Add stats for DiscoveryClient (#1282)

Introduce a nested class for `DiscoveryClient` to make it easier for
users to read helpful attributes that can assist in log analysis and
debugging. Most of these attributes are captured by existing metrics:

- `initTimestampMs`: timestamp when the client was initialized
- `localRegistrySize`: number of instances for all applications
- `lastSuccessfulRegistryFetchTimestampMs`: timestamp for last successful fetch
- `lastSuccessfulHeartbeatTimestampMs`: timestamp for last successful heartbeat

Add a new attribute called `initLocalRegistrySize`, which is the
number of instances for all applications read when the client was
initialized. Include a helper method that uses this attribute so
that users can determine whether the client's initial fetch of registry
information succeeded or failed. Note that other accessors are suffixed
with `Ms` (for milliseconds) to denote the time unit.

Additionally, update the existing `registrySize` with the value from
`initLocalRegistrySize` during initialization instead of waiting until
the first local registry refresh.

Co-authored-by: John Bennett <bennett@netflix.com>

eureka-client/src/main/java/com/netflix/discovery/DiscoveryClient.java

@@ -189,6 +189,9 @@ public class DiscoveryClient implements EurekaClient {
     protected final EurekaTransportConfig transportConfig;
 
     private final long initTimestampMs;
+    private final int initRegistrySize;
+
+    private final Stats stats = new Stats();
 
     private static final class EurekaTransport {
         private ClosableResolver bootstrapResolver;
@@ -381,8 +384,10 @@ public synchronized BackupRegistry get() {
             DiscoveryManager.getInstance().setEurekaClientConfig(config);
 
             initTimestampMs = System.currentTimeMillis();
+            initRegistrySize = this.getApplications().size();
+            registrySize = initRegistrySize;
             logger.info("Discovery Client initialized at timestamp {} with initial instances count: {}",
-                    initTimestampMs, this.getApplications().size());
+                    initTimestampMs, initRegistrySize);
 
             return;  // no need to setup up an network tasks and we are done
         }
@@ -465,8 +470,10 @@ public synchronized BackupRegistry get() {
         DiscoveryManager.getInstance().setEurekaClientConfig(config);
 
         initTimestampMs = System.currentTimeMillis();
+        initRegistrySize = this.getApplications().size();
+        registrySize = initRegistrySize;
         logger.info("Discovery Client initialized at timestamp {} with initial instances count: {}",
-                initTimestampMs, this.getApplications().size());
+                initTimestampMs, initRegistrySize);
     }
 
     private void scheduleServerEndpointTask(EurekaTransport eurekaTransport,
@@ -1743,4 +1750,53 @@ private long computeStalenessMonitorDelay(long delay) {
         }
     }
 
+    /**
+     * Gets stats for the DiscoveryClient.
+     *
+     * @return The DiscoveryClientStats instance.
+     */
+    public Stats getStats() {
+        return stats;
+    }
+
+    /**
+     * Stats is used to track useful attributes of the DiscoveryClient. It includes helpers that can aid
+     * debugging and log analysis.
+     */
+    public class Stats {
+
+        private Stats() {}
+
+        public int initLocalRegistrySize() {
+            return initRegistrySize;
+        }
+
+        public long initTimestampMs() {
+            return initTimestampMs;
+        }
+
+        public int localRegistrySize() {
+            return registrySize;
+        }
+
+        public long lastSuccessfulRegistryFetchTimestampMs() {
+            return lastSuccessfulRegistryFetchTimestamp;
+        }
+
+        public long lastSuccessfulHeartbeatTimestampMs() {
+            return lastSuccessfulHeartbeatTimestamp;
+        }
+
+        /**
+         * Used to determine if the Discovery client's first attempt to fetch from the service registry succeeded with
+         * non-empty results.
+         *
+         * @return true if succeeded, failed otherwise
+         */
+        public boolean initSucceeded() {
+            return initLocalRegistrySize() > 0 && initTimestampMs() > 0;
+        }
+
+    }
+
 }

eureka-client/src/test/java/com/netflix/discovery/DiscoveryClientStatsInitFailedTest.java

@@ -0,0 +1,41 @@
+package com.netflix.discovery;
+
+import com.netflix.discovery.junit.resource.DiscoveryClientResource;
+import org.junit.After;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+/**
+ * Tests for DiscoveryClient stats reported when initial registry fetch fails.
+ */
+public class DiscoveryClientStatsInitFailedTest extends BaseDiscoveryClientTester {
+
+    @Before
+    public void setUp() throws Exception {
+        setupProperties();
+        populateRemoteRegistryAtStartup();
+        setupDiscoveryClient();
+    }
+
+    @After
+    public void tearDown() throws Exception {
+        shutdownDiscoveryClient();
+        DiscoveryClientResource.clearDiscoveryClientConfig();
+    }
+
+    @Test
+    public void testEmptyInitLocalRegistrySize() throws Exception {
+        Assert.assertTrue(client instanceof DiscoveryClient);
+        DiscoveryClient clientImpl = (DiscoveryClient) client;
+        Assert.assertEquals(0, clientImpl.getStats().initLocalRegistrySize());
+    }
+
+    @Test
+    public void testInitFailed() throws Exception {
+        Assert.assertTrue(client instanceof DiscoveryClient);
+        DiscoveryClient clientImpl = (DiscoveryClient) client;
+        Assert.assertFalse(clientImpl.getStats().initSucceeded());
+    }
+
+}

eureka-client/src/test/java/com/netflix/discovery/DiscoveryClientStatsTest.java

@@ -0,0 +1,25 @@
+package com.netflix.discovery;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Tests for DiscoveryClient stats reported when initial registry fetch succeeds.
+ */
+public class DiscoveryClientStatsTest extends AbstractDiscoveryClientTester {
+
+    @Test
+    public void testNonEmptyInitLocalRegistrySize() throws Exception {
+        Assert.assertTrue(client instanceof DiscoveryClient);
+        DiscoveryClient clientImpl = (DiscoveryClient) client;
+        Assert.assertEquals(createLocalApps().size(), clientImpl.getStats().initLocalRegistrySize());
+    }
+
+    @Test
+    public void testInitSucceeded() throws Exception {
+        Assert.assertTrue(client instanceof DiscoveryClient);
+        DiscoveryClient clientImpl = (DiscoveryClient) client;
+        Assert.assertTrue(clientImpl.getStats().initSucceeded());
+    }
+
+}