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/*
 * 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.jackrabbit.oak.segment;

import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Queue;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ConcurrentMap;
import java.util.function.Supplier;

import com.googlecode.concurrentlinkedhashmap.ConcurrentLinkedHashMap;
import org.apache.jackrabbit.oak.segment.file.tar.GCGeneration;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

/**
 * A simple tracker for the source of commits (writes) in
 * {@link SegmentNodeStore}. It provides two basic functionalities:
 * 
 * exposes the number of commits executed per thread
 * exposes the threads (and possibly their details - i.e., stack traces)
 * currently waiting on the commit semaphore
 * 
 * 
 * For the most part, this class delegates thread-safety to its underlying
 * state variables. However, the {@link #trackDequedCommitOf(Thread)} and
 * {@link #trackExecutedCommitOf(Thread)} method must be called in
 * sequence within the same transaction, because they are linked
 * via the {@link #currentCommit} field.
 */
class CommitsTracker {
    private final String[] threadGroups;
    private final int otherWritersLimit;
    private final ConcurrentMap queuedWritersMap;
    private final Queue commits = new ConcurrentLinkedQueue<>();

    /*
     * Read access via getCurrentWriter() happens usually on a separate thread, thus volatile
     */
    private volatile Commit currentCommit;

    static final class Commit {
        private final String threadName;
        private final WeakReference thread;
        private final Supplier gcGeneration;

        private long queued;
        private long dequeued;
        private long applied;

        Commit(Thread thread, Supplier gcGeneration) {
            this.threadName = thread.getName();
            this.gcGeneration = gcGeneration;
            this.thread = new WeakReference<>(thread);
        }

        @NotNull
        Commit queued() {
            queued = System.currentTimeMillis();
            return this;
        }

        @NotNull
        Commit dequeued() {
            dequeued = System.currentTimeMillis();
            return this;
        }

        @NotNull
        Commit applied() {
            applied = System.currentTimeMillis();
            return this;
        }

        @Nullable
        StackTraceElement[] getStackTrace() {
            Thread t = thread.get();
            return t == null
                ? null
                : t.getStackTrace();
        }

        @NotNull
        String getThreadName() {
            return threadName;
        }

        @Nullable
        GCGeneration getGCGeneration() {
            return gcGeneration.get();
        }

        long getQueued() {
            return queued;
        }

        long getDequeued() {
            return dequeued;
        }

        long getApplied() {
            return applied;
        }
    }

    CommitsTracker(String[] threadGroups, int otherWritersLimit) {
        this.threadGroups = threadGroups;
        this.otherWritersLimit = otherWritersLimit;
        this.queuedWritersMap = new ConcurrentHashMap<>();
    }

    public void trackQueuedCommitOf(Thread thread, Supplier gcGeneration) {
        queuedWritersMap.put(thread.getName(), new Commit(thread, gcGeneration).queued());
    }

    public void trackDequedCommitOf(Thread thread) {
        currentCommit = queuedWritersMap.remove(thread.getName());
        if (currentCommit != null) {
            currentCommit.dequeued();
        }
    }

    public void trackExecutedCommitOf(Thread thread) {
        long t = System.currentTimeMillis();
        final Iterator it = commits.iterator();

        // Purge the queue
        // Avoiding removeIf allows us to bail out early. See OAK-7885
        while (it.hasNext()) {
            if (it.next().getQueued() < t - 60000) {
                it.remove();
            } else {
                break;
            }
        }

        if (currentCommit != null) {
            currentCommit.applied();
            commits.offer(currentCommit);
            currentCommit = null;
        }
    }

    public Map getQueuedWritersMap() {
        return new HashMap<>(queuedWritersMap);
    }

    @Nullable
    public Commit getCurrentWriter() {
        return currentCommit;
    }

    private String findGroupFor(String thread) {
        if (threadGroups == null) {
            return "other";
        }

        for (String group : threadGroups) {
            if (thread.matches(group)) {
                return group;
            }
        }

        return "other";
    }

    public Map getCommitsCountPerGroupLastMinute() {
        Map commitsPerGroup = new HashMap<>();
        long t = System.currentTimeMillis() - 60000;
        for (Commit commit : commits) {
            if (commit.getQueued() > t) {
                String group = findGroupFor(commit.threadName);
                if (!"other".equals(group)) {
                    commitsPerGroup.compute(group, (k, v) -> v == null ? 1 : v + 1);
                }
            }
        }
        return commitsPerGroup;
    }

    public Map getCommitsCountOthers() {
        Map commitsOther = new ConcurrentLinkedHashMap.Builder()
                .maximumWeightedCapacity(otherWritersLimit).build();
        long t = System.currentTimeMillis() - 60000;
        for (Commit commit : commits) {
            if (commit.getQueued() > t) {
                String group = findGroupFor(commit.threadName);
                if ("other".equals(group)) {
                    commitsOther.compute(commit.threadName, (k, v) -> v == null ? 1 : v + 1);
                }
            }
        }
        return commitsOther;
    }
}