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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