[GitHub] [phoenix] stoty commented on a diff in pull request #1431: PHOENIX-6699 Phoenix metrics overwriting DefaultMetricsSystem in Regi…

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stoty commented on code in PR #1431:
URL: https://github.com/apache/phoenix/pull/1431#discussion_r861979379


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phoenix-core/src/main/java/org/apache/phoenix/monitoring/MetricUtil.java:
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@@ -38,4 +46,22 @@ public static MetricsStopWatch getMetricsStopWatch(boolean isRequestMetricsEnabl
         return new MetricsStopWatch(true);
     }
 
+    // We need to cover the case when JmxCacheBuster has just stopped the HBase metrics
+    // system, and not accidentally overwrite the DefaultMetricsSystem singleton.
+    // See PHOENIX-6699
+    public static boolean isDefaultMetricsInitialized() {
+        try {
+            MetricsSystemImpl metrics = (MetricsSystemImpl) DefaultMetricsSystem.instance();
+            Field prefixField = MetricsSystemImpl.class.getDeclaredField("prefix");

Review Comment:
   _MetricsSystemImpl.monitoring_ is not useful for this purpose, as it is set to false when the metrics system is stopped.
   This is why we can overwrite the stopped Default Metrics System:
   https://github.com/apache/hadoop/blob/7bd7725532fd139d2e0e1662df7700f7ab95067a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/impl/MetricsSystemImpl.java#L156
   
   _MetricsSystemImpl.prefix_ is only written by the init() method, it is not touched otherwise, so it can be used to check for the codition when the MetricsSystemImpl is initialized, but in a stopped state:
   https://github.com/apache/hadoop/blob/7bd7725532fd139d2e0e1662df7700f7ab95067a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/impl/MetricsSystemImpl.java#L154
   
   I don't think we have to worry about a race around prefix, AFAICT by the time the Phoenix classes are loaded, HBase has long initialized DefaultMetrics, and prefix has been set.
   



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