[从源码学设计]蚂蚁金服SOFARegistry 之 ChangeNotifier
2021/4/26 12:27:24
本文主要是介绍[从源码学设计]蚂蚁金服SOFARegistry 之 ChangeNotifier，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！
SOFARegistry 是蚂蚁金服开源的一个生产级、高时效、高可用的服务注册中心。 本系列文章重点在于分析设计和架构，即利用多篇文章，从多个角度反推总结 DataServer 或者 SOFARegistry 的实现机制和架构思路，让大家借以学习阿里如何设计。 本文为第十五篇，分析如何执行ChangeNotifier 来通知相关模块：hi，这里有新数据变化来到了，兄弟们走起来。
[从源码学设计]蚂蚁金服SOFARegistry 之 ChangeNotifier
目录
- [从源码学设计]蚂蚁金服SOFARegistry 之 ChangeNotifier
- 3.1 放入消息
- 3.2 消费消息&发送通知
- 3.1.1 PublishDataHandler
- 3.1.2 DataChangeEventCenter
- 3.2.7.1 时间轮
- 3.2.1 DataChangeHandler
- 3.2.2 类定义
- 3.2.3 执行引擎
- 3.2.4 业务执行
- 3.2.5 通知
- 3.2.6 BackUpNotifier同步
- 3.2.7 SessionServerNotifier通知数据变化
- 2.1 接口定义
- 2.2 派生类
- 2.3 Bean
- 1.1 概述
- 1.2 数据变化
- 0x00 摘要
- 0x01 业务范畴
- 0x02 数据结构
- 0x03 流程
- 0x04 总结
- 0xFF 参考
0x00 摘要
SOFARegistry 是蚂蚁金服开源的一个生产级、高时效、高可用的服务注册中心。
本系列文章重点在于分析设计和架构，即利用多篇文章，从多个角度反推总结 DataServer 或者 SOFARegistry 的实现机制和架构思路，让大家借以学习阿里如何设计。
本文为第十五篇，分析如何执行ChangeNotifier 来通知相关模块：hi，这里有新数据变化来到了，兄弟们走起来。
0x01 业务范畴
1.1 概述
当有数据发布者 publisher 上下线时，会分别触发 publishDataProcessor 或 unPublishDataHandler，Handler 会往 dataChangeEventCenter 中添加一个数据变更事件，用于异步地通知事件变更中心数据的变更。事件变更中心收到该事件之后，会往队列中加入事件。
此时 dataChangeEventCenter 会根据不同的事件类型异步地对上下线数据进行相应的处理，即把这个事件变更信息变成ChangeNotifier ，进而变成Operator，放到AbstractAcceptorStore；
与此同时 DataChangeHandler 会把这个事件变更信息通过 ChangeNotifier 对外发布，通知其他节点进行数据同步。
因为篇幅限制，前文对 ChangeNotifier 这部分只是略过，本文就详细讲解下事件变更通知ChangeNotifier。 这里会再把整理流程串起来，会涉及到前面某些文章内容。
先给出图示以便大家了解 ChangeNotifier 的作用。
+--------------------+ | PublishDataHandler | +--------+-----------+ | | | publisher | v +---------+------------+ |DataChangeEventCenter | +---------+------------+ | | | ChangeData v +---------+------------+ | DataChangeEventQueue | +---------+------------+ | | | ChangeData v +-------+----------+ | DataChangeHandler| +-------+----------+ | | | ChangeData v +------+--------+ +------------+ | ChangeNotifier| +--------> | datumCache | +------+--------+ +------------+ | | v +---+------+ | notifier | +---+------+ | v +-----------+---------------+ | | v v +----+----------------+ +------+----------+ |SessionServerNotifier| | BackUpNotifier | +----+----------------+ +------+----------+ | | | | | | | v +--v------------+ +------+----------------+ | sessionServer | | AbstractAcceptorStore | +---------------+ +-----------------------+
1.2 数据变化
数据变化有两个方向
数据服务器节点变化；
数据的变化，即Publisher和Scriber的变化；
ChangeNotifier就是负责把 Publisher和Scriber的变化 通知给相关模块。变更通知就是一种解耦。
0x02 数据结构
我们首先需要看看通知的数据结构。
2.1 接口定义
IDataChangeNotifier是通知的接口定义：
public interface IDataChangeNotifier {
SetgetSuitableSource();
/**
*
* @param datum
* @param lastVersion
*/
void notify(Datum datum, Long lastVersion);
}2.2 派生类
IDataChangeNotifier 有四个派生类，分别对应了具体数据变化的四种可能，从名字大约可以判断出用途。
public class BackUpNotifier implements IDataChangeNotifier public class SessionServerNotifier implements IDataChangeNotifier public class SnapshotBackUpNotifier implements IDataChangeNotifier public class TempPublisherNotifier implements IDataChangeNotifier
2.3 Bean
对应的Bean如下：
@Bean(name = "dataChangeNotifiers")
public ListdataChangeNotifiers() {
Listlist = new ArrayList<>();
list.add(sessionServerNotifier());
list.add(tempPublisherNotifier());
list.add(backUpNotifier());
return list;
}0x03 流程
我们从头理一下流程。
3.1 放入消息
当有数据发布者 publisher 上下线时，会分别触发 publishDataProcessor 或 unPublishDataHandler ，Handler 会往 dataChangeEventCenter 中添加一个数据变更事件，用于异步地通知事件变更中心数据的变更。事件变更中心收到该事件之后，会往队列中加入事件。
在DataServer这里，具体流程如下：
3.1.1 PublishDataHandler
PublishDataHandler 响应 PublishDataRequest。当有Publisher时候，就往DataChangeEventCenter放入消息。即调用下面来放入消息
dataChangeEventCenter.onChange(publisher, dataServerConfig.getLocalDataCenter());
具体代码如下：
public class PublishDataHandler extends AbstractServerHandler{
@Autowired
private ForwardService forwardService;
@Autowired
private SessionServerConnectionFactory sessionServerConnectionFactory;
@Autowired
private DataChangeEventCenter dataChangeEventCenter;
@Autowired
private DataServerConfig dataServerConfig;
@Autowired
private DatumLeaseManager datumLeaseManager;
@Autowired
private ThreadPoolExecutor publishProcessorExecutor;
@Override
public Object doHandle(Channel channel, PublishDataRequest request) {
Publisher publisher = Publisher.internPublisher(request.getPublisher());
if (forwardService.needForward()) {
CommonResponse response = new CommonResponse();
response.setSuccess(false);
response.setMessage("Request refused, Server status is not working");
return response;
}
dataChangeEventCenter.onChange(publisher, dataServerConfig.getLocalDataCenter());
if (publisher.getPublishType() != PublishType.TEMPORARY) {
String connectId = WordCache.getInstance().getWordCache(
publisher.getSourceAddress().getAddressString());
sessionServerConnectionFactory.registerConnectId(request.getSessionServerProcessId(),
connectId);
// record the renew timestamp
datumLeaseManager.renew(connectId);
}
return CommonResponse.buildSucce***esponse();
}
}此时具体逻辑如下：
+--------------------+ | PublishDataHandler | +--------+-----------+ | | | publisher | v +---------+------------+ |DataChangeEventCenter | +---------+------------+
3.1.2 DataChangeEventCenter
DataChangeEventCenter 的核心是一个DataChangeEventQueue数组，
DataChangeEventCenter . onChange函数会首先根据Publisher的DataInfoId获取hash，根据这个hash数值来决定把 DataChangeEvent 消息放入哪个queue来处理，就是调用这个 queue的 onChange 函数。
public class DataChangeEventCenter {
/**
* queues of DataChangeEvent
*/
private DataChangeEventQueue[] dataChangeEventQueues;
@Autowired
private DatumCache datumCache;
@PostConstruct
public void init() {
if (isInited.compareAndSet(false, true)) {
queueCount = dataServerConfig.getQueueCount();
dataChangeEventQueues = new DataChangeEventQueue[queueCount];
for (int idx = 0; idx < queueCount; idx++) {
dataChangeEventQueues[idx] = new DataChangeEventQueue(idx, dataServerConfig, this,
datumCache);
dataChangeEventQueues[idx].start();
}
}
}
/**
* receive changed publisher, then wrap it into the DataChangeEvent and put it into dataChangeEventQueue
*
* @param publisher
* @param dataCenter
*/
public void onChange(Publisher publisher, String dataCenter) {
int idx = hash(publisher.getDataInfoId());
Datum datum = new Datum(publisher, dataCenter);
if (publisher instanceof UnPublisher) {
datum.setContainsUnPub(true);
}
if (publisher.getPublishType() != PublishType.TEMPORARY) {
dataChangeEventQueues[idx].onChange(new DataChangeEvent(DataChangeTypeEnum.MERGE,
DataSourceTypeEnum.PUB, datum));
} else {
dataChangeEventQueues[idx].onChange(new DataChangeEvent(DataChangeTypeEnum.MERGE,
DataSourceTypeEnum.PUB_TEMP, datum));
}
}
}DataChangeEventQueue 的主要数据成员如下：
public class DataChangeEventQueue {
/**
* a block queue that stores all data change events
*/
private final BlockingQueueeventQueue;
private final Map<String, Map> CHANGE_DATA_MAP_FOR_MERGE = new ConcurrentHashMap<>();
private final DelayQueueCHANGE_QUEUE = new DelayQueue();
private DataChangeEventCenter dataChangeEventCenter;
private DatumCache datumCache;
}其执行引擎是一个线程，其block在 BlockingQueue eventQueue 之上，当有消息时候，就取出消息，针对消息类型做不同处理。
public void start() {
Executor executor = ExecutorFactory
.newSingleThreadExecutor(String.format("%s_%s", DataChangeEventQueue.class.getSimpleName(), getName()));
executor.execute(() -> {
while (true) {
try {
IDataChangeEvent event = eventQueue.take();
DataChangeScopeEnum scope = event.getScope();
if (scope == DataChangeScopeEnum.DATUM) {
DataChangeEvent dataChangeEvent = (DataChangeEvent) event;
//Temporary push data will be notify as soon as,and not merge to normal pub data;
if (dataChangeEvent.getSourceType() == DataSourceTypeEnum.PUB_TEMP) {
addTempChangeData(dataChangeEvent.getDatum(), dataChangeEvent.getChangeType(),
dataChangeEvent.getSourceType());
} else {
handleDatum(dataChangeEvent.getChangeType(), dataChangeEvent.getSourceType(),
dataChangeEvent.getDatum());
}
} else if (scope == DataChangeScopeEnum.CLIENT) {
handleClientOff((ClientChangeEvent) event);
} else if (scope == DataChangeScopeEnum.SNAPSHOT) {
handleSnapshot((DatumSnapshotEvent) event);
}
}
}
});
}对于 Publisher 消息类型，handleDatum 函数会根据changeType是 COVER 还是 MERGE 来做不同处理。
在此步骤中，也会把 ChangeData 放入 CHANGE_QUEUE.put(changeData);
private void handleDatum(DataChangeTypeEnum changeType, DataSourceTypeEnum sourceType,
Datum targetDatum) {
lock.lock();
try {
//get changed datum
ChangeData changeData = getChangeData(targetDatum.getDataCenter(),
targetDatum.getDataInfoId(), sourceType, changeType);
Datum cacheDatum = changeData.getDatum();
if (changeType == DataChangeTypeEnum.COVER || cacheDatum == null) {
changeData.setDatum(targetDatum);
} else {
MaptargetPubMap = targetDatum.getPubMap();
MapcachePubMap = cacheDatum.getPubMap();
for (Publisher pub : targetPubMap.values()) {
String registerId = pub.getRegisterId();
Publisher cachePub = cachePubMap.get(registerId);
if (cachePub != null) {
// if the registerTimestamp of cachePub is greater than the registerTimestamp of pub, it means
// that pub is not the newest data, should be ignored
if (pub.getRegisterTimestamp() < cachePub.getRegisterTimestamp()) {
continue;
}
// if pub and cachePub both are publisher, and sourceAddress of both are equal,
// and version of cachePub is greater than version of pub, should be ignored
if (!(pub instanceof UnPublisher) && !(cachePub instanceof UnPublisher)
&& pub.getSourceAddress().equals(cachePub.getSourceAddress())
&& cachePub.getVersion() > pub.getVersion()) {
continue;
}
}
cachePubMap.put(registerId, pub);
cacheDatum.setVersion(targetDatum.getVersion());
}
}
} finally {
lock.unlock();
}
}此时具体逻辑如下：
+--------------------+ | PublishDataHandler | +--------+-----------+ | | | publisher | v +---------+------------+ |DataChangeEventCenter | +---------+------------+ | | | ChangeData v +---------+------------+ | DataChangeEventQueue | +---------+------------+
3.2 消费消息&发送通知
DataChangeHandler 会针对每个DataChangeEventQueue进行消费通知。
public class DataChangeHandler {
@Autowired
private DataChangeEventCenter dataChangeEventCenter;
@Autowired
private DatumCache datumCache;
@Resource
private ListdataChangeNotifiers;
@PostConstruct
public void start() {
DataChangeEventQueue[] queues = dataChangeEventCenter.getQueues();
int queueCount = queues.length;
Executor executor = ExecutorFactory.newFixedThreadPool(queueCount, DataChangeHandler.class.getSimpleName());
Executor notifyExecutor = ExecutorFactory
.newFixedThreadPool(dataServerConfig.getQueueCount() * 5, this.getClass().getSimpleName());
for (int idx = 0; idx < queueCount; idx++) {
final DataChangeEventQueue dataChangeEventQueue = queues[idx];
final String name = dataChangeEventQueue.getName();
executor.execute(() -> {
while (true) {
try {
final ChangeData changeData = dataChangeEventQueue.take();
notifyExecutor.execute(new ChangeNotifier(changeData, name));
}
}
});
}
}
}3.2.1 DataChangeHandler
DataChangeHandler 会定期提取DataChangeEventCenter中的消息，然后进行处理。
3.2.2 类定义
public class DataChangeHandler {
@Autowired
private DataServerConfig dataServerConfig;
@Autowired
private DataChangeEventCenter dataChangeEventCenter;
@Autowired
private DatumCache datumCache;
@Resource
private ListdataChangeNotifiers;
}3.2.3 执行引擎
这里是一个双层线程模型。
executor = ExecutorFactory.newFixedThreadPool(queueCount)
notifyExecutor= ExecutorFactory.newFixedThreadPool(dataServerConfig.getQueueCount() * 5)
可以认为 executor 是控制线程，notifierExecutor是工作线程，工作线程是控制线程的5倍。
- DataChangeHandler 会遍历 DataChangeEventCenter 中所有 DataChangeEventQueue，
- 针对每一个dataChangeEventQueue调用executor的一个控制线程，
- 在这个控制线程里面，可以从 DataChangeEventQueue 之中取出ChangeData，针对每一个ChangeData，调用notifyExecutor的一个工作线程，生成一个ChangeNotifier进行处理。
@PostConstruct
public void start() {
DataChangeEventQueue[] queues = dataChangeEventCenter.getQueues();
int queueCount = queues.length;
Executor executor = ExecutorFactory.newFixedThreadPool(queueCount, DataChangeHandler.class.getSimpleName());
Executor notifyExecutor = ExecutorFactory
.newFixedThreadPool(dataServerConfig.getQueueCount() * 5, this.getClass().getSimpleName());
for (int idx = 0; idx < queueCount; idx++) {
final DataChangeEventQueue dataChangeEventQueue = queues[idx];
final String name = dataChangeEventQueue.getName();
executor.execute(() -> {
while (true) {
final ChangeData changeData = dataChangeEventQueue.take();
notifyExecutor.execute(new ChangeNotifier(changeData, name));
}
});
}
}3.2.4 业务执行
对于 ChangeData，会生成 ChangeNotifier 进行处理。会把这个事件变更信息通过 ChangeNotifier 对外发布，通知其他节点进行数据同步。
在 ChangeNotifier 之中，会判断changeData的类型做不同处理。
- 如果是SnapshotData，则：
- 生成SnapshotData；
- 调用 datumCache.putSnapshot 做存储；
- 调用notify做通知；
- 如果是其他类型，则：
- 对于pub or unPub merge，需要datum.updateVersion();
- 如果是 PUB_TEMP，则notifyTempPub(datum, sourceType, changeType);
- 如果是版本更新，则notify(datum, sourceType, lastVersion);
具体如下：
private class ChangeNotifier implements Runnable {
private ChangeData changeData;
private String name;
@Override
public void run() {
if (changeData instanceof SnapshotData) {
......
} else {
Datum datum = changeData.getDatum();
String dataCenter = datum.getDataCenter();
String dataInfoId = datum.getDataInfoId();
DataSourceTypeEnum sourceType = changeData.getSourceType();
DataChangeTypeEnum changeType = changeData.getChangeType();
if (changeType == DataChangeTypeEnum.MERGE
&& sourceType != DataSourceTypeEnum.BACKUP
&& sourceType != DataSourceTypeEnum.SYNC) {
//update version for pub or unPub merge to cache
//if the version product before merge to cache,it may be cause small version override big one
datum.updateVersion();
}
long version = datum.getVersion();
try {
if (sourceType == DataSourceTypeEnum.CLEAN) {
if (datumCache.cleanDatum(dataCenter, dataInfoId)) {
......
}
} else if (sourceType == DataSourceTypeEnum.PUB_TEMP) {
notifyTempPub(datum, sourceType, changeType);
} else {
MergeResult mergeResult = datumCache.putDatum(changeType, datum);
Long lastVersion = mergeResult.getLastVersion();
if (lastVersion != null
&& lastVersion.longValue() == LocalDatumStorage.ERROR_DATUM_VERSION) {
return;
}
//lastVersion null means first add datum
if (lastVersion == null || version != lastVersion) {
if (mergeResult.isChangeFlag()) {
notify(datum, sourceType, lastVersion);
}
}
}
}
}
}
}此时具体逻辑如下：
+--------------------+ | PublishDataHandler | +--------+-----------+ | | | publisher | v +---------+------------+ |DataChangeEventCenter | +---------+------------+ | | | ChangeData v +---------+------------+ | DataChangeEventQueue | +---------+------------+ | | | ChangeData v +-------+----------+ | DataChangeHandler| +-------+----------+ | | | ChangeData v +------+--------+ +------------+ | ChangeNotifier| +--------> | datumCache | +------+--------+ +------------+
3.2.5 通知
notify函数会遍历dataChangeNotifiers，找出可以支持本Datum对应SourceType的Notifier来执行。
具体如何支持哪些函数，是由getSuitableSource设置的。
private void notify(Datum datum, DataSourceTypeEnum sourceType, Long lastVersion) {
for (IDataChangeNotifier notifier : dataChangeNotifiers) {
if (notifier.getSuitableSource().contains(sourceType)) {
notifier.notify(datum, lastVersion);
}
}
}对应的Bean是：
@Bean(name = "dataChangeNotifiers")
public ListdataChangeNotifiers() {
Listlist = new ArrayList<>();
list.add(sessionServerNotifier());
list.add(tempPublisherNotifier());
list.add(backUpNotifier());
return list;
}3.2.6 BackUpNotifier同步
就是调用 syncDataService.appendOperator 进行通知，其实就是把 Datum 变成 Operator，存到AbstractAcceptorStore。
public class BackUpNotifier implements IDataChangeNotifier {
@Autowired
private SyncDataService syncDataService;
@Override
public SetgetSuitableSource() {
Setset = new HashSet<>();
set.add(DataSourceTypeEnum.PUB);
return set;
}
@Override
public void notify(Datum datum, Long lastVersion) {
syncDataService.appendOperator(new Operator(datum.getVersion(), lastVersion, datum,
DataSourceTypeEnum.BACKUP));
}
}3.2.7 SessionServerNotifier通知数据变化
SessionServerNotifier 则要复杂很多。
public class SessionServerNotifier implements IDataChangeNotifier {
private AsyncHashedWheelTimer asyncHashedWheelTimer;
@Autowired
private DataServerConfig dataServerConfig;
@Autowired
private Exchange boltExchange;
@Autowired
private SessionServerConnectionFactory sessionServerConnectionFactory;
@Autowired
private DatumCache datumCache;
@Override
public SetgetSuitableSource() {
Setset = new HashSet<>();
set.add(DataSourceTypeEnum.PUB);
set.add(DataSourceTypeEnum.SYNC);
set.add(DataSourceTypeEnum.SNAPSHOT);
return set;
}
}3.2.7.1 时间轮
建立了一个500毫秒的时间轮。
@PostConstruct
public void init() {
ThreadFactoryBuilder threadFactoryBuilder = new ThreadFactoryBuilder();
threadFactoryBuilder.setDaemon(true);
asyncHashedWheelTimer = new AsyncHashedWheelTimer(threadFactoryBuilder.setNameFormat(
"Registry-SessionServerNotifier-WheelTimer").build(), 500, TimeUnit.MILLISECONDS, 1024,
dataServerConfig.getSessionServerNotifierRetryExecutorThreadSize(),
dataServerConfig.getSessionServerNotifierRetryExecutorQueueSize(), threadFactoryBuilder
.setNameFormat("Registry-SessionServerNotifier-WheelExecutor-%d").build(),
new TaskFailedCallback() {
@Override
public void executionRejected(Throwable e) {
LOGGER.error("executionRejected: " + e.getMessage(), e);
}
@Override
public void executionFailed(Throwable e) {
LOGGER.error("executionFailed: " + e.getMessage(), e);
}
});
}从业务角度看，当有publisher相关消息来临时候，
DataChangeHandler的notify函数会遍历dataChangeNotifiers，找出可以支持本Datum对应SourceType的Notifier来执行。
private void notify(Datum datum, DataSourceTypeEnum sourceType, Long lastVersion) {
for (IDataChangeNotifier notifier : dataChangeNotifiers) {
if (notifier.getSuitableSource().contains(sourceType)) {
notifier.notify(datum, lastVersion);
}
}
}到了SessionServerNotifier这里的notify函数，会遍历目前缓存的所有Connection，逐一通知。
@Override
public void notify(Datum datum, Long lastVersion) {
DataChangeRequest request = new DataChangeRequest(datum.getDataInfoId(),
datum.getDataCenter(), datum.getVersion());
Listconnections = sessionServerConnectionFactory.getSessionConnections();
for (Connection connection : connections) {
doNotify(new NotifyCallback(connection, request));
}
}具体通知函数：
private void doNotify(NotifyCallback notifyCallback) {
Connection connection = notifyCallback.connection;
DataChangeRequest request = notifyCallback.request;
try {
//check connection active
if (!connection.isFine()) {
return;
}
Server sessionServer = boltExchange.getServer(dataServerConfig.getPort());
sessionServer.sendCallback(sessionServer.getChannel(connection.getRemoteAddress()),
request, notifyCallback, dataServerConfig.getRpcTimeout());
} catch (Exception e) {
onFailed(notifyCallback);
}
}而时间轮是在调用失败的重试中使用。
就是当没有达到失败重试最大次数时，进行定时重试。
private void onFailed(NotifyCallback notifyCallback) {
DataChangeRequest request = notifyCallback.request;
Connection connection = notifyCallback.connection;
notifyCallback.retryTimes++;
//check version, if it's fall behind, stop retry
long _currentVersion = datumCache.get(request.getDataCenter(), request.getDataInfoId()).getVersion();
if (request.getVersion() != _currentVersion) {
return;
}
if (notifyCallback.retryTimes {
//check version, if it's fall behind, stop retry
long currentVersion = datumCache.get(request.getDataCenter(), request.getDataInfoId()).getVersion();
if (request.getVersion() == currentVersion) {
doNotify(notifyCallback);
}
}, getDelayTimeForRetry(notifyCallback.retryTimes), TimeUnit.MILLISECONDS);
}
}具体逻辑如下：
+--------------------+ | PublishDataHandler | +--------+-----------+ | | | publisher | v +---------+------------+ |DataChangeEventCenter | +---------+------------+ | | | ChangeData v +---------+------------+ | DataChangeEventQueue | +---------+------------+ | | | ChangeData v +-------+----------+ | DataChangeHandler| +-------+----------+ | | | ChangeData v +------+--------+ +------------+ | ChangeNotifier| +--------> | datumCache | +------+--------+ +------------+ | | v +---+------+ | notifier | +---+------+ | v +-----------+---------------+ | | v v +----+----------------+ +------+----------+ |SessionServerNotifier| | BackUpNotifier | +----+----------------+ +------+----------+ | | | | | | | v +--v------------+ +------+----------------+ | sessionServer | | AbstractAcceptorStore | +---------------+ +-----------------------+
0x04 总结
本文是把注册中的一个点“事件变更通知ChangeNotifie“进行细化展开，以 SessionServerNotifier 和 BackUpNotifier 为例，为大家进行解释ChangeNotifier的原理和使用。把包括 dataChangeEventCenter 等功能也梳理了一下，希望对大家有所帮助。
在 DataServer，数据变化有两个方向：
数据服务器节点变化；
数据的变化，即 Publisher 和 Scriber 的变化；
ChangeNotifier就是负责把 Publisher 和 Scriber 的变化 通知给相关模块。变更通知就是一种解耦。
0xFF 参考
[从源码学设计]蚂蚁金服SOFARegistry之服务上线
[从源码学设计]蚂蚁金服SOFARegistry 之 服务注册和操作日志
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