TopicOverloadPolicy.java

/*
 * Copyright (c) 2008-2017, Hazelcast, Inc. All Rights Reserved.
 *
 * 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 com.hazelcast.topic;

/**
 * A policy to deal with an overloaded topic; so topic where there is no place to store new messages.
 *
 * This policy can only be used in combination with the {@link com.hazelcast.core.HazelcastInstance#getReliableTopic(String)}.
 *
 * The reliable topic uses a {@link com.hazelcast.ringbuffer.Ringbuffer} to store the messages. A ringbuffer doesn't track where
 * readers are, so therefore it has no concept of a slow consumers. This provides many advantages like high performance reads, but
 * it also gives the ability to the reader to reread the same message multiple times in case of error.
 *
 * Because a ringbuffer has a capacity and to prevent that a fast producer overwrites the messages needed by a slow consumer, a
 * time to live time can be set on the ringbuffer. See {@link com.hazelcast.config.RingbufferConfig#setTimeToLiveSeconds(int)}.
 * This {@link TopicOverloadPolicy} controls how the publisher is going to deal with the situation that a ringbuffer is full
 *  and the oldest item in the ringbuffer is not old enough to get overwritten.
 *
 * So keep in mind that a this retention period keep the messages in memory, even though it might by that all readers already
 * have completed reading.
 */
public enum TopicOverloadPolicy {

    /**
     * Using this policy, a message that has not expired can be overwritten. No matter the retention period set, the overwrite
     * will just overwrite the item.
     *
     * This can be a problem for slow consumers because they were promised a certain time window to process messages. But it will
     * benefit producers and fast consumers since they are able to continue. This policy sacrifices the slow producer in favor
     * of fast producers/consumers.
     */
    DISCARD_OLDEST,

    /**
     * The message that was to be published, is discarded.
     */
    DISCARD_NEWEST,

    /**
     * The caller will wait till there space in the ringbuffer.
     */
    BLOCK,

    /**
     * The publish call immediately fails.
     */
    ERROR
}