jolshan commented on code in PR #14387:
URL: https://github.com/apache/kafka/pull/14387#discussion_r1384117312


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/metrics/GroupCoordinatorMetricsShard.java:
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@@ -0,0 +1,325 @@
+/*
+ * 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.kafka.coordinator.group.metrics;
+
+import com.yammer.metrics.core.MetricName;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.metrics.Sensor;
+import org.apache.kafka.common.utils.Utils;
+import org.apache.kafka.coordinator.group.consumer.ConsumerGroup;
+import org.apache.kafka.coordinator.group.generic.GenericGroupState;
+import org.apache.kafka.timeline.SnapshotRegistry;
+import org.apache.kafka.timeline.TimelineLong;
+
+import java.util.Collections;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicLong;
+
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_CONSUMER_GROUPS;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_CONSUMER_GROUPS_ASSIGNING;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_CONSUMER_GROUPS_DEAD;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_CONSUMER_GROUPS_EMPTY;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_CONSUMER_GROUPS_RECONCILING;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_CONSUMER_GROUPS_STABLE;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_GENERIC_GROUPS;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_GENERIC_GROUPS_COMPLETING_REBALANCE;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_GENERIC_GROUPS_DEAD;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_GENERIC_GROUPS_EMPTY;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_GENERIC_GROUPS_PREPARING_REBALANCE;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_GENERIC_GROUPS_STABLE;
+import static 
org.apache.kafka.coordinator.group.metrics.GroupCoordinatorMetrics.NUM_OFFSETS;
+
+/**
+ * This class is mapped to a single {@link 
org.apache.kafka.coordinator.group.GroupCoordinatorShard}. It will
+ * record all metrics that the shard handles with respect to {@link 
org.apache.kafka.coordinator.group.OffsetMetadataManager}
+ * and {@link org.apache.kafka.coordinator.group.GroupMetadataManager} 
operations.
+ *
+ * Local gauges will be recorded in this class which will be gathered by 
{@link GroupCoordinatorMetrics} to
+ * report.
+ */
+public class GroupCoordinatorMetricsShard implements CoordinatorMetricsShard {
+
+    /**
+     * This class represents a gauge counter for this shard. The TimelineLong 
object represents a gauge backed by
+     * the snapshot registry. Once we commit to a certain offset in the 
snapshot registry, we write the given
+     * TimelineLong's value to the AtomicLong. This AtomicLong represents the 
actual gauge counter that is queried
+     * when reporting the value to {@link GroupCoordinatorMetrics}.
+     */
+    private static class TimelineGaugeCounter {
+
+        final TimelineLong timelineLong;
+
+        final AtomicLong atomicLong;
+
+        public TimelineGaugeCounter(TimelineLong timelineLong, AtomicLong 
atomicLong) {
+            this.timelineLong = timelineLong;
+            this.atomicLong = atomicLong;
+        }
+    }
+
+    /**
+     * Local timeline gauge counters keyed by the metric name.
+     */
+    private final Map<String, TimelineGaugeCounter> localGauges;
+
+    /**
+     * All sensors keyed by the sensor name. A Sensor object is shared across 
all metrics shards.
+     */
+    private final Map<String, Sensor> globalSensors;
+
+    /**
+     * Global gauge counters keyed by the metric name. The same counter is 
shared across all metrics shards.
+     */
+    private final Map<String, AtomicLong> globalGauges;
+
+    /**
+     * The topic partition.
+     */
+    private final TopicPartition topicPartition;
+
+    public GroupCoordinatorMetricsShard(
+        SnapshotRegistry snapshotRegistry,
+        Map<String, Sensor> globalSensors,
+        Map<String, AtomicLong> globalGauges,
+        TopicPartition topicPartition
+    ) {
+        Objects.requireNonNull(snapshotRegistry);
+        TimelineLong numOffsetsTimeline = new TimelineLong(snapshotRegistry);
+        TimelineLong numGenericGroupsTimeline = new 
TimelineLong(snapshotRegistry);
+        TimelineLong numConsumerGroupsTimeline = new 
TimelineLong(snapshotRegistry);
+        TimelineLong numConsumerGroupsEmptyTimeline = new 
TimelineLong(snapshotRegistry);
+        TimelineLong numConsumerGroupsAssigningTimeline = new 
TimelineLong(snapshotRegistry);
+        TimelineLong numConsumerGroupsReconcilingTimeline = new 
TimelineLong(snapshotRegistry);
+        TimelineLong numConsumerGroupsStableTimeline = new 
TimelineLong(snapshotRegistry);
+        TimelineLong numConsumerGroupsDeadTimeline = new 
TimelineLong(snapshotRegistry);
+
+        this.localGauges = Collections.unmodifiableMap(Utils.mkMap(
+            Utils.mkEntry(NUM_OFFSETS.getName(),
+                new TimelineGaugeCounter(numOffsetsTimeline, new 
AtomicLong(0))),
+            Utils.mkEntry(NUM_GENERIC_GROUPS.getName(),
+                new TimelineGaugeCounter(numGenericGroupsTimeline, new 
AtomicLong(0))),
+            Utils.mkEntry(NUM_CONSUMER_GROUPS.getName(),
+                new TimelineGaugeCounter(numConsumerGroupsTimeline, new 
AtomicLong(0))),
+            Utils.mkEntry(NUM_CONSUMER_GROUPS_EMPTY.getName(),
+                new TimelineGaugeCounter(numConsumerGroupsEmptyTimeline, new 
AtomicLong(0))),
+            Utils.mkEntry(NUM_CONSUMER_GROUPS_ASSIGNING.getName(),
+                new TimelineGaugeCounter(numConsumerGroupsAssigningTimeline, 
new AtomicLong(0))),
+            Utils.mkEntry(NUM_CONSUMER_GROUPS_RECONCILING.getName(),
+                new TimelineGaugeCounter(numConsumerGroupsReconcilingTimeline, 
new AtomicLong(0))),
+            Utils.mkEntry(NUM_CONSUMER_GROUPS_STABLE.getName(),
+                new TimelineGaugeCounter(numConsumerGroupsStableTimeline, new 
AtomicLong(0))),
+            Utils.mkEntry(NUM_CONSUMER_GROUPS_DEAD.getName(),
+                new TimelineGaugeCounter(numConsumerGroupsDeadTimeline, new 
AtomicLong(0)))
+        ));
+
+        this.globalSensors = Objects.requireNonNull(globalSensors);
+        this.globalGauges = Objects.requireNonNull(globalGauges);
+        this.topicPartition = Objects.requireNonNull(topicPartition);
+    }
+
+    @Override
+    public void incrementGlobalGauge(MetricName metricName) {
+        AtomicLong gaugeCounter = globalGauges.get(metricName.getName());
+        if (gaugeCounter != null) {
+            gaugeCounter.incrementAndGet();
+        }
+    }
+
+    @Override
+    public void incrementLocalGauge(MetricName metricName) {
+        TimelineGaugeCounter gaugeCounter = 
localGauges.get(metricName.getName());
+        if (gaugeCounter != null) {
+            synchronized (gaugeCounter.timelineLong) {
+                gaugeCounter.timelineLong.increment();
+            }
+        }
+    }
+
+    @Override
+    public void decrementGlobalGauge(MetricName metricName) {
+        AtomicLong gaugeCounter = globalGauges.get(metricName.getName());
+        if (gaugeCounter != null) {
+            gaugeCounter.decrementAndGet();
+        }
+    }
+
+    @Override
+    public void decrementLocalGauge(MetricName metricName) {
+        TimelineGaugeCounter gaugeCounter = 
localGauges.get(metricName.getName());
+        if (gaugeCounter != null) {
+            synchronized (gaugeCounter.timelineLong) {
+                gaugeCounter.timelineLong.decrement();
+            }
+        }
+    }
+
+    @Override
+    public long globalGaugeValue(MetricName metricName) {
+        AtomicLong gaugeCounter = globalGauges.get(metricName.getName());
+        if (gaugeCounter != null) {
+            return gaugeCounter.get();
+        }
+        return 0;
+    }
+
+    @Override
+    public long localGaugeValue(MetricName metricName) {
+        TimelineGaugeCounter gaugeCounter = 
localGauges.get(metricName.getName());
+        if (gaugeCounter != null) {
+            return gaugeCounter.atomicLong.get();
+        }
+        return 0;
+    }
+
+    @Override
+    public void record(String sensorName) {
+        Sensor sensor = globalSensors.get(sensorName);
+        if (sensor != null) {
+            sensor.record();
+        }
+    }
+
+    @Override
+    public void record(String sensorName, double val) {
+        Sensor sensor = globalSensors.get(sensorName);
+        if (sensor != null) {
+            sensor.record(val);
+        }
+    }
+
+    @Override
+    public TopicPartition topicPartition() {
+        return this.topicPartition;
+    }
+
+    @Override
+    public void commitUpTo(long offset) {
+        this.localGauges.forEach((__, gaugeCounter) -> {
+            long value;
+            synchronized (gaugeCounter.timelineLong) {
+                value = gaugeCounter.timelineLong.get(offset);
+            }
+            gaugeCounter.atomicLong.set(value);
+        });
+    }
+
+    /**
+     * Called when a generic group's state has changed. Increment/decrement
+     * the counter accordingly.
+     *
+     * @param oldState The previous state. null value means that it's a new 
group.
+     * @param newState The next state. null value means that the group has 
been removed.
+     */
+    public void onGenericGroupStateTransition(GenericGroupState oldState, 
GenericGroupState newState) {
+        if (newState != null) {
+            switch (newState) {
+                case PREPARING_REBALANCE:
+                    
incrementGlobalGauge(NUM_GENERIC_GROUPS_PREPARING_REBALANCE);
+                    break;
+                case COMPLETING_REBALANCE:
+                    
incrementGlobalGauge(NUM_GENERIC_GROUPS_COMPLETING_REBALANCE);
+                    break;
+                case STABLE:
+                    incrementGlobalGauge(NUM_GENERIC_GROUPS_STABLE);
+                    break;
+                case DEAD:
+                    incrementGlobalGauge(NUM_GENERIC_GROUPS_DEAD);
+                    break;
+                case EMPTY:
+                    incrementGlobalGauge(NUM_GENERIC_GROUPS_EMPTY);
+            }
+        } else {
+            decrementLocalGauge(NUM_GENERIC_GROUPS);
+        }
+
+        if (oldState != null) {
+            switch (oldState) {
+                case PREPARING_REBALANCE:
+                    
decrementGlobalGauge(NUM_GENERIC_GROUPS_PREPARING_REBALANCE);
+                    break;
+                case COMPLETING_REBALANCE:
+                    
decrementGlobalGauge(NUM_GENERIC_GROUPS_COMPLETING_REBALANCE);
+                    break;
+                case STABLE:
+                    decrementGlobalGauge(NUM_GENERIC_GROUPS_STABLE);
+                    break;
+                case DEAD:
+                    decrementGlobalGauge(NUM_GENERIC_GROUPS_DEAD);
+                    break;
+                case EMPTY:
+                    decrementGlobalGauge(NUM_GENERIC_GROUPS_EMPTY);
+            }
+        } else {
+            incrementLocalGauge(NUM_GENERIC_GROUPS);
+        }
+    }
+
+    /**
+     * Called when a consumer group's state has changed. Increment/decrement
+     * the counter accordingly.
+     *
+     * @param oldState The previous state. null value means that it's a new 
group.
+     * @param newState The next state. null value means that the group has 
been removed.
+     */
+    public void onConsumerGroupStateTransition(

Review Comment:
   whoops. ok cool so no blocking needed.



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