Package org.apache.curator.framework.recipes.locks

Class InterProcessSemaphoreV2

  • public class InterProcessSemaphoreV2
extends Object

    A counting semaphore that works across JVMs. All processes in all JVMs that use the same lock path will achieve an inter-process limited set of leases. Further, this semaphore is mostly "fair" - each user will get a lease in the order requested (from ZK's point of view).

    There are two modes for determining the max leases for the semaphore. In the first mode the max leases is a convention maintained by the users of a given path. In the second mode a SharedCountReader is used as the method for semaphores of a given path to determine the max leases.

    If a SharedCountReader is not used, no internal checks are done to prevent Process A acting as if there are 10 leases and Process B acting as if there are 20. Therefore, make sure that all instances in all processes use the same numberOfLeases value.

    The various acquire methods return Lease objects that represent acquired leases. Clients must take care to close lease objects (ideally in a finally block) else the lease will be lost. However, if the client session drops (crash, etc.), any leases held by the client are automatically closed and made available to other clients.

    Thanks to Ben Bangert (ben@groovie.org) for the algorithm used.

    • Field Detail

      • LOCK_SCHEMA

        public static final Set<String> LOCK_SCHEMA

    • Constructor Detail

      • InterProcessSemaphoreV2

        public InterProcessSemaphoreV2​(CuratorFramework client,
                               String path,
                               int maxLeases)
        Parameters:
        client - the client
        path - path for the semaphore
        maxLeases - the max number of leases to allow for this instance

      • InterProcessSemaphoreV2

        public InterProcessSemaphoreV2​(CuratorFramework client,
                               String path,
                               SharedCountReader count)
        Parameters:
        client - the client
        path - path for the semaphore
        count - the shared count to use for the max leases

    • Method Detail

      • setNodeData

        public void setNodeData​(byte[] nodeData)
        Set the data to put for the node created by this semaphore. This must be called prior to calling one of the acquire() methods.
        Parameters:
        nodeData - node data

      • getParticipantNodes

        public Collection<String> getParticipantNodes()
                                       throws Exception
        Return a list of all current nodes participating in the semaphore
        Returns:
        list of nodes
        Throws:
        Exception - ZK errors, interruptions, etc.

      • returnAll

        public void returnAll​(Collection<Lease> leases)
        Convenience method. Closes all leases in the given collection of leases
        Parameters:
        leases - leases to close

      • returnLease

        public void returnLease​(Lease lease)
        Convenience method. Closes the lease
        Parameters:
        lease - lease to close

      • acquire

        public Lease acquire()
              throws Exception

        Acquire a lease. If no leases are available, this method blocks until either the maximum number of leases is increased or another client/process closes a lease.

        The client must close the lease when it is done with it. You should do this in a finally block.

        Returns:
        the new lease
        Throws:
        Exception - ZK errors, interruptions, etc.

      • acquire

        public Collection<Lease> acquire​(int qty)
                          throws Exception

        Acquire qty leases. If there are not enough leases available, this method blocks until either the maximum number of leases is increased enough or other clients/processes close enough leases.

        The client must close the leases when it is done with them. You should do this in a finally block. NOTE: You can use returnAll(Collection) for this.

        Parameters:
        qty - number of leases to acquire
        Returns:
        the new leases
        Throws:
        Exception - ZK errors, interruptions, etc.

      • acquire

        public Lease acquire​(long time,
                     TimeUnit unit)
              throws Exception

        Acquire a lease. If no leases are available, this method blocks until either the maximum number of leases is increased or another client/process closes a lease. However, this method will only block to a maximum of the time parameters given.

        The client must close the lease when it is done with it. You should do this in a finally block.

        Parameters:
        time - time to wait
        unit - time unit
        Returns:
        the new lease or null if time ran out
        Throws:
        Exception - ZK errors, interruptions, etc.

      • acquire

        public Collection<Lease> acquire​(int qty,
                                 long time,
                                 TimeUnit unit)
                          throws Exception

        Acquire qty leases. If there are not enough leases available, this method blocks until either the maximum number of leases is increased enough or other clients/processes close enough leases. However, this method will only block to a maximum of the time parameters given. If time expires before all leases are acquired, the subset of acquired leases are automatically closed.

        The client must close the leases when it is done with them. You should do this in a finally block. NOTE: You can use returnAll(Collection) for this.

        Parameters:
        qty - number of leases to acquire
        time - time to wait
        unit - time unit
        Returns:
        the new leases or null if time ran out
        Throws:
        Exception - ZK errors, interruptions, etc.