sashidhar commented on a change in pull request #8038: Making optimal usage of multiple segment cache locations URL: https://github.com/apache/incubator-druid/pull/8038#discussion_r328927992
########## File path: server/src/main/java/org/apache/druid/segment/loading/RoundRobinStorageLocationSelectorStrategy.java ########## @@ -0,0 +1,48 @@ +/* + * 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.druid.segment.loading; + +import com.google.common.collect.ImmutableList; +import com.google.common.collect.Iterators; +import org.apache.druid.timeline.DataSegment; + +import java.util.Iterator; + +/** + * A {@link StorageLocation} selector strategy that selects a segment cache location in a round-robin fashion each time + * among the available storage locations. + */ +public class RoundRobinStorageLocationSelectorStrategy implements StorageLocationSelectorStrategy +{ + private Iterator<StorageLocation> cyclicIterator; + + @Override + public void setStorageLocations(ImmutableList<StorageLocation> storageLocations) + { + // cyclicIterator remembers the marker internally + cyclicIterator = Iterators.cycle(storageLocations); + } + + @Override + public Iterator<StorageLocation> getLocations(DataSegment dataSegment, String storageDirStr) + { + return cyclicIterator; Review comment: I've a test case (single thread sequential calls to the iterator) where the round robin iteration becomes incorrect with or without the fix in point 2. The fix I think is to use a single variable alone `startIndex`, `i` may not be needed. A dry run of the test case testRoundRobinLocationSelectorStrategy() with calls to next() gives below indices. The locations are not being iterated in proper round robin fashion. > i=0, startIndex=1 > i=1, startIndex=1 > i=2, startIndex=1 > i=1, startIndex=2 > i=2, startIndex=2 > i=0, startIndex=2 > i=2, startIndex=0 > i=0, startIndex=0 > i=1, startIndex=0 > i=0, startIndex=1 > i=1, startIndex=1 > i=2, startIndex=1 > i=1, startIndex=2 > i=2, startIndex=2 > i=0, startIndex=2 The code will look like, ``` public class RoundRobinStorageLocationSelectorStrategy implements StorageLocationSelectorStrategy { private final List<StorageLocation> storageLocations; private final AtomicInteger startIndex = new AtomicInteger(0); public RoundRobinStorageLocationSelectorStrategy(List<StorageLocation> storageLocations) { this.storageLocations = storageLocations; } @Override public Iterator<StorageLocation> getLocations() { return new Iterator<StorageLocation>() { private final int numStorageLocations = storageLocations.size(); private int remainingIterations = numStorageLocations; @Override public boolean hasNext() { return remainingIterations > 0; } @Override public StorageLocation next() { if (!hasNext()) { throw new NoSuchElementException(); } remainingIterations--; final StorageLocation nextLocation = storageLocations.get(startIndex.getAndUpdate(n -> (n + 1) % numStorageLocations)); return nextLocation; } }; } } ``` This should likely resolve the issues. I'm further testing this. ``` final StorageLocation nextLocation = storageLocations.get(startIndex.getAndUpdate(n -> (n + 1) % numStorageLocations)); ``` ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: [email protected] With regards, Apache Git Services --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
