feat: instrument Spanner client with OpenCensus metrics (#54)

* feat: add session metrics
Add active_sessions (The number of active sessions in use) and max_sessions (The number of max sessions configured the user) metrics

* Fix code reviews

Use maxSessionsInUse instead of numSessionsInUse and update the description.

* Change active sessions description

* add numSessionsInUse metric

* Fix package structure

* rename metric name and description

* fix nits

* createMockSession for metrics validations

Author: mayurkale22

google-cloud-spanner/src/main/java/com/google/cloud/spanner/MetricRegistryConstants.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2020 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.spanner;
+
+import com.google.common.collect.ImmutableList;
+import io.opencensus.metrics.LabelKey;
+import io.opencensus.metrics.LabelValue;
+
+/** A helper class that holds OpenCensus's related constants. */
+class MetricRegistryConstants {
+
+  // The label keys are used to uniquely identify timeseries.
+  private static final LabelKey DATABASE = LabelKey.create("database", "Target database");
+  private static final LabelKey INSTANCE_ID =
+      LabelKey.create("instance_id", "Name of the instance");
+  private static final LabelKey LIBRARY_VERSION =
+      LabelKey.create("library_version", "Library version");
+
+  /** The label value is used to represent missing value. */
+  private static final LabelValue UNSET_LABEL = LabelValue.create(null);
+
+  static final ImmutableList<LabelKey> SPANNER_LABEL_KEYS =
+      ImmutableList.of(DATABASE, INSTANCE_ID, LIBRARY_VERSION);
+
+  static final ImmutableList<LabelValue> SPANNER_DEFAULT_LABEL_VALUES =
+      ImmutableList.of(UNSET_LABEL, UNSET_LABEL, UNSET_LABEL);
+
+  /** Unit to represent counts. */
+  static final String COUNT = "1";
+
+  // The Metric name and description
+  static final String MAX_IN_USE_SESSIONS = "cloud.google.com/java/spanner/max_in_use_session";
+  static final String MAX_ALLOWED_SESSIONS = "cloud.google.com/java/spanner/max_allowed_sessions";
+  static final String IN_USE_SESSIONS = "cloud.google.com/java/spanner/in_use_sessions";
+  static final String MAX_IN_USE_SESSIONS_DESCRIPTION =
+      "The maximum number of sessions in use during the last 10 minute interval.";
+  static final String MAX_ALLOWED_SESSIONS_DESCRIPTION =
+      "The maximum number of sessions allowed. Configurable by the user.";
+  static final String IN_USE_SESSIONS_DESCRIPTION = "The number of sessions currently in use.";
+}

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java

@@ -16,6 +16,15 @@
 
 package com.google.cloud.spanner;
 
+import static com.google.cloud.spanner.MetricRegistryConstants.COUNT;
+import static com.google.cloud.spanner.MetricRegistryConstants.IN_USE_SESSIONS;
+import static com.google.cloud.spanner.MetricRegistryConstants.IN_USE_SESSIONS_DESCRIPTION;
+import static com.google.cloud.spanner.MetricRegistryConstants.MAX_ALLOWED_SESSIONS;
+import static com.google.cloud.spanner.MetricRegistryConstants.MAX_ALLOWED_SESSIONS_DESCRIPTION;
+import static com.google.cloud.spanner.MetricRegistryConstants.MAX_IN_USE_SESSIONS;
+import static com.google.cloud.spanner.MetricRegistryConstants.MAX_IN_USE_SESSIONS_DESCRIPTION;
+import static com.google.cloud.spanner.MetricRegistryConstants.SPANNER_DEFAULT_LABEL_VALUES;
+import static com.google.cloud.spanner.MetricRegistryConstants.SPANNER_LABEL_KEYS;
 import static com.google.cloud.spanner.SpannerExceptionFactory.newSpannerException;
 
 import com.google.api.core.ApiFuture;
@@ -40,6 +49,12 @@
 import com.google.common.util.concurrent.Uninterruptibles;
 import com.google.protobuf.Empty;
 import io.opencensus.common.Scope;
+import io.opencensus.common.ToLongFunction;
+import io.opencensus.metrics.DerivedLongGauge;
+import io.opencensus.metrics.LabelValue;
+import io.opencensus.metrics.MetricOptions;
+import io.opencensus.metrics.MetricRegistry;
+import io.opencensus.metrics.Metrics;
 import io.opencensus.trace.Annotation;
 import io.opencensus.trace.AttributeValue;
 import io.opencensus.trace.Span;
@@ -49,6 +64,7 @@
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.LinkedList;
+import java.util.List;
 import java.util.Queue;
 import java.util.Random;
 import java.util.Set;
@@ -1116,11 +1132,15 @@ private static enum Position {
    * Return pool is immediately ready for use, though getting a session might block for sessions to
    * be created.
    */
-  static SessionPool createPool(SpannerOptions spannerOptions, SessionClient sessionClient) {
+  static SessionPool createPool(
+      SpannerOptions spannerOptions, SessionClient sessionClient, List<LabelValue> labelValues) {
     return createPool(
         spannerOptions.getSessionPoolOptions(),
         ((GrpcTransportOptions) spannerOptions.getTransportOptions()).getExecutorFactory(),
-        sessionClient);
+        sessionClient,
+        new Clock(),
+        Metrics.getMetricRegistry(),
+        labelValues);
   }
 
   static SessionPool createPool(
@@ -1135,8 +1155,31 @@ static SessionPool createPool(
       ExecutorFactory<ScheduledExecutorService> executorFactory,
       SessionClient sessionClient,
       Clock clock) {
+    return createPool(
+        poolOptions,
+        executorFactory,
+        sessionClient,
+        clock,
+        Metrics.getMetricRegistry(),
+        SPANNER_DEFAULT_LABEL_VALUES);
+  }
+
+  static SessionPool createPool(
+      SessionPoolOptions poolOptions,
+      ExecutorFactory<ScheduledExecutorService> executorFactory,
+      SessionClient sessionClient,
+      Clock clock,
+      MetricRegistry metricRegistry,
+      List<LabelValue> labelValues) {
     SessionPool pool =
-        new SessionPool(poolOptions, executorFactory, executorFactory.get(), sessionClient, clock);
+        new SessionPool(
+            poolOptions,
+            executorFactory,
+            executorFactory.get(),
+            sessionClient,
+            clock,
+            metricRegistry,
+            labelValues);
     pool.initPool();
     return pool;
   }
@@ -1146,13 +1189,16 @@ private SessionPool(
       ExecutorFactory<ScheduledExecutorService> executorFactory,
       ScheduledExecutorService executor,
       SessionClient sessionClient,
-      Clock clock) {
+      Clock clock,
+      MetricRegistry metricRegistry,
+      List<LabelValue> labelValues) {
     this.options = options;
     this.executorFactory = executorFactory;
     this.executor = executor;
     this.sessionClient = sessionClient;
     this.clock = clock;
     this.poolMaintainer = new PoolMaintainer();
+    this.initMetricsCollection(metricRegistry, labelValues);
   }
 
   @VisibleForTesting
@@ -1766,4 +1812,73 @@ public void onSessionCreateFailure(Throwable t, int createFailureForSessionCount
       }
     }
   }
+
+  /**
+   * Initializes and creates Spanner session relevant metrics. When coupled with an exporter, it
+   * allows users to monitor client behavior.
+   */
+  private void initMetricsCollection(MetricRegistry metricRegistry, List<LabelValue> labelValues) {
+    DerivedLongGauge maxInUseSessionsMetric =
+        metricRegistry.addDerivedLongGauge(
+            MAX_IN_USE_SESSIONS,
+            MetricOptions.builder()
+                .setDescription(MAX_IN_USE_SESSIONS_DESCRIPTION)
+                .setUnit(COUNT)
+                .setLabelKeys(SPANNER_LABEL_KEYS)
+                .build());
+
+    DerivedLongGauge maxAllowedSessionsMetric =
+        metricRegistry.addDerivedLongGauge(
+            MAX_ALLOWED_SESSIONS,
+            MetricOptions.builder()
+                .setDescription(MAX_ALLOWED_SESSIONS_DESCRIPTION)
+                .setUnit(COUNT)
+                .setLabelKeys(SPANNER_LABEL_KEYS)
+                .build());
+
+    DerivedLongGauge numInUseSessionsMetric =
+        metricRegistry.addDerivedLongGauge(
+            IN_USE_SESSIONS,
+            MetricOptions.builder()
+                .setDescription(IN_USE_SESSIONS_DESCRIPTION)
+                .setUnit(COUNT)
+                .setLabelKeys(SPANNER_LABEL_KEYS)
+                .build());
+
+    // The value of a maxSessionsInUse is observed from a callback function. This function is
+    // invoked whenever metrics are collected.
+    maxInUseSessionsMetric.createTimeSeries(
+        labelValues,
+        this,
+        new ToLongFunction<SessionPool>() {
+          @Override
+          public long applyAsLong(SessionPool sessionPool) {
+            return sessionPool.maxSessionsInUse;
+          }
+        });
+
+    // The value of a maxSessions is observed from a callback function. This function is invoked
+    // whenever metrics are collected.
+    maxAllowedSessionsMetric.createTimeSeries(
+        labelValues,
+        options,
+        new ToLongFunction<SessionPoolOptions>() {
+          @Override
+          public long applyAsLong(SessionPoolOptions options) {
+            return options.getMaxSessions();
+          }
+        });
+
+    // The value of a numSessionsInUse is observed from a callback function. This function is
+    // invoked whenever metrics are collected.
+    numInUseSessionsMetric.createTimeSeries(
+        labelValues,
+        this,
+        new ToLongFunction<SessionPool>() {
+          @Override
+          public long applyAsLong(SessionPool sessionPool) {
+            return sessionPool.numSessionsInUse;
+          }
+        });
+  }
 }

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java

@@ -16,6 +16,7 @@
 
 package com.google.cloud.spanner;
 
+import com.google.api.gax.core.GaxProperties;
 import com.google.api.gax.paging.Page;
 import com.google.cloud.BaseService;
 import com.google.cloud.PageImpl;
@@ -27,8 +28,10 @@
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableList;
 import com.google.common.util.concurrent.Futures;
 import com.google.common.util.concurrent.ListenableFuture;
+import io.opencensus.metrics.LabelValue;
 import io.opencensus.trace.Tracer;
 import io.opencensus.trace.Tracing;
 import java.util.ArrayList;
@@ -142,8 +145,14 @@ public DatabaseClient getDatabaseClient(DatabaseId db) {
       if (dbClients.containsKey(db)) {
         return dbClients.get(db);
       } else {
+        List<LabelValue> labelValues =
+            ImmutableList.of(
+                LabelValue.create(db.getDatabase()),
+                LabelValue.create(db.getInstanceId().getName()),
+                LabelValue.create(GaxProperties.getLibraryVersion(getOptions().getClass())));
         SessionPool pool =
-            SessionPool.createPool(getOptions(), SpannerImpl.this.getSessionClient(db));
+            SessionPool.createPool(
+                getOptions(), SpannerImpl.this.getSessionClient(db), labelValues);
         DatabaseClientImpl dbClient = createDatabaseClient(pool);
         dbClients.put(db, dbClient);
         return dbClient;