OAK-11452 : exported full gc OSGi configs as metrics

oak-core-spi/src/main/java/org/apache/jackrabbit/oak/stats/DefaultStatisticsProvider.java

@@ -22,6 +22,7 @@
 import java.util.Map;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Supplier;
 
 import org.apache.jackrabbit.api.stats.RepositoryStatistics;
 import org.apache.jackrabbit.api.stats.RepositoryStatistics.Type;
@@ -60,6 +61,12 @@ public HistogramStats getHistogram(String name, StatsOptions options) {
         return getStats(name, true, SimpleStats.Type.HISTOGRAM, options);
     }
 
+    @Override
+    public <T> GaugeStats<T> getGauge(String name, Supplier<T> supplier) {
+        return statsMeters.computeIfAbsent(name,
+                k -> new SimpleStats<>(new AtomicLong(), SimpleStats.Type.GAUGE, supplier.get()));
+    }
+
     private synchronized SimpleStats getStats(String type, boolean resetValueEachSecond, SimpleStats.Type statsType,
                                               StatsOptions options){
         SimpleStats stats = statsMeters.get(type);

oak-core-spi/src/main/java/org/apache/jackrabbit/oak/stats/GaugeStats.java

@@ -0,0 +1,33 @@
+/*
+ * 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.jackrabbit.oak.stats;
+
+import org.osgi.annotation.versioning.ProviderType;
+
+@ProviderType
+public interface GaugeStats<T> extends Stats {
+
+    /**
+     * Returns the metric's current value.
+     *
+     * @return the metric's current value
+     */
+    T getValue();
+}

oak-core-spi/src/main/java/org/apache/jackrabbit/oak/stats/SimpleStats.java

@@ -22,10 +22,12 @@
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
 
-public final class SimpleStats implements TimerStats, MeterStats, CounterStats, HistogramStats {
-    public enum Type {COUNTER, METER, TIMER, HISTOGRAM}
+public final class SimpleStats<T> implements TimerStats, MeterStats, CounterStats, HistogramStats, GaugeStats<T> {
+
+    public enum Type {COUNTER, METER, TIMER, HISTOGRAM, GAUGE}
     private final AtomicLong statsHolder;
     private long counter;
+    private T value;
 
     /*
         Using 2 different variables for managing the sum in meter calls
@@ -44,8 +46,13 @@ This is done to ensure that more frequent mark() is fast (used for Session reads
     private final Type type;
 
     public SimpleStats(AtomicLong statsHolder, Type type) {
+        this(statsHolder, type, null);
+    }
+
+    public SimpleStats(AtomicLong statsHolder, Type type, T value) {
         this.statsHolder = statsHolder;
         this.type = type;
+        this.value = value;
     }
 
     @Override
@@ -62,6 +69,9 @@ public long getCount() {
                 //For Meter it can happen that backing statsHolder gets
                 //reset each second. So need to manage that sum separately
                 return meterSum + meterSumRef.get();
+            case GAUGE:
+                // For gauge type there is no count, it only returns the constant value set
+                return 0;
         }
         throw new IllegalStateException();
     }
@@ -115,6 +125,11 @@ public void update(long value) {
         statsHolder.getAndAdd(value);
     }
 
+    @Override
+    public T getValue() {
+        return value;
+    }
+
     private static final class SimpleContext implements Context {
         private final TimerStats timer;
         private final long startTime;

oak-core-spi/src/main/java/org/apache/jackrabbit/oak/stats/StatisticsProvider.java

@@ -22,6 +22,8 @@
 import org.osgi.annotation.versioning.ProviderType;
 import org.apache.jackrabbit.api.stats.RepositoryStatistics;
 
+import java.util.function.Supplier;
+
 @ProviderType
 public interface StatisticsProvider {
     StatisticsProvider NOOP = new StatisticsProvider() {
@@ -49,6 +51,11 @@ public TimerStats getTimer(String name, StatsOptions options) {
         public HistogramStats getHistogram(String name, StatsOptions options) {
             return NoopStats.INSTANCE;
         }
+
+        @Override
+        public <T> GaugeStats<T> getGauge(String name, Supplier<T> supplier) {
+            return null;
+        }
     };
 
 
@@ -61,4 +68,14 @@ public HistogramStats getHistogram(String name, StatsOptions options) {
     TimerStats getTimer(String name, StatsOptions options);
 
     HistogramStats getHistogram(String name, StatsOptions options);
+
+    /**
+     * Creates a new {@link GaugeStats} and registers it under the given name.
+     * If a gauge with the same exists already then the same instance is returned.
+     * @param name the name of the gauge
+     * @param supplier provides the values which are returned by the gauge
+     * @param <T> the type of the metric
+     * @return the gauge
+     */
+    <T> GaugeStats<T> getGauge(String name, Supplier<T> supplier);
 }

oak-core-spi/src/test/java/org/apache/jackrabbit/oak/stats/DefaultStatisticsProviderTest.java

@@ -35,7 +35,6 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 
 public class DefaultStatisticsProviderTest {
@@ -61,6 +60,14 @@ public void meter() throws Exception {
         assertTrue(getRegisteredTimeSeries(statsProvider).contains("test"));
     }
 
+    @Test
+    public void gauge() {
+        GaugeStats<String> gaugeStats = statsProvider.getGauge("test", () -> "value");
+
+        assertNotNull(gaugeStats);
+        assertEquals("value", gaugeStats.getValue());
+    }
+
     @Test
     public void counter() throws Exception {
         CounterStats counterStats = statsProvider.getCounterStats("test", StatsOptions.DEFAULT);

oak-core-spi/src/test/java/org/apache/jackrabbit/oak/stats/StatisticManagerTest.java

@@ -24,6 +24,7 @@
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Supplier;
 
 import org.apache.jackrabbit.api.jmx.QueryStatManagerMBean;
 import org.apache.jackrabbit.api.stats.RepositoryStatistics;
@@ -131,7 +132,7 @@ public void cleanup() {
         executorService.shutdownNow();
     }
 
-    private class DummyStatsProvider implements StatisticsProvider {
+    private static class DummyStatsProvider implements StatisticsProvider {
 
         @Override
         public RepositoryStatistics getStats() {
@@ -157,5 +158,10 @@ public TimerStats getTimer(String name, StatsOptions options) {
         public HistogramStats getHistogram(String name, StatsOptions options) {
             return NoopStats.INSTANCE;
         }
+
+        @Override
+        public <T> GaugeStats<T> getGauge(String name, Supplier<T> supplier) {
+            return null;
+        }
     }
 }

oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/metric/GaugeImpl.java

@@ -0,0 +1,36 @@
+/*
+ * 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.jackrabbit.oak.plugins.metric;
+
+import com.codahale.metrics.Gauge;
+import org.apache.jackrabbit.oak.stats.GaugeStats;
+
+public class GaugeImpl<T> implements GaugeStats<T> {
+
+    private final Gauge<T> gauge;
+
+    public GaugeImpl(Gauge<T> gauge) {
+        this.gauge = gauge;
+    }
+    @Override
+    public T getValue() {
+        return this.gauge.getValue();
+    }
+}

oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/metric/MetricStatisticsProvider.java

@@ -26,11 +26,13 @@
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Supplier;
 
 import javax.management.MBeanServer;
 import javax.management.MalformedObjectNameException;
 import javax.management.ObjectName;
 
+import com.codahale.metrics.Gauge;
 import com.codahale.metrics.JmxReporter;
 import com.codahale.metrics.Meter;
 import com.codahale.metrics.MetricRegistry;
@@ -43,6 +45,7 @@
 import org.apache.jackrabbit.oak.spi.whiteboard.WhiteboardUtils;
 import org.apache.jackrabbit.oak.stats.Clock;
 import org.apache.jackrabbit.oak.stats.CounterStats;
+import org.apache.jackrabbit.oak.stats.GaugeStats;
 import org.apache.jackrabbit.oak.stats.HistogramStats;
 import org.apache.jackrabbit.oak.stats.MeterStats;
 import org.apache.jackrabbit.oak.stats.SimpleStats;
@@ -117,6 +120,11 @@ public HistogramStats getHistogram(String name, StatsOptions options) {
         return getStats(name, StatsBuilder.HISTOGRAMS, options);
     }
 
+    @Override
+    public <T> GaugeStats<T> getGauge(String name, Supplier<T> supplier) {
+        return getOrAddGauge(name, supplier);
+    }
+
     public MetricRegistry getRegistry() {
         return registry;
     }
@@ -125,6 +133,36 @@ RepositoryStatisticsImpl getRepoStats() {
         return repoStats;
     }
 
+    @SuppressWarnings("unchecked")
+    private <T> GaugeStats<T> getOrAddGauge(final String name, final Supplier<T> supplier) {
+        final Stats stats = statsRegistry.get(name);
+        if (stats instanceof GaugeStats<?>) {
+            return (GaugeStats<T>) stats;
+        } else {
+            try {
+                return registerGauge(name, supplier);
+            } catch (IllegalArgumentException e) {
+                final Stats added = statsRegistry.get(name);
+                if (added instanceof GaugeStats<?>) {
+                    return (GaugeStats<T>) added;
+                }
+            }
+        }
+        throw new IllegalArgumentException(name + " is already used for a different type of stats");
+    }
+
+    private <T> GaugeStats<T> registerGauge(final String name, final Supplier<T> supplier) {
+        final Gauge<T> codahaleGauge = supplier::get;
+        @SuppressWarnings("rawtypes")
+        MetricRegistry.MetricSupplier<Gauge> metricSupplier = () -> codahaleGauge;
+
+        @SuppressWarnings("unchecked")
+        Gauge<T> g = registry.gauge(name, metricSupplier);
+        GaugeImpl<T> gauge = new GaugeImpl<>(g);
+        statsRegistry.put(name, gauge);
+        return gauge;
+    }
+
     private <T extends Stats> T getStats(String name, StatsBuilder<T> builder, StatsOptions options) {
         Stats stats = statsRegistry.get(name);
         //Use double locking pattern. The map should get populated with required set

oak-core/src/test/java/org/apache/jackrabbit/oak/plugins/metric/MetricStatisticsProviderTest.java

@@ -41,6 +41,7 @@
 import org.apache.jackrabbit.api.stats.RepositoryStatistics.Type;
 import org.apache.jackrabbit.oak.commons.collections.SetUtils;
 import org.apache.jackrabbit.oak.stats.CounterStats;
+import org.apache.jackrabbit.oak.stats.GaugeStats;
 import org.apache.jackrabbit.oak.stats.HistogramStats;
 import org.apache.jackrabbit.oak.stats.MeterStats;
 import org.apache.jackrabbit.oak.stats.NoopStats;
@@ -111,6 +112,20 @@ public void histogram() throws Exception {
         assertTrue(((CompositeStats) histoStats).isHistogram());
     }
 
+    @Test
+    public void gauge() {
+        GaugeStats<String> gaugeStats = statsProvider.getGauge("test", () -> "value");
+
+        assertNotNull(gaugeStats);
+        assertEquals("value", statsProvider.getRegistry().getGauges().get("test").getValue());
+
+        // updating gauge is not possible
+        gaugeStats = statsProvider.getGauge("test", () -> "value2");
+        assertNotNull(gaugeStats);
+        assertEquals("value", statsProvider.getRegistry().getGauges().get("test").getValue());
+
+    }
+
     @Test
     public void timeSeriesIntegration() throws Exception {
         MeterStats meterStats = statsProvider.getMeter(Type.SESSION_COUNT.name(), StatsOptions.DEFAULT);

oak-run/src/main/java/org/apache/jackrabbit/oak/run/FullGCMetricsExporterFixture.java

@@ -24,5 +24,5 @@
  * Fixture encapsulating FullGC metrics exporter instance of T
  * @param <T>
  */
-public interface FullGCMetricsExporterFixture<T> extends FullGCMetricsExporter<T>, MetricsExporterFixture<T> {
+public interface FullGCMetricsExporterFixture<T> extends FullGCMetricsExporter, MetricsExporterFixture<T> {
 }

oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/spi/monitor/RoleStatisticsProvider.java

@@ -20,12 +20,15 @@
 import org.apache.jackrabbit.api.stats.RepositoryStatistics;
 import org.apache.jackrabbit.api.stats.TimeSeries;
 import org.apache.jackrabbit.oak.stats.CounterStats;
+import org.apache.jackrabbit.oak.stats.GaugeStats;
 import org.apache.jackrabbit.oak.stats.HistogramStats;
 import org.apache.jackrabbit.oak.stats.MeterStats;
 import org.apache.jackrabbit.oak.stats.StatisticsProvider;
 import org.apache.jackrabbit.oak.stats.StatsOptions;
 import org.apache.jackrabbit.oak.stats.TimerStats;
 
+import java.util.function.Supplier;
+
 public class RoleStatisticsProvider implements StatisticsProvider{
 
     private final StatisticsProvider delegate;
@@ -75,6 +78,11 @@ public HistogramStats getHistogram(String name, StatsOptions options) {
         return delegate.getHistogram(addRoleToName(name, role), options);
     }
 
+    @Override
+    public <T> GaugeStats<T> getGauge(String name, Supplier<T> supplier) {
+        return delegate.getGauge(addRoleToName(name, role), supplier);
+    }
+
     private static String addRoleToName(String name, String role) {
         return role + '.' + name;
     }

oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/SegmentBufferMonitorTest.java

@@ -28,10 +28,12 @@
 import java.util.HashMap;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Supplier;
 
 import org.apache.jackrabbit.api.stats.RepositoryStatistics;
 import org.apache.jackrabbit.oak.commons.Buffer;
 import org.apache.jackrabbit.oak.stats.CounterStats;
+import org.apache.jackrabbit.oak.stats.GaugeStats;
 import org.apache.jackrabbit.oak.stats.HistogramStats;
 import org.apache.jackrabbit.oak.stats.MeterStats;
 import org.apache.jackrabbit.oak.stats.SimpleStats;
@@ -71,6 +73,11 @@ public TimerStats getTimer(String name, StatsOptions options) {
         public HistogramStats getHistogram(String name, StatsOptions options) {
             throw new IllegalStateException();
         }
+
+        @Override
+        public <T> GaugeStats<T> getGauge(String name, Supplier<T> supplier) {
+            throw new IllegalStateException();
+        }
     });
 
     @Test