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From ctubbsii <...@git.apache.org>
Subject [GitHub] accumulo pull request #223: ACCUMULO-4409 Create AccumuloMonitorAppender
Date Thu, 02 Mar 2017 22:59:13 GMT
Github user ctubbsii commented on a diff in the pull request:

    https://github.com/apache/accumulo/pull/223#discussion_r104052130
  
    --- Diff: server/monitor/src/main/java/org/apache/accumulo/monitor/util/AccumuloMonitorAppender.java
---
    @@ -0,0 +1,153 @@
    +/*
    + * 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.accumulo.monitor.util;
    +
    +import static java.nio.charset.StandardCharsets.UTF_8;
    +
    +import java.util.concurrent.Executors;
    +import java.util.concurrent.ScheduledExecutorService;
    +import java.util.concurrent.TimeUnit;
    +import java.util.concurrent.atomic.AtomicBoolean;
    +
    +import org.apache.accumulo.core.Constants;
    +import org.apache.accumulo.core.client.Instance;
    +import org.apache.accumulo.core.conf.Property;
    +import org.apache.accumulo.core.zookeeper.ZooUtil;
    +import org.apache.accumulo.fate.zookeeper.ZooCache;
    +import org.apache.accumulo.fate.zookeeper.ZooCacheFactory;
    +import org.apache.accumulo.server.client.HdfsZooInstance;
    +import org.apache.log4j.AsyncAppender;
    +import org.apache.log4j.net.SocketAppender;
    +import org.apache.zookeeper.data.Stat;
    +
    +import com.google.common.net.HostAndPort;
    +
    +public class AccumuloMonitorAppender extends AsyncAppender {
    +
    +  private final ScheduledExecutorService executorService;
    +  private final AtomicBoolean trackerScheduled;
    +
    +  /**
    +   * A Log4j Appender which follows the registered location of the active Accumulo monitor
service, and forwards log messages to it
    +   */
    +  public AccumuloMonitorAppender() {
    +    // create the background thread to watch for updates to monitor location
    +    trackerScheduled = new AtomicBoolean(false);
    +    executorService = Executors.newSingleThreadScheduledExecutor(runnable -> {
    +      Thread t = new Thread(runnable, AccumuloMonitorAppender.class.getSimpleName() +
" Location Tracker");
    +      t.setDaemon(true);
    +      return t;
    +    });
    +  }
    +
    +  @Override
    +  public void activateOptions() {
    +    // only schedule it once (in case options get activated more than once); not sure
if this is possible
    +    if (trackerScheduled.compareAndSet(false, true)) {
    +      // wait 5 seconds, then run every 5 seconds
    +      executorService.scheduleAtFixedRate(new MonitorTracker(), 5, 5, TimeUnit.SECONDS);
    +    }
    +    super.activateOptions();
    +  }
    +
    +  @Override
    +  public void close() {
    +    if (!executorService.isShutdown()) {
    +      executorService.shutdownNow();
    +    }
    +    super.close();
    +  }
    +
    +  private class MonitorTracker implements Runnable {
    +
    +    private String path;
    +    private ZooCache zooCache;
    +
    +    private long lastModifiedTransactionId;
    +    private SocketAppender lastSocketAppender;
    +
    +    public MonitorTracker() {
    +
    +      // path and zooCache are lazily set the first time this tracker is run
    +      // this allows the tracker to be constructed and scheduled during log4j initialization
without
    +      // triggering any actual logs from the Accumulo or ZooKeeper code
    +      this.path = null;
    +      this.zooCache = null;
    +
    +      this.lastModifiedTransactionId = 0;
    +      this.lastSocketAppender = null;
    +    }
    +
    +    @Override
    +    public void run() {
    +      try {
    +        // lazily set up path and zooCache (see comment in constructor)
    +        if (this.zooCache == null) {
    +          Instance instance = HdfsZooInstance.getInstance();
    +          this.path = ZooUtil.getRoot(instance) + Constants.ZMONITOR_LOG4J_ADDR;
    +          this.zooCache = new ZooCacheFactory().getZooCache(instance.getZooKeepers(),
instance.getZooKeepersSessionTimeOut());
    +        }
    +
    +        // get the current location from the cache and update if necessary
    +        Stat stat = new Stat();
    +        byte[] loc = zooCache.get(path, stat);
    +        long modifiedTransactionId = stat.getMzxid();
    +
    +        // modifiedTransactionId will be 0 if no current location
    +        // lastModifiedTransactionId will be 0 if we've never seen a location
    +        if (modifiedTransactionId != lastModifiedTransactionId) {
    +          // replace old socket on every change, even if new location is the same as
old location
    +          // if modifiedTransactionId changed, then the monitor restarted and the old
socket is dead now
    +          switchAppender(loc, modifiedTransactionId);
    +        }
    +      } catch (Exception e) {
    +        // dump any non-fatal problems to the console, but let it run again
    +        e.printStackTrace();
    +      }
    +    }
    +
    +    private void switchAppender(byte[] newLocation, long newModifiedTransactionId) {
    +      // remove and close the last one, if it was non-null
    +      if (lastSocketAppender != null) {
    --- End diff --
    
    It won't be called multiple times with null. If `newLocation` is null, it means the ZK
node didn't exist, and the modified transaction ID will be 0. So the code outside guards against,
because the last modified transaction id is updated to 0 the first time `newLocation` is null.
    
    However, even if it were called multiple times, both the remove of the last appender and
the close are both NOOPs if it has already been removed and closed.


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