlock</codeph>) are not
remembered in this way.</p>
</conbody></concept>