| /* |
| * Licensed to the Apache Software Foundation (ASF) under one |
| * or more contributor license agreements. See the NOTICE file |
| * distributed with this work for additional information |
| * regarding copyright ownership. The ASF licenses this file |
| * to you 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.apache.gossip.crdt; |
| |
| import org.apache.gossip.manager.Clock; |
| import org.apache.gossip.manager.SystemClock; |
| |
| import java.util.Arrays; |
| import java.util.HashMap; |
| import java.util.HashSet; |
| import java.util.Map; |
| import java.util.Set; |
| import java.util.function.Function; |
| import java.util.stream.Collectors; |
| import java.util.stream.Stream; |
| |
| /* |
| Last write wins CrdtSet |
| Each operation has timestamp: when you add or remove SystemClock is used to get current time in nanoseconds. |
| When all add/remove operations are within the only node LWWSet is guaranteed to work like a Set. |
| If you have multiple nodes with ideally synchronized clocks: |
| You will observe operations on all machines later than on the initiator, but the last operations on cluster will win. |
| If you have some significant clock drift you will suffer from data loss. |
| |
| Read more: https://github.com/aphyr/meangirls#lww-element-set |
| |
| You can view examples of usage in tests: |
| LwwSetTest - unit tests |
| DataTest - integration test with 2 nodes, LWWSet was serialized/deserialized, sent between nodes, merged |
| */ |
| |
| public class LwwSet<ElementType> implements CrdtAddRemoveSet<ElementType, Set<ElementType>, LwwSet<ElementType>> { |
| static private Clock clock = new SystemClock(); |
| |
| private final Map<ElementType, Timestamps> struct; |
| |
| static class Timestamps { |
| private final long latestAdd; |
| private final long latestRemove; |
| |
| Timestamps(){ |
| latestAdd = 0; |
| latestRemove = 0; |
| } |
| |
| Timestamps(long add, long remove){ |
| latestAdd = add; |
| latestRemove = remove; |
| } |
| |
| long getLatestAdd(){ |
| return latestAdd; |
| } |
| |
| long getLatestRemove(){ |
| return latestRemove; |
| } |
| |
| // consider element present when addTime >= removeTime, so we prefer add to remove |
| boolean isPresent(){ |
| return latestAdd >= latestRemove; |
| } |
| |
| Timestamps updateAdd(){ |
| return new Timestamps(clock.nanoTime(), latestRemove); |
| } |
| |
| Timestamps updateRemove(){ |
| return new Timestamps(latestAdd, clock.nanoTime()); |
| } |
| |
| Timestamps merge(Timestamps other){ |
| if (other == null){ |
| return this; |
| } |
| return new Timestamps(Math.max(latestAdd, other.latestAdd), Math.max(latestRemove, other.latestRemove)); |
| } |
| } |
| |
| |
| public LwwSet(){ |
| struct = new HashMap<>(); |
| } |
| |
| @SafeVarargs |
| public LwwSet(ElementType... elements){ |
| this(new HashSet<>(Arrays.asList(elements))); |
| } |
| |
| public LwwSet(Set<ElementType> set){ |
| struct = new HashMap<>(); |
| for (ElementType e : set){ |
| struct.put(e, new Timestamps().updateAdd()); |
| } |
| } |
| |
| public LwwSet(LwwSet<ElementType> first, LwwSet<ElementType> second){ |
| Function<ElementType, Timestamps> timestampsFor = p -> { |
| Timestamps firstTs = first.struct.get(p); |
| Timestamps secondTs = second.struct.get(p); |
| if (firstTs == null){ |
| return secondTs; |
| } |
| return firstTs.merge(secondTs); |
| }; |
| struct = Stream.concat(first.struct.keySet().stream(), second.struct.keySet().stream()) |
| .distinct().collect(Collectors.toMap(p -> p, timestampsFor)); |
| } |
| |
| public LwwSet<ElementType> add(ElementType e){ |
| return this.merge(new LwwSet<>(e)); |
| } |
| |
| // for serialization |
| LwwSet(Map<ElementType, Timestamps> struct){ |
| this.struct = struct; |
| } |
| |
| Map<ElementType, Timestamps> getStruct(){ |
| return struct; |
| } |
| |
| |
| public LwwSet<ElementType> remove(ElementType e){ |
| Timestamps eTimestamps = struct.get(e); |
| if (eTimestamps == null || !eTimestamps.isPresent()){ |
| return this; |
| } |
| Map<ElementType, Timestamps> changeMap = new HashMap<>(); |
| changeMap.put(e, eTimestamps.updateRemove()); |
| return this.merge(new LwwSet<>(changeMap)); |
| } |
| |
| @Override |
| public LwwSet<ElementType> merge(LwwSet<ElementType> other){ |
| return new LwwSet<>(this, other); |
| } |
| |
| @Override |
| public Set<ElementType> value(){ |
| return struct.entrySet().stream() |
| .filter(entry -> entry.getValue().isPresent()) |
| .map(Map.Entry::getKey) |
| .collect(Collectors.toSet()); |
| } |
| |
| @Override |
| public LwwSet<ElementType> optimize(){ |
| return this; |
| } |
| |
| @Override |
| public boolean equals(Object obj){ |
| return this == obj || (obj != null && getClass() == obj.getClass() && value().equals(((LwwSet) obj).value())); |
| } |
| } |