/*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.jctools_voltpatches.queues;
import java.util.Queue;
/**
* This is a tagging interface for the queues in this library which implement a subset of the {@link Queue}
* interface sufficient for concurrent message passing.<br>
* Message passing queues provide happens before semantics to messages passed through, namely that writes made
* by the producer before offering the message are visible to the consuming thread after the message has been
* polled out of the queue.
*
* @author nitsanw
*
* @param <T>
* the event/message type
*/
public interface MessagePassingQueue<T> {
int UNBOUNDED_CAPACITY = -1;
interface Supplier<T> {
/**
* This method will return the next value to be written to the queue. As such the queue
* implementations are commited to insert the value once the call is made.
* <p>
* Users should be aware that underlying queue implementations may upfront claim parts of the queue
* for batch operations and this will effect the view on the queue from the supplier method. In
* particular size and any offer methods may take the view that the full batch has already happened.
*
* @return new element, NEVER null
*/
T get();
}
interface Consumer<T> {
/**
* This method will process an element already removed from the queue. This method is expected to
* never throw an exception.
* <p>
* Users should be aware that underlying queue implementations may upfront claim parts of the queue
* for batch operations and this will effect the view on the queue from the accept method. In
* particular size and any poll/peek methods may take the view that the full batch has already
* happened.
*
* @param e not null
*/
void accept(T e);
}
interface WaitStrategy {
/**
* This method can implement static or dynamic backoff. Dynamic backoff will rely on the counter for
* estimating how long the caller has been idling. The expected usage is:
*
* <pre>
* <code>
* int ic = 0;
* while(true) {
* if(!isGodotArrived()) {
* ic = w.idle(ic);
* continue;
* }
* ic = 0;
* // party with Godot until he goes again
* }
* </code>
* </pre>
*
* @param idleCounter idle calls counter, managed by the idle method until reset
* @return new counter value to be used on subsequent idle cycle
*/
int idle(int idleCounter);
}
interface ExitCondition {
/**
* This method should be implemented such that the flag read or determination cannot be hoisted out of
* a loop which notmally means a volatile load, but with JDK9 VarHandles may mean getOpaque.
*
* @return true as long as we should keep running
*/
boolean keepRunning();
}
/**
* Called from a producer thread subject to the restrictions appropriate to the implementation and
* according to the {@link Queue#offer(Object)} interface.
*
* @param e not null, will throw NPE if it is
* @return true if element was inserted into the queue, false iff full
*/
boolean offer(T e);
/**
* Called from the consumer thread subject to the restrictions appropriate to the implementation and
* according to the {@link Queue#poll()} interface.
*
* @return a message from the queue if one is available, null iff empty
*/
T poll();
/**
* Called from the consumer thread subject to the restrictions appropriate to the implementation and
* according to the {@link Queue#peek()} interface.
*
* @return a message from the queue if one is available, null iff empty
*/
T peek();
/**
* This method's accuracy is subject to concurrent modifications happening as the size is estimated and as
* such is a best effort rather than absolute value. For some implementations this method may be O(n)
* rather than O(1).
*
* @return number of messages in the queue, between 0 and {@link Integer#MAX_VALUE} but less or equals to
* capacity (if bounded).
*/
int size();
/**
* Removes all items from the queue. Called from the consumer thread subject to the restrictions
* appropriate to the implementation and according to the {@link Queue#clear()} interface.
*/
void clear();
/**
* This method's accuracy is subject to concurrent modifications happening as the observation is carried
* out.
*
* @return true if empty, false otherwise
*/
boolean isEmpty();
/**
* @return the capacity of this queue or UNBOUNDED_CAPACITY if not bounded
*/
int capacity();
/**
* Called from a producer thread subject to the restrictions appropriate to the implementation. As opposed
* to {@link Queue#offer(Object)} this method may return false without the queue being full.
*
* @param e not null, will throw NPE if it is
* @return true if element was inserted into the queue, false if unable to offer
*/
boolean relaxedOffer(T e);
/**
* Called from the consumer thread subject to the restrictions appropriate to the implementation. As
* opposed to {@link Queue#poll()} this method may return null without the queue being empty.
*
* @return a message from the queue if one is available, null if unable to poll
*/
T relaxedPoll();
/**
* Called from the consumer thread subject to the restrictions appropriate to the implementation. As
* opposed to {@link Queue#peek()} this method may return null without the queue being empty.
*
* @return a message from the queue if one is available, null if unable to peek
*/
T relaxedPeek();
/**
* Remove all available item from the queue and hand to consume. This should be semantically similar to:
* <code><br/>
* M m;</br>
* while((m = relaxedPoll()) != null){</br>
* c.accept(m);</br>
* }</br>
* </code> There's no strong commitment to the queue being empty at the end of a drain. Called from a
* consumer thread subject to the restrictions appropriate to the implementation.
*
* @return the number of polled elements
*/
int drain(Consumer<T> c);
/**
* Stuff the queue with elements from the supplier. Semantically similar to: <code><br/>
* while(relaxedOffer(s.get());</br>
* </code> There's no strong commitment to the queue being full at the end of a fill. Called from a
* producer thread subject to the restrictions appropriate to the implementation.
*
* @return the number of offered elements
*/
int fill(Supplier<T> s);
/**
* Remove up to <i>limit</i> elements from the queue and hand to consume. This should be semantically
* similar to:
*
* <pre>
* <code>
* M m;
* while((m = relaxedPoll()) != null){
* c.accept(m);
* }
* </code>
* </pre>
*
* There's no strong commitment to the queue being empty at the end of a drain. Called from a consumer
* thread subject to the restrictions appropriate to the implementation.
*
* @return the number of polled elements
*/
int drain(Consumer<T> c, int limit);
/**
* Stuff the queue with up to <i>limit</i> elements from the supplier. Semantically similar to:
*
* <pre>
* <code>
* for(int i=0; i < limit && relaxedOffer(s.get(); i++);</br>
* </code>
* </pre>
*
* There's no strong commitment to the queue being full at the end of a fill. Called from a producer
* thread subject to the restrictions appropriate to the implementation.
*
* @return the number of offered elements
*/
int fill(Supplier<T> s, int limit);
/**
* Remove elements from the queue and hand to consume forever. Semantically similar to:
*
* <pre>
* <code>
* int idleCounter = 0;
* while (exit.keepRunning()) {
* E e = relaxedPoll();
* if(e==null){
* idleCounter = wait.idle(idleCounter);
* continue;
* }
* idleCounter = 0;
* c.accept(e);
* }
* </code>
* </pre>
*
* Called from a consumer thread subject to the restrictions appropriate to the implementation.
*
*/
void drain(Consumer<T> c, WaitStrategy wait, ExitCondition exit);
/**
* Stuff the queue with elements from the supplier forever. Semantically similar to:
*
* <pre>
* <code>
* int idleCounter = 0;
* while (exit.keepRunning()) {
* E e = s.get();
* while (!relaxedOffer(e)) {
* idleCounter = wait.idle(idleCounter);
* continue;
* }
* idleCounter = 0;
* }
* </code>
* </pre>
*
* Called from a producer thread subject to the restrictions appropriate to the implementation.
*
*/
void fill(Supplier<T> s, WaitStrategy wait, ExitCondition exit);
}