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

Class InterProcessSemaphoreV2

java.lang.Object
org.apache.curator.framework.recipes.locks.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 Details

    • LOCK_SCHEMA

      public static final Set<String> LOCK_SCHEMA

  • Constructor Details

    • 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 Details

    • 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.