Support for watching changes to static cluster metadata file

Change-Id: I5f9f89997288ca9a33a9e41f7520b875aceeffbe

@@ -15,11 +15,20 @@
  */
 package org.onosproject.cluster.impl;
 
+import static org.onlab.util.Tools.groupedThreads;
 import static org.slf4j.LoggerFactory.getLogger;
 
 import java.io.File;
 import java.io.IOException;
+import java.nio.file.FileSystems;
+import java.nio.file.Path;
+import java.nio.file.StandardWatchEventKinds;
+import java.nio.file.WatchEvent;
+import java.nio.file.WatchKey;
+import java.nio.file.WatchService;
 import java.util.Set;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicReference;
 
 import org.apache.felix.scr.annotations.Activate;
@@ -71,13 +80,17 @@ public class ConfigFileBasedClusterMetadataProvider implements ClusterMetadataPr
     private static final String PORT = "port";
     private static final String IP = "ip";
 
-    private static final File CONFIG_FILE = new File("../config/cluster.json");
+    private static final String CONFIG_DIR = "../config";
+    private static final String CONFIG_FILE_NAME = "cluster.json";
+    private static final File CONFIG_FILE = new File(CONFIG_DIR, CONFIG_FILE_NAME);
 
     @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
     protected ClusterMetadataProviderRegistry providerRegistry;
 
     private static final ProviderId PROVIDER_ID = new ProviderId("config", "none");
-    private AtomicReference<Versioned<ClusterMetadata>> cachedMetadata = new AtomicReference<>();
+    private final AtomicReference<Versioned<ClusterMetadata>> cachedMetadata = new AtomicReference<>();
+    private final ExecutorService configFileChangeDetector =
+            Executors.newSingleThreadExecutor(groupedThreads("onos/cluster/metadata/config-watcher", ""));
 
     private ObjectMapper mapper;
     private ClusterMetadataProviderService providerService;
@@ -95,11 +108,20 @@ public class ConfigFileBasedClusterMetadataProvider implements ClusterMetadataPr
         module.addDeserializer(PartitionId.class, new PartitionIdDeserializer());
         mapper.registerModule(module);
         providerService = providerRegistry.register(this);
+        configFileChangeDetector.execute(() -> {
+            try {
+                watchConfigFile();
+            } catch (IOException e) {
+                log.warn("Failure in setting up a watch for config "
+                        + "file updates. updates to {} will be ignored", CONFIG_FILE, e);
+            }
+        });
         log.info("Started");
     }
 
     @Deactivate
     public void deactivate() {
+        configFileChangeDetector.shutdown();
         providerRegistry.unregister(this);
         log.info("Stopped");
     }
@@ -114,7 +136,7 @@ public class ConfigFileBasedClusterMetadataProvider implements ClusterMetadataPr
         checkState(isAvailable());
         synchronized (this) {
             if (cachedMetadata.get() == null) {
-                loadMetadata();
+                cachedMetadata.set(fetchMetadata());
             }
             return cachedMetadata.get();
         }
@@ -151,20 +173,20 @@ public class ConfigFileBasedClusterMetadataProvider implements ClusterMetadataPr
         return CONFIG_FILE.exists();
     }
 
-    private void loadMetadata() {
+    private Versioned<ClusterMetadata> fetchMetadata() {
         ClusterMetadata metadata = null;
         long version = 0;
         try {
             metadata = mapper.readValue(CONFIG_FILE, ClusterMetadata.class);
-            version = metadata.hashCode();
+            version = CONFIG_FILE.lastModified();
         } catch (IOException e) {
             Throwables.propagate(e);
         }
-        cachedMetadata.set(new Versioned<>(new ClusterMetadata(PROVIDER_ID,
+        return new Versioned<>(new ClusterMetadata(PROVIDER_ID,
                                                    metadata.getName(),
                                                    Sets.newHashSet(metadata.getNodes()),
                                                    Sets.newHashSet(metadata.getPartitions())),
-                                           version));
+                               version);
     }
 
     private static class PartitionDeserializer extends JsonDeserializer<Partition> {
@@ -232,4 +254,34 @@ public class ConfigFileBasedClusterMetadataProvider implements ClusterMetadataPr
             return new NodeId(node.asText());
         }
     }
+
+    /**
+     * Monitors the config file for any updates and notifies providerService accordingly.
+     * @throws IOException
+     */
+    private void watchConfigFile() throws IOException {
+        WatchService watcher = FileSystems.getDefault().newWatchService();
+        Path configFilePath = FileSystems.getDefault().getPath(CONFIG_DIR);
+        configFilePath.register(watcher, StandardWatchEventKinds.ENTRY_MODIFY);
+        while (true) {
+            try {
+                final WatchKey watchKey = watcher.take();
+                for (WatchEvent<?> event : watchKey.pollEvents()) {
+                    final Path changed = (Path) event.context();
+                    log.info("{} was updated", changed);
+                    // TODO: Fix concurrency issues
+                    Versioned<ClusterMetadata> latestMetadata = fetchMetadata();
+                    cachedMetadata.set(latestMetadata);
+                    providerService.clusterMetadataChanged(latestMetadata);
+                }
+                if (!watchKey.reset()) {
+                    log.debug("WatchKey has been unregistered");
+                    break;
+                }
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+                break;
+            }
+        }
+    }
 }
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