org.apache.excalibur.event
Interface Sink

All Known Subinterfaces:

Queue

All Known Implementing Classes:

AbstractQueue, LossyMultiCastSink, MultiCastSink

public interface Sink

A Sink implements the end of a finite-length event queue where elements are enqueued. These operations can throw a SinkException if the sink is closed or becomes full, allowing event queues to support thresholding and backpressure.

The interface design is heavily influenced by Matt Welsh's SandStorm server, his demonstration of the SEDA architecture. We have deviated where we felt the design differences where better.

Author:

Avalon Development Team

Field Summary

static java.lang.String

ROLE

Method Summary

int	canAccept() Deprecated. Use the EnqueuePredicate to control this instead.
void	enqueue(java.lang.Object element) Enqueues the given element onto the Sink.
void	enqueue(java.lang.Object[] elements) Given an array of elements, atomically enqueues all of the elements in the array.
boolean	isFull() Deprecated. Use the EnqueuePredicate to control this instead
int	maxSize() Deprecated. Use the EnqueuePredicate to control this instead.
PreparedEnqueue	prepareEnqueue(java.lang.Object[] elements) Support for transactional enqueue.
int	size() Returns the number of elements waiting in this Sink.
boolean	tryEnqueue(java.lang.Object element) Tries to enqueue an event, but instead of throwing exceptions, it returns a boolean value of whether the attempt was successful.

Field Detail

ROLE

public static final java.lang.String ROLE

Method Detail

enqueue

public void enqueue(java.lang.Object element)
             throws SinkException

Enqueues the given element onto the Sink.

Parameters:

element - The elements to enqueue

Throws:

SinkFullException - Indicates that the sink is temporarily full.

SinkClosedException - Indicates that the sink is no longer being serviced.

SinkException

enqueue

public void enqueue(java.lang.Object[] elements)
             throws SinkException

Given an array of elements, atomically enqueues all of the elements in the array. This guarantees that no other thread can interleave its own elements with those being inserted from this array. The implementation must enqueue all of the elements or none of them; if a SinkFullException or SinkClosedException is thrown, none of the elements will have been enqueued.

Parameters:

elements - The element array to enqueue

Throws:

SinkFullException - Indicates that the sink is temporarily full.

SinkClosedException - Indicates that the sink is no longer being serviced.

SinkException

tryEnqueue

public boolean tryEnqueue(java.lang.Object element)

Tries to enqueue an event, but instead of throwing exceptions, it returns a boolean value of whether the attempt was successful.

Parameters:

element - The element to attempt to enqueue

Returns:

true if successful, false if not.

prepareEnqueue

public PreparedEnqueue prepareEnqueue(java.lang.Object[] elements)
                               throws SinkException

Support for transactional enqueue.

This method allows a client to provisionally enqueue a number of elements onto the queue, and then later commit the enqueue (with a commitEnqueue call), or abort (with an abortEnqueue call). This mechanism can be used to perform "split-phase" enqueues, where a client first enqueues a set of elements on the queue and then performs some work to "fill in" those elements before performing a commit. This can also be used to perform multi-queue transactional enqueue operations, with an "all-or-nothing" strategy for enqueueing events on multiple Sinks.

This method would generally be used in the following manner:

   PreparedEnqueue enqueue = sink.prepareEnqueue(someElements);
   if (canCommit) {
     enqueue.commit();
   } else {
     enqueue.abort();
   }
 

Note that this method does not protect against "dangling prepares" -- that is, a prepare without an associated commit or abort operation. This method should be used with care. In particular, be sure that all code paths (such as exceptions) after a prepare include either a commit or an abort.

Parameters:

elements - The element array to provisionally enqueue

Returns:

A PreparedEnqueue that may be used to commit or abort the provisional enqueue

Throws:

SinkFullException - Indicates that the sink is temporarily full and that the requested elements could not be provisionally enqueued.

SinkClosedException - Indicates that the sink is no longer being serviced.

SinkException

See Also:

PreparedEnqueue

maxSize

public int maxSize()

Deprecated. Use the EnqueuePredicate to control this instead.

Returns the length threshold of the sink. This is for informational purposes only; an implementation may allow more (or fewer) new entries to be enqueued than maxSize() - size(). This may be the case, for example, if the sink implements some form of dynamic thresholding, and does not always accurately report maxSize().

Returns:

-1 if the sink has no length threshold.

isFull

public boolean isFull()

Deprecated. Use the EnqueuePredicate to control this instead

Returns true if this sink has reached its threshold; false otherwise. Like maxSize(), this is also informational, and isFull() returning false does not guarantee that future enqueue operations will succeed. Clearly, isFull() returning true does not guarantee that they will fail, since the Sink may be serviced in the meantime.

Returns:

true if the Sink is full

canAccept

public int canAccept()

Deprecated. Use the EnqueuePredicate to control this instead.

Returns the number of elements it can currently accept. This is typically the difference between size() and maxSize(). It will return -1 if the sink is unbounded.

Returns:

the number of elements the Sink can accept

size

public int size()

Returns the number of elements waiting in this Sink.

Important: The contract for this method was updated to account for any elements that were prepared for enqueueing. It provides a more predictable and consistent environment, as well as making it easier for EnqueuePredicates to account for those elements.

Returns:

the number of elements in the Sink