Add stubs for channel pool health checking

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/gaxx/grpc/BigtableChannelPool.java

@@ -61,9 +61,13 @@ public class BigtableChannelPool extends ManagedChannel {
   private final BigtableChannelPoolSettings settings;
   private final ChannelFactory channelFactory;
   private final ScheduledExecutorService executor;
+  private final ScheduledExecutorService channelHealthProbingExecutor =
+      Executors.newSingleThreadScheduledExecutor();
 
   private final Object entryWriteLock = new Object();
   @VisibleForTesting final AtomicReference<ImmutableList<Entry>> entries = new AtomicReference<>();
+  private final ChannelPoolHealthChecker channelPoolHealthChecker =
+      new ChannelPoolHealthChecker(() -> entries.get());
   private final AtomicInteger indexTicker = new AtomicInteger();
   private final String authority;
 
@@ -92,7 +96,8 @@ public static BigtableChannelPool create(
     ImmutableList.Builder<Entry> initialListBuilder = ImmutableList.builder();
 
     for (int i = 0; i < settings.getInitialChannelCount(); i++) {
-      initialListBuilder.add(new Entry(channelFactory.createSingleChannel()));
+      initialListBuilder.add(
+          new Entry(channelFactory.createSingleChannel(), channelHealthProbingExecutor));
     }
 
     entries.set(initialListBuilder.build());
@@ -150,6 +155,9 @@ public ManagedChannel shutdown() {
       // shutdownNow will cancel scheduled tasks
       executor.shutdownNow();
     }
+    if (channelHealthProbingExecutor != null) {
+      channelHealthProbingExecutor.shutdownNow();
+    }
     return this;
   }
 
@@ -190,6 +198,9 @@ public ManagedChannel shutdownNow() {
     if (executor != null) {
       executor.shutdownNow();
     }
+    if (channelHealthProbingExecutor != null) {
+      channelHealthProbingExecutor.shutdownNow();
+    }
     return this;
   }
 
@@ -316,7 +327,8 @@ private void expand(int desiredSize) {
 
     for (int i = 0; i < desiredSize - localEntries.size(); i++) {
       try {
-        newEntries.add(new Entry(channelFactory.createSingleChannel()));
+        newEntries.add(
+            new Entry(channelFactory.createSingleChannel(), channelHealthProbingExecutor));
       } catch (IOException e) {
         LOG.log(Level.WARNING, "Failed to add channel", e);
       }
@@ -354,7 +366,8 @@ void refresh() {
 
       for (int i = 0; i < newEntries.size(); i++) {
         try {
-          newEntries.set(i, new Entry(channelFactory.createSingleChannel()));
+          newEntries.set(
+              i, new Entry(channelFactory.createSingleChannel(), channelHealthProbingExecutor));
         } catch (IOException e) {
           LOG.log(Level.WARNING, "Failed to refresh channel, leaving old channel", e);
         }
@@ -430,14 +443,26 @@ static class Entry {
     @VisibleForTesting final AtomicInteger outstandingRpcs = new AtomicInteger(0);
 
     private final AtomicInteger maxOutstanding = new AtomicInteger();
+    private final AtomicInteger probesInFlight = new AtomicInteger(0);
 
     // Flag that the channel should be closed once all of the outstanding RPC complete.
     private final AtomicBoolean shutdownRequested = new AtomicBoolean();
     // Flag that the channel has been closed.
     private final AtomicBoolean shutdownInitiated = new AtomicBoolean();
+    private final ChannelHealthChecker healthChecker;
 
-    private Entry(ManagedChannel channel) {
+    private Entry(ManagedChannel channel, ScheduledExecutorService executor) {
       this.channel = channel;
+      this.healthChecker = new ChannelHealthChecker(this, executor);
+    }
+
+    ManagedChannel getManagedChannel() {
+      return this.channel;
+    }
+
+    // Add a getter for the healthChecker
+    public ChannelHealthChecker getHealthChecker() {
+      return healthChecker;
     }
 
     int getAndResetMaxOutstanding() {
@@ -454,7 +479,7 @@ private boolean retain() {
       // register desire to start RPC
       int currentOutstanding = outstandingRpcs.incrementAndGet();
 
-      // Rough book keeping
+      // Rough bookkeeping
       int prevMax = maxOutstanding.get();
       if (currentOutstanding > prevMax) {
         maxOutstanding.incrementAndGet();
@@ -491,6 +516,9 @@ private void release() {
      */
     private void requestShutdown() {
       shutdownRequested.set(true);
+      if (healthChecker != null) {
+        healthChecker.stop();
+      }
       if (outstandingRpcs.get() == 0) {
         shutdown();
       }

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/gaxx/grpc/ChannelHealthChecker.java

@@ -0,0 +1,106 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed 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
+ *
+ *     https://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 com.google.cloud.bigtable.gaxx.grpc;
+
+import com.google.cloud.bigtable.gaxx.grpc.BigtableChannelPool.Entry;
+import com.google.common.collect.EvictingQueue;
+import java.time.Instant;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+/**
+ * Stub for a class that checks the health of individual channels in a channel pool by sending
+ * PingAndWarm probes at a fixed interval
+ */
+public class ChannelHealthChecker {
+
+  // Configuration constants
+  private static final int WINDOW_DURATION_MINUTES = 5;
+  private static final int PROBE_RATE_SECONDS = 30;
+  private static final int PROBE_DEADLINE_MILLISECONDS = 500;
+  private static final int MIN_PROBES_FOR_EVALUATION = 4;
+  private static final int FAILURE_PERCENT_THRESHOLD = 60;
+
+  // Class fields
+  final Entry entry;
+  private final ScheduledExecutorService probeExecutor;
+  private volatile ScheduledFuture<?> scheduledProbeFuture;
+  private final ReadWriteLock probeResultsLock = new ReentrantReadWriteLock();
+  private final EvictingQueue<ProbeResult> probeResults;
+
+  // private final AtomicInteger probesInFlight = new AtomicInteger(0);
+
+  /** Inner class to represent the result of a single probe. */
+  class ProbeResult {
+    final Instant startTime;
+    final boolean success;
+
+    ProbeResult(Instant startTime, boolean success) {
+      this.startTime = startTime;
+      this.success = success;
+    }
+
+    public boolean isSuccessful() {
+      return success;
+    }
+  }
+
+  /** Constructor for the health checker. */
+  public ChannelHealthChecker(Entry entry, ScheduledExecutorService executor) {
+    int queueCapacity = (WINDOW_DURATION_MINUTES * 60) / PROBE_RATE_SECONDS;
+    this.probeResults = EvictingQueue.create(queueCapacity);
+    this.entry = entry;
+    this.probeExecutor = executor;
+    // Scheduling runProbe will go here
+  }
+
+  /** Cancel health checking future (No-op stub) */
+  public void stop() {
+    // Method stub, no operation.
+  }
+
+  /** Runs a single health probe. (No-op stub) */
+  private void runProbe() {
+    // Method stub, no operation.
+  }
+
+  /** Callback for when a probe finishes. (No-op stub) */
+  void probeFinished(Instant startTime, boolean success) {
+    // Method stub, no operation.
+  }
+
+  /** Number of probes in flight plus number of probe results. (No-op stub) */
+  AtomicInteger recentProbesSent() {
+    return new AtomicInteger(0);
+  }
+
+  /** Number of recently failed probes. (No-op stub) */
+  AtomicInteger recentlyFailedProbes() {
+    return new AtomicInteger(0);
+  }
+
+  /**
+   * Determines if the channel is healthy. (No-op stub)
+   *
+   * @return A default value of true.
+   */
+  public boolean healthy() {
+    return true;
+  }
+}

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/gaxx/grpc/ChannelPoolHealthChecker.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed 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
+ *
+ *     https://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 com.google.cloud.bigtable.gaxx.grpc;
+
+import com.google.cloud.bigtable.gaxx.grpc.BigtableChannelPool.Entry;
+import com.google.common.collect.ImmutableList;
+import java.time.Instant;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.function.Supplier;
+import javax.annotation.Nullable;
+
+/** Stub for a class that will manage the health checking in the BigtableChannelPool */
+public class ChannelPoolHealthChecker {
+
+  // Class fields
+  private final Supplier<ImmutableList<Entry>> entrySupplier;
+  private Instant lastEviction;
+  private ScheduledExecutorService executor;
+
+  /** Constructor for the pool health checker. */
+  public ChannelPoolHealthChecker(Supplier<ImmutableList<Entry>> entrySupplier) {
+    this.entrySupplier = entrySupplier;
+    this.lastEviction = Instant.MIN;
+    this.executor = Executors.newSingleThreadScheduledExecutor();
+    // Scheduling for detectAndRemoveOutlierChannels goes here
+  }
+
+  /**
+   * Finds a channel that is an outlier in terms of health. (No-op stub)
+   *
+   * @return A default value of null.
+   */
+  @Nullable
+  private Entry findOutlierEntry() {
+    return null;
+  }
+
+  /** Periodically detects and removes outlier channels from the pool. (No-op stub) */
+  private void detectAndRemoveOutlierEntries() {
+    // Method stub, no operation.
+  }
+}