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/*
* Copyright 2015 Open Networking Laboratory
*
* 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.onosproject.store.service;
import java.util.concurrent.CompletableFuture;
/**
* A distributed collection designed for holding elements prior to processing.
* A queue provides insertion, extraction and inspection operations. The extraction operation
* is designed to be non-blocking.
*
* @param <E> queue entry type
*/
public interface DistributedQueue<E> {
/**
* Returns total number of entries in the queue.
* @return queue size
*/
long size();
/**
* Returns true if queue has elements in it.
* @return true is queue has elements, false otherwise
*/
default boolean isEmpty() {
return size() == 0;
}
/**
* Inserts an entry into the queue.
* @param entry entry to insert
*/
void push(E entry);
/**
* If the queue is non-empty, an entry will be removed from the queue and the returned future
* will be immediately completed with it. If queue is empty when this call is made, the returned
* future will be eventually completed when an entry is added to the queue.
* @return queue entry
*/
CompletableFuture<E> pop();
/**
* Returns an entry from the queue without removing it. If the queue is empty returns null.
* @return queue entry or null if queue is empty
*/
E peek();
}
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