Package com.pushtechnology.diffusion.client.session

Interface Session.SessionLock

Enclosing interface:
Session
public static interface Session.SessionLock
A session lock is a server-managed resource that can be used to coordinate exclusive access to shared resources across sessions. For example, to ensure a single session has the right to update a topic; to ensure at most one session responds to an event; or to select a single session to perform a housekeeping task. Session locks support general collaborative locking schemes. The application architect is responsible for designing a suitable locking scheme and for ensuring each application component follows the scheme appropriately.

Session locks are identified by a lock name. Lock names are arbitrary and chosen at will to suit the application. Each lock is owned by at most one session. Locks are established on demand; there is no separate operation to create or destroy a lock.

A session lock is acquired using the Session.lock(String) method. If no other session owns the lock, the server will assign the lock to the calling session immediately. Otherwise, the server will record that the session is waiting to acquire the lock. A session can call lock more than once for a given session lock – if the lock is acquired, all calls will complete successfully with equal SessionLocks.

If a session closes, the session locks it owns are automatically released. A session can also release a lock. When a session lock is released and other sessions are waiting to acquire the lock, the server will arbitrarily select one of the waiting sessions and notify it that it has acquired the lock. All of the newly selected session's pending lock calls will complete normally. Other sessions will continue to wait.

The Session.lock(String, SessionLockScope) variant of this method takes a scope parameter that provides the further option of automatically releasing the lock when the session loses its connection to the server.

The acquisition life cycle of a session lock from the perspective of a session is shown in the following diagram.

Session Lock life cycle.

Differences to java.util.concurrent.locks.Lock

Unlike the Lock API, there is no association between a lock and a thread. If a session calls this method for a lock it already owns, the call will complete normally and immediately with a SessionLock that is equal to the one returned when the lock was originally acquired. A single call to unlock() will release this session's claim to a lock.

A further difference to java.util.concurrent.locks.Lock is that lock ownership can be lost due to an independent event such as loss of connection, and not only due to the use of the locking API by the owner. Consequently, the session should poll using isOwned() to check that it still owns the lock before accessing the protected resource.

Race conditions

This session lock API has inherent race conditions. Even if an application is coded correctly to protect a shared resource using session locks, there may be a period where two or more sessions concurrently access the resource. The races arise for several reasons including

  • due to the check-then-act approach of polling isOwned(), the lock can be lost after the check has succeeded but before the resource is accessed;
  • the server can detect a session is disconnected and assign the lock to another session before the original session has detected the disconnection.

Despite this imprecision, session locks provide a useful way to coordinate session actions.

Since:
6.1

  • Method Details

    • getName

      String getName()
      Returns:
      the name of the session lock

    • getSequence

      long getSequence()
      A value that identifies the acquisition of the lock with the given name. SessionLocks that are acquired later are guaranteed to have bigger sequence values, allowing the sequence number to be used as a fencing token.
      Returns:
      a value that identifies the acquisition of this lock

    • isOwned

      boolean isOwned()
      Test whether the session lock is still owned.
      Returns:
      true if the session lock is still owned by the session

    • getScope

      Session.SessionLockScope getScope()
      The scope of the lock.

      The scope determines when the lock will be released automatically.

      If a session makes multiple requests for a lock using different scopes, and the server assigns the lock to the session fulfilling the requests, the lock will be given the weakest scope (UNLOCK_ON_CONNECTION_LOSS). Consequently, an individual request can complete with a lock that has a different scope to that requested.

      Returns:
      the lock scope
      See Also:

    • unlock

      CompletableFuture<Boolean> unlock()
      Release a session lock, if owned.
      Returns:
      a CompletableFuture that completes when a response is received from the server.

      On completion, this session will no longer own the named session lock. If CompletableFuture completes normally, a true value indicates this session previously owned the lock and a false value indicates it did not.

      If the CompletableFuture completes exceptionally, this session does not own the session lock. Common reasons for failure, indicated by the exception reported as the cause, include:

      Since:
      6.1
      See Also: