yandrey321 commented on code in PR #11719:
URL: https://github.com/apache/ozone/pull/11719#discussion_r4379053355


##########
hadoop-hdds/client/src/main/java/org/apache/hadoop/hdds/scm/XceiverClientMetrics.java:
##########
@@ -79,13 +94,67 @@ public void init() {
     }
   }
 
+  /**
+   * Set by each {@link XceiverClientCreator} from its configuration; read 
when the metrics are created.
+   */
+  public static void setMetricsMode(MetricsMode mode) {
+    metricsMode = mode;
+  }
+
   public static XceiverClientMetrics create() {
-    DefaultMetricsSystem.initialize(SOURCE_NAME);
+    if (startsMetricsSystem(metricsMode, classLoader())) {
+      DefaultMetricsSystem.initialize(SOURCE_NAME);
+    }
+    // Registering is cheap without a started metrics system: the source goes 
live if the system starts later.
     MetricsSystem ms = DefaultMetricsSystem.instance();
     return ms.register(SOURCE_NAME, "Storage Container Client Metrics",
         new XceiverClientMetrics());
   }
 
+  static boolean startsMetricsSystem(MetricsMode mode, ClassLoader loader) {
+    switch (mode) {
+    case ALWAYS:
+      return true;
+    case NEVER:
+      return false;
+    default:
+      return isSinkConfigured(SOURCE_NAME, loader);
+    }
+  }
+
+  /**
+   * Whether the metrics system would configure a sink for the prefix. Same 
lookup as Hadoop's MetricsConfig:
+   * the first of hadoop-metrics2-[prefix].properties and 
hadoop-metrics2.properties found on the classpath, with
+   * [prefix].sink.[instance].[option] keys; "*.sink" keys only supply 
defaults. Uses java.util.Properties to avoid
+   * loading Commons Configuration.
+   */
+  static boolean isSinkConfigured(String prefix, ClassLoader loader) {
+    final String lowerPrefix = StringUtils.toLowerCase(prefix);
+    for (String name : new String[] {"hadoop-metrics2-" + lowerPrefix + 
".properties", "hadoop-metrics2.properties"}) {
+      final URL url = loader.getResource(name);
+      if (url == null) {
+        continue;
+      }
+      final Properties properties = new Properties();
+      try (InputStream in = url.openStream()) {
+        properties.load(in);

Review Comment:
   Fixed conservatively. If the file AUTO finds contains an include or 
includeoptional directive (case-insensitive, as Commons Configuration matches 
it), the client now starts the metrics system as before, since the included 
files may configure the sink. I didn't resolve the includes: MetricsConfig 
splits the value on commas, interpolates it and resolves it relative to the 
including file, which would mean reimplementing part of Commons Configuration 
for a rare setup. TestXceiverClientMetricsMode covers both directives, mixed 
case, and an include in either file.



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