Experimental max. active connections metric.

src/main/java/com/zaxxer/hikari/metrics/PoolStats.java

@@ -16,31 +16,40 @@
 
 package com.zaxxer.hikari.metrics;
 
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
+import java.util.concurrent.atomic.AtomicLong;
+
 import static com.zaxxer.hikari.util.ClockSource.currentTime;
 import static com.zaxxer.hikari.util.ClockSource.plusMillis;
 
-import java.util.concurrent.atomic.AtomicLong;
-
 /**
  *
  * @author Brett Wooldridge
  */
+@SuppressWarnings("unused")
 public abstract class PoolStats
 {
+   private static final AtomicIntegerFieldUpdater<PoolStats> fieldUpdater;
+
    private final AtomicLong reloadAt;
    private final long timeoutMs;
+   private volatile int peakActiveConnections;
 
    protected volatile int totalConnections;
    protected volatile int idleConnections;
    protected volatile int activeConnections;
    protected volatile int pendingThreads;
 
+   static {
+      fieldUpdater = AtomicIntegerFieldUpdater.newUpdater(PoolStats.class, "peakActiveConnections");
+   }
+
    public PoolStats(final long timeoutMs)
    {
       this.timeoutMs = timeoutMs;
       this.reloadAt = new AtomicLong();
    }
-   
+
    public int getTotalConnections()
    {
       if (shouldLoad()) {
@@ -77,6 +86,16 @@ public abstract class PoolStats
       return pendingThreads;
    }
 
+   public int getPeakActiveConnections()
+   {
+      return fieldUpdater.getAndSet(this, 0);
+   }
+
+   public void setPeakActiveConnections(final int peak)
+   {
+      fieldUpdater.lazySet(this, peak);
+   }
+
    protected abstract void update();
 
    private boolean shouldLoad()

src/main/java/com/zaxxer/hikari/pool/HikariPool.java

@@ -291,7 +291,8 @@ public final class HikariPool extends PoolBase implements HikariPoolMXBean, IBag
    public void setMetricsTrackerFactory(MetricsTrackerFactory metricsTrackerFactory)
    {
       if (metricsTrackerFactory != null) {
-         this.metricsTracker = new MetricsTrackerDelegate(metricsTrackerFactory.create(config.getPoolName(), getPoolStats()));
+         final PoolStats poolStats = getPoolStats();
+         this.metricsTracker = new MetricsTrackerDelegate(metricsTrackerFactory.create(config.getPoolName(), poolStats), poolStats);
       }
       else {
          this.metricsTracker = new NopMetricsTrackerDelegate();

src/main/java/com/zaxxer/hikari/pool/PoolBase.java

@@ -30,6 +30,7 @@ import java.util.concurrent.Executor;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 import java.util.concurrent.atomic.AtomicReference;
 
 import javax.management.MBeanServer;
@@ -38,6 +39,7 @@ import javax.naming.InitialContext;
 import javax.naming.NamingException;
 import javax.sql.DataSource;
 
+import com.zaxxer.hikari.metrics.PoolStats;
 import com.zaxxer.hikari.pool.HikariPool.PoolInitializationException;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -693,16 +695,29 @@ abstract class PoolBase
     */
    static class MetricsTrackerDelegate implements IMetricsTrackerDelegate
    {
+      private static final AtomicIntegerFieldUpdater<MetricsTrackerDelegate> activeUpdater;
+      private static final AtomicIntegerFieldUpdater<MetricsTrackerDelegate> maxActiveUpdater;
+
       final IMetricsTracker tracker;
+      final PoolStats poolStats;
+      volatile int activeCount;
+      volatile int maxActiveCount;
+
+      static {
+         activeUpdater = AtomicIntegerFieldUpdater.newUpdater(MetricsTrackerDelegate.class, "activeCount");
+         maxActiveUpdater = AtomicIntegerFieldUpdater.newUpdater(MetricsTrackerDelegate.class, "maxActiveCount");
+      }
 
-      MetricsTrackerDelegate(IMetricsTracker tracker)
+      MetricsTrackerDelegate(IMetricsTracker tracker, PoolStats poolStats)
       {
          this.tracker = tracker;
+         this.poolStats = poolStats;
       }
 
       @Override
       public void recordConnectionUsage(final PoolEntry poolEntry)
       {
+         activeUpdater.decrementAndGet(this);
          tracker.recordConnectionUsageMillis(poolEntry.getMillisSinceBorrowed());
       }
 
@@ -723,6 +738,15 @@ abstract class PoolBase
       {
          final long now = currentTime();
          poolEntry.lastBorrowed = now;
+
+         final int active = activeUpdater.incrementAndGet(this);
+         final int maxActive = maxActiveUpdater.get(this);
+         if (active > maxActive) {
+            if (maxActive < maxActiveUpdater.getAndUpdate(this, (current) -> (active > current) ? active : current)) {
+               poolStats.setPeakActiveConnections(active);
+            }
+         }
+
          tracker.recordConnectionAcquiredNanos(elapsedNanos(startTime, now));
       }