Class BasicDaemon

java.lang.Object
org.apache.derby.impl.services.daemon.BasicDaemon
All Implemented Interfaces:
Runnable, DaemonService
public class BasicDaemon extends Object implements DaemonService, 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 Details

    • numClients

      private int numClients

    • OPTIMAL_QUEUE_SIZE

      private static final int OPTIMAL_QUEUE_SIZE
      See Also:

    • subscription

      private final Vector<ServiceRecord> subscription

    • contextService

      protected final ContextService contextService

    • contextMgr

      protected final ContextManager contextMgr

    • highPQ

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

    • normPQ

      private final List<ServiceRecord> normPQ

    • 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 Details

    • BasicDaemon

      public BasicDaemon(ContextService contextService)
      make a BasicDaemon

  • Method Details

    • 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 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

    • waitUntilQueueIsEmpty

      public void waitUntilQueueIsEmpty()
      Specified by:
      waitUntilQueueIsEmpty 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()

    • setThreadPriority

      private static void setThreadPriority(ModuleFactory mf, int priority)
      Change the priority of the current thread, but only if it was created by ModuleFactory.getDaemonThread(Runnable, String, boolean).

    • getMonitor

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