Package org.apache.derby.impl.services.daemon

Class BasicDaemon

  • All Implemented Interfaces:
    java.lang.Runnable, DaemonService
    public class BasicDaemon
extends java.lang.Object
implements DaemonService, java.lang.Runnable
    A BasicDaemon is a background worker thread which does asynchronous I/O and general clean up. It should not be used as a general worker thread for parallel execution. One cannot count on the order of request or count on when the daemon will wake up, even with serviceNow requests. Request are not persistent and not recoverable, they are all lost when the system crashes or is shutdown. System shutdown, even orderly ones, do not wait for daemons to finish its work or empty its queue. Furthermore, any Serviceable subscriptions, including onDemandOnly, must tolerate spurious services. The BasicDaemon will setup a context manager with no context on it. The Serviceable object's performWork must provide useful context on the context manager to do its work. The BasicDaemon will wrap performWork call with try / catch block and will use the ContextManager's error handling to clean up any error. The BasicDaemon will guarentee serviceNow request will not be lost as long as the jbms does not crash - however, if N serviceNow requests are made by the same client, it may only be serviced once, not N times. Many Serviceable object will subscribe to the same BasicDaemon. Their performWork method should be well behaved - in other words, it should not take too long or hog too many resources or deadlock with anyone else. And it cannot (should not) error out. The BasicDaemon implementation manages the DaemonService's data structure, handles subscriptions and enqueues requests, and determine the service schedule for its Serviceable objects. The BasicDaemon keeps an array (Vector) of Serviceable subscriptions it also keeps 2 queues for clients that uses it for one time service - the 1st queue is for a serviceNow enqueue request, the 2nd queue is for non serviceNow enqueue request. This BasicDaemon services its clients in the following order: 1. any subscribed client that have made a serviceNow request that has not been fulfilled 2. serviceable clients on the 1st queue 3. all subscribed clients that are not onDemandOnly 4. serviceable clients 2nd queue

    • Field Detail

      • numClients

        private int numClients

      • subscription

        private final java.util.Vector<ServiceRecord> subscription

      • highPQ

        private final java.util.List<ServiceRecord> highPQ
        Queues for the work to be done. These are synchronized by this object.

      • nextService

        private int nextService
        which subscribed clients to service next? only accessed by daemon thread

      • awakened

        private boolean awakened

      • waiting

        private boolean waiting
        true if I'm waiting, if this is false then I am running and a notify is not required.

      • inPause

        private boolean inPause

      • running

        private boolean running

      • stopRequested

        private boolean stopRequested

      • stopped

        private boolean stopped

      • lastServiceTime

        private long lastServiceTime

      • earlyWakeupCount

        private int earlyWakeupCount

    • Constructor Detail

      • BasicDaemon

        public BasicDaemon​(ContextService contextService)
        make a BasicDaemon

    • Method Detail

      • subscribe

        public int subscribe​(Serviceable newClient,
                     boolean onDemandOnly)
        Description copied from interface: DaemonService
        Add a new client that this daemon needs to service
        Specified by:
        subscribe in interface DaemonService
        Parameters:
        newClient - a Serviceable object this daemon will service from time to time
        onDemandOnly - only service this client when it ask for service with a serviceNow request
        Returns:
        a client number that uniquely identifies this client (this subscription)

      • unsubscribe

        public void unsubscribe​(int clientNumber)
        Removes a client from the list of subscribed clients. The call does not wait for the daemon to finish the work it is currently performing. Therefore, the client must tolerate that its performWork() method could be invoked even after the call to unsubscribe() has returned (but not more than once).
        Specified by:
        unsubscribe in interface DaemonService
        Parameters:
        clientNumber - client identifier

      • serviceNow

        public void serviceNow​(int clientNumber)
        Description copied from interface: DaemonService
        Service this subscription ASAP. When this method is called, the subscriber's performWork() method is guaranteed to be invoked at some point in the future. However, there is no guarantee that a subscriber's performWork() is called the same number of times as the subscriber calls this method. More precisely, if a subscriber is waiting for this daemon service to invoke its performWork() method, the daemon service may, but is not required to, ignore requests from that subscriber until the performWork() method has been invoked.
        Specified by:
        serviceNow in interface DaemonService
        Parameters:
        clientNumber - the number that uniquely identifies the client

      • enqueue

        public boolean enqueue​(Serviceable newClient,
                       boolean serviceNow)
        Description copied from interface: DaemonService
        Request a one time service from the Daemon. Unless performWork returns REQUEUE (see Serviceable), the daemon will service this client once and then it will get rid of this client. Since no client number is associated with this client, it cannot request to be serviced or be unsubscribed. The work is always added to the deamon, regardless of the state it returns.
        Specified by:
        enqueue in interface DaemonService
        Parameters:
        newClient - the object that needs a one time service
        serviceNow - if true, this client should be serviced ASAP, as if a serviceNow has been issued. If false, then this client will be serviced with the normal scheduled.
        Returns:
        true if the daemon indicates it is being overloaded, false it's happy.

      • clear

        public void clear()
        Get rid of all queued up Serviceable tasks.
        Specified by:
        clear in interface DaemonService

      • nextAssignment

        protected ServiceRecord nextAssignment​(boolean urgent)

      • serviceClient

        protected void serviceClient​(ServiceRecord clientRecord)

      • run

        public void run()
        Specified by:
        run in interface java.lang.Runnable

      • pause

        public void pause()
        Description copied from interface: DaemonService
        Pause. No new service is performed until a resume is issued.
        Specified by:
        pause in interface DaemonService

      • resume

        public void resume()
        Description copied from interface: DaemonService
        Resume service after a pause
        Specified by:
        resume in interface DaemonService

      • stop

        public void stop()
        Finish what we are doing and at the next convenient moment, get rid of the thread and make the daemon object goes away if possible. remember we are calling from another thread
        Specified by:
        stop in interface DaemonService

      • stopRequested

        private boolean stopRequested()

      • inPause

        private boolean inPause()

      • wakeUp

        protected void wakeUp()

      • rest

        private boolean rest()
        Returns true if awakened by some notification, false if wake up by timer

      • work

        private void work​(boolean urgentOnly)

      • yield

        private void yield()

      • getMonitor

        static ModuleFactory getMonitor()
        Privileged Monitor lookup. Must be package private so that user code can't call this entry point.