opendaylight/restconf/sal-rest-connector/src/main/java/org/opendaylight/netconf/sal/streams/listeners/ListenerAdapter.java

   1 /*
   2  * Copyright (c) 2014 Cisco Systems, Inc. and others.  All rights reserved.
   3  *
   4  * This program and the accompanying materials are made available under the
   5  * terms of the Eclipse Public License v1.0 which accompanies this distribution,
   6  * and is available at http://www.eclipse.org/legal/epl-v10.html
   7  */
   8 package org.opendaylight.netconf.sal.streams.listeners;
   9
  10 import com.google.common.base.Charsets;
  11 import com.google.common.base.Preconditions;
  12 import com.google.common.eventbus.AsyncEventBus;
  13 import com.google.common.eventbus.EventBus;
  14 import com.google.common.eventbus.Subscribe;
  15 import io.netty.channel.Channel;
  16 import io.netty.handler.codec.http.websocketx.TextWebSocketFrame;
  17 import io.netty.util.internal.ConcurrentSet;
  18 import java.io.ByteArrayOutputStream;
  19 import java.io.IOException;
  20 import java.io.OutputStreamWriter;
  21 import java.io.UnsupportedEncodingException;
  22 import java.text.SimpleDateFormat;
  23 import java.util.Collection;
  24 import java.util.Date;
  25 import java.util.HashMap;
  26 import java.util.Map;
  27 import java.util.Map.Entry;
  28 import java.util.Random;
  29 import java.util.Set;
  30 import java.util.concurrent.Executors;
  31 import java.util.regex.Pattern;
  32 import javax.xml.parsers.DocumentBuilder;
  33 import javax.xml.parsers.DocumentBuilderFactory;
  34 import javax.xml.parsers.ParserConfigurationException;
  35 import javax.xml.stream.XMLOutputFactory;
  36 import javax.xml.stream.XMLStreamException;
  37 import javax.xml.stream.XMLStreamWriter;
  38 import javax.xml.transform.OutputKeys;
  39 import javax.xml.transform.Transformer;
  40 import javax.xml.transform.TransformerException;
  41 import javax.xml.transform.TransformerFactory;
  42 import javax.xml.transform.dom.DOMResult;
  43 import javax.xml.transform.dom.DOMSource;
  44 import javax.xml.transform.stream.StreamResult;
  45 import org.opendaylight.controller.md.sal.common.api.data.AsyncDataChangeEvent;
  46 import org.opendaylight.controller.md.sal.dom.api.DOMDataChangeListener;
  47 import org.opendaylight.netconf.sal.restconf.impl.ControllerContext;
  48 import org.opendaylight.yangtools.concepts.ListenerRegistration;
  49 import org.opendaylight.yangtools.yang.common.QName;
  50 import org.opendaylight.yangtools.yang.data.api.YangInstanceIdentifier;
  51 import org.opendaylight.yangtools.yang.data.api.YangInstanceIdentifier.NodeIdentifierWithPredicates;
  52 import org.opendaylight.yangtools.yang.data.api.YangInstanceIdentifier.NodeWithValue;
  53 import org.opendaylight.yangtools.yang.data.api.YangInstanceIdentifier.PathArgument;
  54 import org.opendaylight.yangtools.yang.data.api.schema.NormalizedNode;
  55 import org.opendaylight.yangtools.yang.data.api.schema.NormalizedNodeContainer;
  56 import org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeStreamWriter;
  57 import org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter;
  58 import org.opendaylight.yangtools.yang.data.impl.codec.xml.XMLStreamNormalizedNodeStreamWriter;
  59 import org.opendaylight.yangtools.yang.data.impl.codec.xml.XmlDocumentUtils;
  60 import org.opendaylight.yangtools.yang.data.util.DataSchemaContextTree;
  61 import org.opendaylight.yangtools.yang.model.api.SchemaContext;
  62 import org.slf4j.Logger;
  63 import org.slf4j.LoggerFactory;
  64 import org.w3c.dom.Document;
  65 import org.w3c.dom.Element;
  66 import org.w3c.dom.Node;
  67
  68 /**
  69  * {@link ListenerAdapter} is responsible to track events, which occurred by changing data in data source.
  70  */
  71 public class ListenerAdapter implements DOMDataChangeListener {
  72
  73     private static final Logger LOG = LoggerFactory.getLogger(ListenerAdapter.class);
  74     private static final DocumentBuilderFactory DBF = DocumentBuilderFactory.newInstance();
  75     private static final TransformerFactory FACTORY = TransformerFactory.newInstance();
  76     private static final Pattern RFC3339_PATTERN = Pattern.compile("(\\d\\d)(\\d\\d)$");
  77
  78     private final SimpleDateFormat rfc3339 = new SimpleDateFormat("yyyy-MM-dd'T'hh:mm:ssZ");
  79
  80     private final YangInstanceIdentifier path;
  81     private ListenerRegistration<DOMDataChangeListener> registration;
  82     private final String streamName;
  83     private Set<Channel> subscribers = new ConcurrentSet<>();
  84     private final EventBus eventBus;
  85     private final EventBusChangeRecorder eventBusChangeRecorder;
  86
  87     /**
  88      * Creates new {@link ListenerAdapter} listener specified by path and stream name.
  89      *
  90      * @param path
  91      *            Path to data in data store.
  92      * @param streamName
  93      *            The name of the stream.
  94      */
  95     ListenerAdapter(final YangInstanceIdentifier path, final String streamName) {
  96         Preconditions.checkNotNull(path);
  97         Preconditions.checkArgument(streamName != null && !streamName.isEmpty());
  98         this.path = path;
  99         this.streamName = streamName;
 100         eventBus = new AsyncEventBus(Executors.newSingleThreadExecutor());
 101         eventBusChangeRecorder = new EventBusChangeRecorder();
 102         eventBus.register(eventBusChangeRecorder);
 103     }
 104
 105     @Override
 106     public void onDataChanged(final AsyncDataChangeEvent<YangInstanceIdentifier, NormalizedNode<?, ?>> change) {
 107         if (!change.getCreatedData().isEmpty() || !change.getUpdatedData().isEmpty()
 108                 || !change.getRemovedPaths().isEmpty()) {
 109             final String xml = prepareXmlFrom(change);
 110             final Event event = new Event(EventType.NOTIFY);
 111             event.setData(xml);
 112             eventBus.post(event);
 113         }
 114     }
 115
 116     /**
 117      * Tracks events of data change by customer.
 118      */
 119     private final class EventBusChangeRecorder {
 120         @Subscribe
 121         public void recordCustomerChange(final Event event) {
 122             if (event.getType() == EventType.REGISTER) {
 123                 final Channel subscriber = event.getSubscriber();
 124                 if (!subscribers.contains(subscriber)) {
 125                     subscribers.add(subscriber);
 126                 }
 127             } else if (event.getType() == EventType.DEREGISTER) {
 128                 subscribers.remove(event.getSubscriber());
 129                 Notificator.removeListenerIfNoSubscriberExists(ListenerAdapter.this);
 130             } else if (event.getType() == EventType.NOTIFY) {
 131                 for (final Channel subscriber : subscribers) {
 132                     if (subscriber.isActive()) {
 133                         LOG.debug("Data are sent to subscriber {}:", subscriber.remoteAddress());
 134                         subscriber.writeAndFlush(new TextWebSocketFrame(event.getData()));
 135                     } else {
 136                         LOG.debug("Subscriber {} is removed - channel is not active yet.", subscriber.remoteAddress());
 137                         subscribers.remove(subscriber);
 138                     }
 139                 }
 140             }
 141         }
 142     }
 143
 144     /**
 145      * Represents event of specific {@link EventType} type, holds data and {@link Channel} subscriber.
 146      */
 147     private final class Event {
 148         private final EventType type;
 149         private Channel subscriber;
 150         private String data;
 151
 152         /**
 153          * Creates new event specified by {@link EventType} type.
 154          *
 155          * @param type
 156          *            EventType
 157          */
 158         public Event(final EventType type) {
 159             this.type = type;
 160         }
 161
 162         /**
 163          * Gets the {@link Channel} subscriber.
 164          *
 165          * @return Channel
 166          */
 167         public Channel getSubscriber() {
 168             return subscriber;
 169         }
 170
 171         /**
 172          * Sets subscriber for event.
 173          *
 174          * @param subscriber
 175          *            Channel
 176          */
 177         public void setSubscriber(final Channel subscriber) {
 178             this.subscriber = subscriber;
 179         }
 180
 181         /**
 182          * Gets event String.
 183          *
 184          * @return String representation of event data.
 185          */
 186         public String getData() {
 187             return data;
 188         }
 189
 190         /**
 191          * Sets event data.
 192          *
 193          * @param data String.
 194          */
 195         public void setData(final String data) {
 196             this.data = data;
 197         }
 198
 199         /**
 200          * Gets event type.
 201          *
 202          * @return The type of the event.
 203          */
 204         public EventType getType() {
 205             return type;
 206         }
 207     }
 208
 209     /**
 210      * Type of the event.
 211      */
 212     private enum EventType {
 213         REGISTER,
 214         DEREGISTER,
 215         NOTIFY;
 216     }
 217
 218     /**
 219      * Prepare data in printable form and transform it to String.
 220      *
 221      * @param change
 222      *            DataChangeEvent
 223      * @return Data in printable form.
 224      */
 225     private String prepareXmlFrom(final AsyncDataChangeEvent<YangInstanceIdentifier, NormalizedNode<?, ?>> change) {
 226         final SchemaContext schemaContext = ControllerContext.getInstance().getGlobalSchema();
 227         final DataSchemaContextTree dataContextTree =  DataSchemaContextTree.from(schemaContext);
 228         final Document doc = createDocument();
 229         final Element notificationElement = doc.createElementNS("urn:ietf:params:xml:ns:netconf:notification:1.0",
 230                 "notification");
 231         doc.appendChild(notificationElement);
 232
 233         final Element eventTimeElement = doc.createElement("eventTime");
 234         eventTimeElement.setTextContent(toRFC3339(new Date()));
 235         notificationElement.appendChild(eventTimeElement);
 236
 237         final Element dataChangedNotificationEventElement = doc.createElementNS(
 238                 "urn:opendaylight:params:xml:ns:yang:controller:md:sal:remote", "data-changed-notification");
 239         addValuesToDataChangedNotificationEventElement(doc, dataChangedNotificationEventElement, change,
 240                 schemaContext, dataContextTree);
 241         notificationElement.appendChild(dataChangedNotificationEventElement);
 242
 243         try {
 244             final ByteArrayOutputStream out = new ByteArrayOutputStream();
 245             final Transformer transformer = FACTORY.newTransformer();
 246             transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no");
 247             transformer.setOutputProperty(OutputKeys.METHOD, "xml");
 248             transformer.setOutputProperty(OutputKeys.INDENT, "yes");
 249             transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");
 250             transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "4");
 251             transformer.transform(new DOMSource(doc), new StreamResult(new OutputStreamWriter(out, Charsets.UTF_8)));
 252             final byte[] charData = out.toByteArray();
 253             return new String(charData, "UTF-8");
 254         } catch (TransformerException | UnsupportedEncodingException e) {
 255             final String msg = "Error during transformation of Document into String";
 256             LOG.error(msg, e);
 257             return msg;
 258         }
 259     }
 260
 261     /**
 262      * Formats data specified by RFC3339.
 263      *
 264      * @param d
 265      *            Date
 266      * @return Data specified by RFC3339.
 267      */
 268     private String toRFC3339(final Date d) {
 269         return RFC3339_PATTERN.matcher(rfc3339.format(d)).replaceAll("$1:$2");
 270     }
 271
 272     /**
 273      * Creates {@link Document} document.
 274      * @return {@link Document} document.
 275      */
 276     private Document createDocument() {
 277         final DocumentBuilder bob;
 278         try {
 279             bob = DBF.newDocumentBuilder();
 280         } catch (final ParserConfigurationException e) {
 281             return null;
 282         }
 283         return bob.newDocument();
 284     }
 285
 286     /**
 287      * Adds values to data changed notification event element.
 288      *
 289      * @param doc
 290      *            {@link Document}
 291      * @param dataChangedNotificationEventElement
 292      *            {@link Element}
 293      * @param change
 294      *            {@link AsyncDataChangeEvent}
 295      */
 296     private void addValuesToDataChangedNotificationEventElement(final Document doc,
 297             final Element dataChangedNotificationEventElement,
 298             final AsyncDataChangeEvent<YangInstanceIdentifier, NormalizedNode<?, ?>> change,
 299             final SchemaContext  schemaContext, final DataSchemaContextTree dataSchemaContextTree) {
 300
 301         addCreatedChangedValuesFromDataToElement(doc, change.getCreatedData().entrySet(),
 302                 dataChangedNotificationEventElement,
 303                 Operation.CREATED, schemaContext, dataSchemaContextTree);
 304         if (change.getCreatedData().isEmpty()) {
 305             addCreatedChangedValuesFromDataToElement(doc, change.getUpdatedData().entrySet(),
 306                     dataChangedNotificationEventElement,
 307                     Operation.UPDATED, schemaContext, dataSchemaContextTree);
 308         }
 309         addValuesFromDataToElement(doc, change.getRemovedPaths(), dataChangedNotificationEventElement,
 310                 Operation.DELETED);
 311     }
 312
 313     /**
 314      * Adds values from data to element.
 315      *
 316      * @param doc
 317      *            {@link Document}
 318      * @param data
 319      *            Set of {@link YangInstanceIdentifier}.
 320      * @param element
 321      *            {@link Element}
 322      * @param operation
 323      *            {@link Operation}
 324      */
 325     private void addValuesFromDataToElement(final Document doc, final Set<YangInstanceIdentifier> data, final Element element,
 326             final Operation operation) {
 327         if (data == null || data.isEmpty()) {
 328             return;
 329         }
 330         for (final YangInstanceIdentifier path : data) {
 331             if (!ControllerContext.getInstance().isNodeMixin(path)) {
 332                 final Node node = createDataChangeEventElement(doc, path, operation);
 333                 element.appendChild(node);
 334             }
 335         }
 336     }
 337
 338     private void addCreatedChangedValuesFromDataToElement(final Document doc, final Set<Entry<YangInstanceIdentifier,
 339                 NormalizedNode<?,?>>> data, final Element element, final Operation operation, final SchemaContext
 340             schemaContext, final DataSchemaContextTree dataSchemaContextTree) {
 341         if (data == null || data.isEmpty()) {
 342             return;
 343         }
 344         for (Entry<YangInstanceIdentifier, NormalizedNode<?, ?>> entry : data) {
 345             if (!ControllerContext.getInstance().isNodeMixin(entry.getKey())) {
 346                 final Node node = createCreatedChangedDataChangeEventElement(doc, entry, operation, schemaContext,
 347                         dataSchemaContextTree);
 348                 element.appendChild(node);
 349             }
 350         }
 351     }
 352
 353     /**
 354      * Creates changed event element from data.
 355      *
 356      * @param doc
 357      *            {@link Document}
 358      * @param path
 359      *            Path to data in data store.
 360      * @param operation
 361      *            {@link Operation}
 362      * @return {@link Node} node represented by changed event element.
 363      */
 364     private Node createDataChangeEventElement(final Document doc, final YangInstanceIdentifier path, final Operation operation) {
 365         final Element dataChangeEventElement = doc.createElement("data-change-event");
 366         final Element pathElement = doc.createElement("path");
 367         addPathAsValueToElement(path, pathElement);
 368         dataChangeEventElement.appendChild(pathElement);
 369
 370         final Element operationElement = doc.createElement("operation");
 371         operationElement.setTextContent(operation.value);
 372         dataChangeEventElement.appendChild(operationElement);
 373
 374         return dataChangeEventElement;
 375     }
 376
 377     private Node createCreatedChangedDataChangeEventElement(final Document doc, final Entry<YangInstanceIdentifier,
 378             NormalizedNode<?, ?>> entry, final Operation operation, final SchemaContext
 379             schemaContext, final DataSchemaContextTree dataSchemaContextTree) {
 380         final Element dataChangeEventElement = doc.createElement("data-change-event");
 381         final Element pathElement = doc.createElement("path");
 382         final YangInstanceIdentifier path = entry.getKey();
 383         addPathAsValueToElement(path, pathElement);
 384         dataChangeEventElement.appendChild(pathElement);
 385
 386         final Element operationElement = doc.createElement("operation");
 387         operationElement.setTextContent(operation.value);
 388         dataChangeEventElement.appendChild(operationElement);
 389
 390         try {
 391             final DOMResult domResult = writeNormalizedNode((NormalizedNodeContainer<?,?,?>) entry.getValue(), path,
 392                     schemaContext, dataSchemaContextTree);
 393             final Node result = doc.importNode(domResult.getNode().getFirstChild(), true);
 394             dataChangeEventElement.appendChild(result);
 395         } catch (IOException e) {
 396             LOG.error("Error in writer ", e);
 397         } catch (XMLStreamException e) {
 398             LOG.error("Error processing stream", e);
 399         }
 400
 401         return dataChangeEventElement;
 402     }
 403
 404     private static DOMResult writeNormalizedNode(final NormalizedNodeContainer<?,?,?> normalized, final
 405         YangInstanceIdentifier path, final SchemaContext context, final DataSchemaContextTree dataSchemaContextTree) throws
 406             IOException, XMLStreamException {
 407         final XMLOutputFactory XML_FACTORY = XMLOutputFactory.newFactory();
 408         final Document doc = XmlDocumentUtils.getDocument();
 409         final DOMResult result = new DOMResult(doc, "data");
 410         NormalizedNodeWriter normalizedNodeWriter = null;
 411         NormalizedNodeStreamWriter normalizedNodeStreamWriter = null;
 412         XMLStreamWriter writer = null;
 413         try {
 414             writer = XML_FACTORY.createXMLStreamWriter(result);
 415             normalizedNodeStreamWriter = XMLStreamNormalizedNodeStreamWriter.create(writer, context, dataSchemaContextTree
 416                     .getChild(path).getDataSchemaNode().getPath().getParent());
 417             normalizedNodeWriter = NormalizedNodeWriter.forStreamWriter(normalizedNodeStreamWriter);
 418
 419             normalizedNodeWriter.write(normalized);
 420
 421             normalizedNodeWriter.flush();
 422         } finally {
 423             if (normalizedNodeWriter != null) {
 424                 normalizedNodeWriter.close();
 425             }
 426             if (normalizedNodeStreamWriter != null) {
 427                 normalizedNodeStreamWriter.close();
 428             }
 429             if (writer != null) {
 430                 writer.close();
 431             }
 432         }
 433
 434         return result;
 435     }
 436
 437     /**
 438      * Adds path as value to element.
 439      *
 440      * @param path
 441      *            Path to data in data store.
 442      * @param element
 443      *            {@link Element}
 444      */
 445     private void addPathAsValueToElement(final YangInstanceIdentifier path, final Element element) {
 446         // Map< key = namespace, value = prefix>
 447         final Map<String, String> prefixes = new HashMap<>();
 448         final YangInstanceIdentifier normalizedPath = ControllerContext.getInstance().toXpathRepresentation(path);
 449         final StringBuilder textContent = new StringBuilder();
 450
 451         // FIXME: BUG-1281: this is duplicated code from yangtools (BUG-1275)
 452         for (final PathArgument pathArgument : normalizedPath.getPathArguments()) {
 453             if (pathArgument instanceof YangInstanceIdentifier.AugmentationIdentifier) {
 454                 continue;
 455             }
 456             textContent.append("/");
 457             writeIdentifierWithNamespacePrefix(element, textContent, pathArgument.getNodeType(), prefixes);
 458             if (pathArgument instanceof NodeIdentifierWithPredicates) {
 459                 final Map<QName, Object> predicates = ((NodeIdentifierWithPredicates) pathArgument).getKeyValues();
 460                 for (final QName keyValue : predicates.keySet()) {
 461                     final String predicateValue = String.valueOf(predicates.get(keyValue));
 462                     textContent.append("[");
 463                     writeIdentifierWithNamespacePrefix(element, textContent, keyValue, prefixes);
 464                     textContent.append("='");
 465                     textContent.append(predicateValue);
 466                     textContent.append("'");
 467                     textContent.append("]");
 468                 }
 469             } else if (pathArgument instanceof NodeWithValue) {
 470                 textContent.append("[.='");
 471                 textContent.append(((NodeWithValue) pathArgument).getValue());
 472                 textContent.append("'");
 473                 textContent.append("]");
 474             }
 475         }
 476         element.setTextContent(textContent.toString());
 477     }
 478
 479     /**
 480      * Writes identifier that consists of prefix and QName.
 481      *
 482      * @param element
 483      *            {@link Element}
 484      * @param textContent
 485      *            StringBuilder
 486      * @param qName
 487      *            QName
 488      * @param prefixes
 489      *            Map of namespaces and prefixes.
 490      */
 491     private static void writeIdentifierWithNamespacePrefix(final Element element, final StringBuilder textContent,
 492             final QName qName, final Map<String, String> prefixes) {
 493         final String namespace = qName.getNamespace().toString();
 494         String prefix = prefixes.get(namespace);
 495         if (prefix == null) {
 496             prefix = generateNewPrefix(prefixes.values());
 497         }
 498
 499         element.setAttribute("xmlns:" + prefix, namespace);
 500         textContent.append(prefix);
 501         prefixes.put(namespace, prefix);
 502
 503         textContent.append(":");
 504         textContent.append(qName.getLocalName());
 505     }
 506
 507     /**
 508      * Generates new prefix which consists of four random characters <a-z>.
 509      *
 510      * @param prefixes
 511      *            Collection of prefixes.
 512      * @return New prefix which consists of four random characters <a-z>.
 513      */
 514     private static String generateNewPrefix(final Collection<String> prefixes) {
 515         StringBuilder result = null;
 516         final Random random = new Random();
 517         do {
 518             result = new StringBuilder();
 519             for (int i = 0; i < 4; i++) {
 520                 final int randomNumber = 0x61 + (Math.abs(random.nextInt()) % 26);
 521                 result.append(Character.toChars(randomNumber));
 522             }
 523         } while (prefixes.contains(result.toString()));
 524
 525         return result.toString();
 526     }
 527
 528     /**
 529      * Gets path pointed to data in data store.
 530      *
 531      * @return Path pointed to data in data store.
 532      */
 533     public YangInstanceIdentifier getPath() {
 534         return path;
 535     }
 536
 537     /**
 538      * Sets {@link ListenerRegistration} registration.
 539      *
 540      * @param registration DOMDataChangeListener registration
 541      */
 542     public void setRegistration(final ListenerRegistration<DOMDataChangeListener> registration) {
 543         this.registration = registration;
 544     }
 545
 546     /**
 547      * Gets the name of the stream.
 548      *
 549      * @return The name of the stream.
 550      */
 551     public String getStreamName() {
 552         return streamName;
 553     }
 554
 555     /**
 556      * Removes all subscribers and unregisters event bus change recorder form event bus.
 557      */
 558     public void close() throws Exception {
 559         subscribers = new ConcurrentSet<>();
 560         registration.close();
 561         registration = null;
 562         eventBus.unregister(eventBusChangeRecorder);
 563     }
 564
 565     /**
 566      * Checks if {@link ListenerRegistration} registration exist.
 567      *
 568      * @return True if exist, false otherwise.
 569      */
 570     public boolean isListening() {
 571         return registration == null ? false : true;
 572     }
 573
 574     /**
 575      * Creates event of type {@link EventType#REGISTER}, set {@link Channel} subscriber to the event and post event into
 576      * event bus.
 577      *
 578      * @param subscriber
 579      *            Channel
 580      */
 581     public void addSubscriber(final Channel subscriber) {
 582         if (!subscriber.isActive()) {
 583             LOG.debug("Channel is not active between websocket server and subscriber {}" + subscriber.remoteAddress());
 584         }
 585         final Event event = new Event(EventType.REGISTER);
 586         event.setSubscriber(subscriber);
 587         eventBus.post(event);
 588     }
 589
 590     /**
 591      * Creates event of type {@link EventType#DEREGISTER}, sets {@link Channel} subscriber to the event and posts event
 592      * into event bus.
 593      *
 594      * @param subscriber
 595      */
 596     public void removeSubscriber(final Channel subscriber) {
 597         LOG.debug("Subscriber {} is removed.", subscriber.remoteAddress());
 598         final Event event = new Event(EventType.DEREGISTER);
 599         event.setSubscriber(subscriber);
 600         eventBus.post(event);
 601     }
 602
 603     /**
 604      * Checks if exists at least one {@link Channel} subscriber.
 605      *
 606      * @return True if exist at least one {@link Channel} subscriber, false otherwise.
 607      */
 608     public boolean hasSubscribers() {
 609         return !subscribers.isEmpty();
 610     }
 611
 612     /**
 613      * Consists of two types {@link Store#CONFIG} and {@link Store#OPERATION}.
 614      */
 615     private static enum Store {
 616         CONFIG("config"),
 617         OPERATION("operation");
 618
 619         private final String value;
 620
 621         private Store(final String value) {
 622             this.value = value;
 623         }
 624     }
 625
 626     /**
 627      * Consists of three types {@link Operation#CREATED}, {@link Operation#UPDATED} and {@link Operation#DELETED}.
 628      */
 629     private static enum Operation {
 630         CREATED("created"),
 631         UPDATED("updated"),
 632         DELETED("deleted");
 633
 634         private final String value;
 635
 636         private Operation(final String value) {
 637             this.value = value;
 638         }
 639     }
 640
 641 }