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<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>