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Introduce
WeightedRandomDistributionSelector (xDS-endpoint pt 1) (#…
…5501) Motivation: `EdfLoadBalancer` is a basic load balancer which distributes entries based on weight using [Earliest Deadline First Scheduling](https://en.wikipedia.org/wiki/Earliest_deadline_first_scheduling). ref: https://github.com/envoyproxy/envoy/blob/b7818b0df716af47ec22982c5a1cbbace5f2ae15/source/common/upstream/load_balancer_impl.h#L508 This is used not only for request load balancing, but also load balancing based on `Locality` ref: https://github.com/envoyproxy/envoy/blob/b7818b0df716af47ec22982c5a1cbbace5f2ae15/source/common/upstream/upstream_impl.cc#L625 Within our codebase, it seems like we could use `WeightedRandomDistributionEndpointSelector` easily for this purpose. I propose that we extract a `WeightedRandomDistributionSelector` and make it publicly available. See the following for a sample use-case in #5450 ref: https://github.com/line/armeria/blob/bd4968fda63089b2c309583b21201dff1a1119bf/xds/src/main/java/com/linecorp/armeria/xds/client/endpoint/HostSet.java#L38-L39 POC: #5450 Modifications: - Extract `WeightedRandomDistributionSelector` and move it to the `internal` package. Result: - `WeightedRandomDistributionSelector` is now ready to be reused. <!-- Visit this URL to learn more about how to write a pull request description: https://armeria.dev/community/developer-guide#how-to-write-pull-request-description -->
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...ava/com/linecorp/armeria/internal/client/endpoint/WeightedRandomDistributionSelector.java
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| /* | ||
| * Copyright 2020 LINE Corporation | ||
| * | ||
| * LINE Corporation 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: | ||
| * | ||
| * https://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.linecorp.armeria.internal.client.endpoint; | ||
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| import java.util.ArrayList; | ||
| import java.util.Iterator; | ||
| import java.util.List; | ||
| import java.util.concurrent.ThreadLocalRandom; | ||
| import java.util.concurrent.locks.ReentrantLock; | ||
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| import com.google.common.annotations.VisibleForTesting; | ||
| import com.google.common.collect.ImmutableList; | ||
| import com.google.errorprone.annotations.concurrent.GuardedBy; | ||
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| import com.linecorp.armeria.common.annotation.Nullable; | ||
| import com.linecorp.armeria.internal.client.endpoint.WeightedRandomDistributionSelector.AbstractEntry; | ||
| import com.linecorp.armeria.internal.common.util.ReentrantShortLock; | ||
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| /** | ||
| * This selector selects an {@link AbstractEntry} using random and the weight of the {@link AbstractEntry}. | ||
| * If there are A(weight 10), B(weight 4) and C(weight 6) {@link AbstractEntry}s, the chances that | ||
| * {@link AbstractEntry}s are selected are 10/20, 4/20 and 6/20, respectively. If {@link AbstractEntry} | ||
| * A is selected 10 times and B and C are not selected as much as their weight, then A is removed temporarily | ||
| * and the chances that B and C are selected are 4/10 and 6/10. | ||
| */ | ||
| public class WeightedRandomDistributionSelector<T extends AbstractEntry> { | ||
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| private final ReentrantLock lock = new ReentrantShortLock(); | ||
| private final List<T> allEntries; | ||
| @GuardedBy("lock") | ||
| private final List<T> currentEntries; | ||
| private final long total; | ||
| private long remaining; | ||
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| public WeightedRandomDistributionSelector(List<T> endpoints) { | ||
| final ImmutableList.Builder<T> builder = ImmutableList.builderWithExpectedSize(endpoints.size()); | ||
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| long total = 0; | ||
| for (T entry : endpoints) { | ||
| if (entry.weight() <= 0) { | ||
| continue; | ||
| } | ||
| builder.add(entry); | ||
| total += entry.weight(); | ||
| } | ||
| this.total = total; | ||
| remaining = total; | ||
| allEntries = builder.build(); | ||
| currentEntries = new ArrayList<>(allEntries); | ||
| } | ||
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| @VisibleForTesting | ||
| public List<T> entries() { | ||
| return allEntries; | ||
| } | ||
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| @Nullable | ||
| public T select() { | ||
| if (allEntries.isEmpty()) { | ||
| return null; | ||
| } | ||
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| final ThreadLocalRandom threadLocalRandom = ThreadLocalRandom.current(); | ||
| lock.lock(); | ||
| try { | ||
| long target = threadLocalRandom.nextLong(remaining); | ||
| final Iterator<T> it = currentEntries.iterator(); | ||
| while (it.hasNext()) { | ||
| final T entry = it.next(); | ||
| final int weight = entry.weight(); | ||
| target -= weight; | ||
| if (target < 0) { | ||
| entry.increment(); | ||
| if (entry.isFull()) { | ||
| it.remove(); | ||
| entry.reset(); | ||
| remaining -= weight; | ||
| if (remaining == 0) { | ||
| // As all entries are full, reset `currentEntries` and `remaining`. | ||
| currentEntries.addAll(allEntries); | ||
| remaining = total; | ||
| } else { | ||
| assert remaining > 0 : remaining; | ||
| } | ||
| } | ||
| return entry; | ||
| } | ||
| } | ||
| } finally { | ||
| lock.unlock(); | ||
| } | ||
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| // Since `allEntries` is not empty, should subselect one Endpoint from `allEntries`. | ||
| throw new Error("Should never reach here"); | ||
| } | ||
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| public abstract static class AbstractEntry { | ||
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| private int counter; | ||
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| public final void increment() { | ||
| assert counter < weight(); | ||
| counter++; | ||
| } | ||
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| public abstract int weight(); | ||
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| public final void reset() { | ||
| counter = 0; | ||
| } | ||
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| public final int counter() { | ||
| return counter; | ||
| } | ||
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| public final boolean isFull() { | ||
| return counter >= weight(); | ||
| } | ||
| } | ||
| } |