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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3208) Callers of DaemonService.serviceNow() assume behaviour not guaranteed by the interface
Date Thu, 15 Nov 2007 21:17:43 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3208?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12542884

Knut Anders Hatlen commented on DERBY-3208:

The easiest solution is probably to extend the javadoc comment for
DaemonService.serviceNow() with the necessary guarantee for this code
to work. My suggestion is:

 * Service this subscription ASAP. When this method is called, the subscriber's
 * <code>performWork()</code> method is guaranteed to be invoked at some point
 * in the future. However, there is no guarantee that a subscriber's
 * <code>performWork()</code> is called the same number of times as the
 * subscriber calls this method. More precisely, if a subscriber is already
 * waiting for this daemon service to invoke its <code>performWork()</code>
 * method, a call to this method may or may not be a no-op.

I think the above description exactly matches how it is implemented in
BasicDaemon, except that "may or may not be" in the last sentence can
be replaced with "is".

> Callers of DaemonService.serviceNow() assume behaviour not guaranteed by the interface
> --------------------------------------------------------------------------------------
>                 Key: DERBY-3208
>                 URL: https://issues.apache.org/jira/browse/DERBY-3208
>             Project: Derby
>          Issue Type: Bug
>          Components: Services, Store
>    Affects Versions:
>            Reporter: Knut Anders Hatlen
>            Priority: Minor
> There are two callers of DaemonService.serviceNow() in the code:
>   - Clock.rotateClock()
>   - LogToFile.flush()
> They implicitly assume that if they are not currently waiting to be serviced by the daemon
service, a call to serviceNow() is guaranteed to result in their performWork() method being
invoked at some point in the future. They maintain flags (Clock.cleanerRunning and LogToFile.checkpointDaemonCalled)
which tell whether serviceNow() has been called, and if they are true, serviceNow() is not
called again. When performWork() is called, they reset the flag, allowing others to call serviceNow()
> However, DaemonService.serviceNow() does not guarantee that performWork() always gets
called in these cases. Its javadoc says the following:
> 	/**
> 	    Service this subscription ASAP.  Does not guarantee that the daemon
> 		will actually do anything about it.
> 		@param clientNumber the number that uniquely identify the client
> 	 */
> If one of the calls to serviceNow() does not result in a call to performWork(), the flags
which prevent new calls to serviceNow() are not reset. Since both Clock and LogToFile are
subscribed to the daemon service in on-demand-only mode, performWork() is only called if the
subscriber calls serviceNow(). That means there is no way to reset the flags, and Clock/LogToFile
are never allowed to call serviceNow() again.
> Although BasicDaemon's implementation of the DaemonService interface does in fact give
the necessary guarantees (without explicitly stating them, though) to make this protocol work,
Clock and LogToFile should be coded against the specification of the interface rather than
against the undocumented behaviour of an implementation of the interface.

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