ZoneFallbackIterator.java

/**
 * Copyright 2013 Netflix, Inc.
 *
 * 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.netflix.evcache.util;

import java.util.Iterator;
import java.util.Set;

/**
 * A Zone Based fallback circular iterator. This ensures that during a fallback
 * scenario the requests are spread out across all zones evenly.
 * 
 * @author smadappa
 */
public class ZoneFallbackIterator {
    private Entry<String> entry;
    private int size = 0;

    /**
     * Creates an instance of ZoneFallbackIterator given all the zones.
     * 
     * @param allZones
     *            Set of all available zones.
     */
    public ZoneFallbackIterator(Set<String> allZones) {
        if (allZones == null || allZones.size() == 0) return;
        Entry<String> pEntry = null;
        for (Iterator<String> itr = allZones.iterator(); itr.hasNext();) {
            size++;
            final String zone = itr.next();
            final Entry<String> newEntry = new Entry<String>(zone, pEntry);
            if (entry == null) entry = newEntry;
            pEntry = newEntry;
        }

        /*
         * Connect the first and the last entry to form a circular list
         */
        if (pEntry != null) {
            entry.next = pEntry;
        }
    }

    /**
     * Returns the next zone from the set which should get the request.
     * 
     * @return - the next zone in the iterator. If there are none then null is
     *         returned.
     */
    public String next() {
        if (entry == null) return null;
        entry = entry.next;
        return entry.element;
    }

    /**
     * Returns the next zone from the set excluding the given zone which should
     * get the request.
     * 
     * @return - the next zone in the iterator. If there are none then null is
     *         returned.
     */
    public String next(String ignoreZone) {
        if (entry == null) return null;
        entry = entry.next;
        if (entry.element.equals(ignoreZone)) {
            return entry.next.element;
        } else {
            return entry.element;
        }
    }

    public int getSize() {
        return size;
    }

    /**
     * The Entry keeps track of the current element and next element in the
     * list.
     * 
     * @author smadappa
     *
     * @param <E>
     */
    static class Entry<E> {
        private E element;
        private Entry<E> next;

        /**
         * Creates an instance of Entry.
         */
        Entry(E element, Entry<E> next) {
            this.element = element;
            this.next = next;
        }
    }
}