ZOOKEEPER-4324: dropwizard-metrics should be optional

pom.xml

@@ -574,6 +574,7 @@
     <commons-io.version>2.17.0</commons-io.version>
     <burningwave.mockdns.version>0.25.4</burningwave.mockdns.version>
     <dnsjava.version>3.5.1</dnsjava.version>
+    <archunit.version>1.4.2</archunit.version>
     <clover-maven-plugin.version>4.4.1</clover-maven-plugin.version>
     <sonar-maven-plugin.version>3.7.0.1746</sonar-maven-plugin.version>
 
@@ -780,6 +781,11 @@
         <artifactId>dnsjava</artifactId>
         <version>${dnsjava.version}</version>
       </dependency>
+      <dependency>
+        <groupId>com.tngtech.archunit</groupId>
+        <artifactId>archunit-junit5</artifactId>
+        <version>${archunit.version}</version>
+      </dependency>
     </dependencies>
   </dependencyManagement>
 

zookeeper-server/pom.xml

@@ -180,6 +180,11 @@
       <artifactId>tools</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>com.tngtech.archunit</groupId>
+      <artifactId>archunit-junit5</artifactId>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>org.xerial.snappy</groupId>
       <artifactId>snappy-java</artifactId>

zookeeper-server/src/main/java/org/apache/zookeeper/server/ServerMetrics.java

@@ -25,7 +25,6 @@
 import org.apache.zookeeper.metrics.MetricsProvider;
 import org.apache.zookeeper.metrics.Summary;
 import org.apache.zookeeper.metrics.SummarySet;
-import org.apache.zookeeper.metrics.impl.DefaultMetricsProvider;
 import org.apache.zookeeper.metrics.impl.NullMetricsProvider;
 import org.apache.zookeeper.server.util.QuotaMetricsUtils;
 import org.slf4j.Logger;
@@ -40,16 +39,11 @@ public final class ServerMetrics {
      */
     public static final ServerMetrics NULL_METRICS = new ServerMetrics(NullMetricsProvider.INSTANCE);
 
-    /**
-     * Dummy instance useful for tests.
-     */
-    public static final ServerMetrics DEFAULT_METRICS_FOR_TESTS = new ServerMetrics(new DefaultMetricsProvider());
-
     /**
      * Real instance used for tracking server side metrics. The final value is
      * assigned after the {@link MetricsProvider} bootstrap.
      */
-    private static volatile ServerMetrics CURRENT = DEFAULT_METRICS_FOR_TESTS;
+    private static volatile ServerMetrics CURRENT = NULL_METRICS;
 
     /**
      * Access current ServerMetrics.

zookeeper-server/src/test/java/org/apache/zookeeper/DropwizardMetricsOptionalArchTest.java

@@ -0,0 +1,56 @@
+/*
+ * 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.zookeeper;
+
+import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.noClasses;
+import com.tngtech.archunit.base.DescribedPredicate;
+import com.tngtech.archunit.core.domain.JavaClass;
+import com.tngtech.archunit.core.domain.JavaClasses;
+import com.tngtech.archunit.core.importer.ClassFileImporter;
+import com.tngtech.archunit.core.importer.ImportOption;
+import com.tngtech.archunit.lang.ArchRule;
+import java.util.Collections;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Architectural test to enforce that Dropwizard metrics is an optional dependency.
+ *
+ * <p>Only {@link org.apache.zookeeper.server.metric.AvgMinMaxPercentileCounter} may depend
+ * on {@code com.codahale.metrics} packages. All other ZooKeeper classes must remain Dropwizard-free so that it can be
+ * an optional dependency.
+ */
+public class DropwizardMetricsOptionalArchTest {
+
+    @Test
+    public void onlyAvgMinMaxPercentileCounterShouldDependOnDropwizard() {
+        JavaClasses importedClasses =
+            new ClassFileImporter(Collections.singletonList(new ImportOption.DoNotIncludeTests())).importPackages(
+                "org.apache.zookeeper").that(new DescribedPredicate<JavaClass>("ZK non-Dropwizard classes") {
+                @Override
+                public boolean test(JavaClass javaClass) {
+                    return !javaClass.getName().contains("AvgMinMaxPercentileCounter");
+                }
+            });
+
+        ArchRule rule = noClasses().should().dependOnClassesThat().resideInAnyPackage("com.codahale..");
+
+        rule.check(importedClasses);
+    }
+
+}

zookeeper-server/src/test/java/org/apache/zookeeper/ZKTestCase.java

@@ -24,6 +24,8 @@
 import java.time.Instant;
 import java.util.concurrent.CompletableFuture;
 import org.apache.zookeeper.metrics.MetricsUtils;
+import org.apache.zookeeper.metrics.impl.DefaultMetricsProvider;
+import org.apache.zookeeper.server.ServerMetrics;
 import org.apache.zookeeper.util.ServiceUtils;
 import org.hamcrest.CustomMatcher;
 import org.hamcrest.Description;
@@ -93,6 +95,8 @@ public static void before() {
 
     @BeforeEach
     public void starting(TestInfo testInfo) {
+        ServerMetrics.metricsProviderInitialized(new DefaultMetricsProvider());
+
         // By default, disable starting a JettyAdminServer in tests to avoid
         // accidentally attempting to start multiple admin servers on the
         // same port.

zookeeper-server/src/test/java/org/apache/zookeeper/server/quorum/LearnerHandlerMetricsTest.java

@@ -35,6 +35,7 @@
 import java.util.concurrent.TimeUnit;
 import org.apache.jute.BinaryOutputArchive;
 import org.apache.zookeeper.metrics.MetricsUtils;
+import org.apache.zookeeper.metrics.impl.DefaultMetricsProvider;
 import org.apache.zookeeper.server.ServerMetrics;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -55,6 +56,8 @@ class MockLearnerHandler extends LearnerHandler {
 
     @BeforeEach
     public void setup() throws IOException {
+        ServerMetrics.metricsProviderInitialized(new DefaultMetricsProvider());
+
         Leader leader = mock(Leader.class);
         when(leader.getQuorumAuthServer()).thenReturn(null);
 

zookeeper-server/src/test/java/org/apache/zookeeper/server/quorum/SyncRequestProcessorMetricTest.java

@@ -31,9 +31,11 @@
 import java.util.concurrent.TimeUnit;
 import org.apache.zookeeper.ZooDefs;
 import org.apache.zookeeper.metrics.MetricsUtils;
+import org.apache.zookeeper.metrics.impl.DefaultMetricsProvider;
 import org.apache.zookeeper.server.Request;
 import org.apache.zookeeper.server.RequestProcessor;
 import org.apache.zookeeper.server.RequestRecord;
+import org.apache.zookeeper.server.ServerMetrics;
 import org.apache.zookeeper.server.SyncRequestProcessor;
 import org.apache.zookeeper.server.ZKDatabase;
 import org.apache.zookeeper.server.ZooKeeperServer;
@@ -48,6 +50,8 @@ public class SyncRequestProcessorMetricTest {
 
     @BeforeEach
     public void setup() throws Exception {
+        ServerMetrics.metricsProviderInitialized(new DefaultMetricsProvider());
+
         ZKDatabase db = mock(ZKDatabase.class);
         when(db.append(any(Request.class))).thenReturn(true);
         doAnswer(invocation -> {