Etcd Data Synchronization Source Code Analysis

Apache ShenYu is an asynchronous, high-performance, cross-language, responsive API gateway.

In ShenYu gateway, data synchronization refers to how to synchronize the updated data to the gateway after the data is sent in the background management system. The Apache ShenYu gateway currently supports data synchronization for ZooKeeper, WebSocket, http long poll, Nacos, Etcd and Consul. The main content of this article is based on Etcd data synchronization source code analysis.

This paper based on shenyu-2.4.0 version of the source code analysis, the official website of the introduction of please refer to the Data Synchronization Design .

1. About Etcd

Etcd is a strongly consistent, distributed key-value store that provides a reliable way to store data that needs to be accessed by a distributed system or cluster of machines.

2. Admin Data Sync

We traced the source code from a real case, such as updating a selector data in the Divide plugin to a weight of 90 in a background administration system:

2.1 Accept Data

• SelectorController.createSelector()

Enter the createSelector() method of the SelectorController class, which validates data, adds or updates data, and returns results.

@Validated@RequiredArgsConstructor@RestController@RequestMapping("/selector")public class SelectorController {        @PutMapping("/{id}")    public ShenyuAdminResult updateSelector(@PathVariable("id") final String id, @Valid @RequestBody final SelectorDTO selectorDTO) {        // set the current selector data ID        selectorDTO.setId(id);        // create or update operation        Integer updateCount = selectorService.createOrUpdate(selectorDTO);        // return result         return ShenyuAdminResult.success(ShenyuResultMessage.UPDATE_SUCCESS, updateCount);    }        // ......}

2.2 Handle Data

• SelectorServiceImpl.createOrUpdate()

Convert data in the SelectorServiceImpl class using the createOrUpdate() method, save it to the database, publish the event, update upstream.

@RequiredArgsConstructor@Servicepublic class SelectorServiceImpl implements SelectorService {    // eventPublisher    private final ApplicationEventPublisher eventPublisher;        @Override    @Transactional(rollbackFor = Exception.class)    public int createOrUpdate(final SelectorDTO selectorDTO) {        int selectorCount;        // build data DTO --> DO        SelectorDO selectorDO = SelectorDO.buildSelectorDO(selectorDTO);        List<SelectorConditionDTO> selectorConditionDTOs = selectorDTO.getSelectorConditions();        // insert or update ?        if (StringUtils.isEmpty(selectorDTO.getId())) {            //  insert into data            selectorCount = selectorMapper.insertSelective(selectorDO);            // insert into condition data            selectorConditionDTOs.forEach(selectorConditionDTO -> {                selectorConditionDTO.setSelectorId(selectorDO.getId());                selectorConditionMapper.insertSelective(SelectorConditionDO.buildSelectorConditionDO(selectorConditionDTO));            });            // check selector add            if (dataPermissionMapper.listByUserId(JwtUtils.getUserInfo().getUserId()).size() > 0) {                DataPermissionDTO dataPermissionDTO = new DataPermissionDTO();                dataPermissionDTO.setUserId(JwtUtils.getUserInfo().getUserId());                dataPermissionDTO.setDataId(selectorDO.getId());                dataPermissionDTO.setDataType(AdminConstants.SELECTOR_DATA_TYPE);                dataPermissionMapper.insertSelective(DataPermissionDO.buildPermissionDO(dataPermissionDTO));            }
        } else {            // update data, delete and then insert            selectorCount = selectorMapper.updateSelective(selectorDO);            //delete rule condition then add            selectorConditionMapper.deleteByQuery(new SelectorConditionQuery(selectorDO.getId()));            selectorConditionDTOs.forEach(selectorConditionDTO -> {                selectorConditionDTO.setSelectorId(selectorDO.getId());                SelectorConditionDO selectorConditionDO = SelectorConditionDO.buildSelectorConditionDO(selectorConditionDTO);                selectorConditionMapper.insertSelective(selectorConditionDO);            });        }        // publish event        publishEvent(selectorDO, selectorConditionDTOs);
        // update upstream        updateDivideUpstream(selectorDO);        return selectorCount;    }        // ......    }

In the Service class to persist data, i.e. to the database, this should be familiar, not expand. The update upstream operation is analyzed in the corresponding section below, focusing on the publish event operation, which performs data synchronization.

The logic of the publishEvent() method is to find the plugin corresponding to the selector, build the conditional data, and publish the change data.

       private void publishEvent(final SelectorDO selectorDO, final List<SelectorConditionDTO> selectorConditionDTOs) {        // find plugin of selector        PluginDO pluginDO = pluginMapper.selectById(selectorDO.getPluginId());        // build condition data        List<ConditionData> conditionDataList =                selectorConditionDTOs.stream().map(ConditionTransfer.INSTANCE::mapToSelectorDTO).collect(Collectors.toList());        // publish event        eventPublisher.publishEvent(new DataChangedEvent(ConfigGroupEnum.SELECTOR, DataEventTypeEnum.UPDATE,                Collections.singletonList(SelectorDO.transFrom(selectorDO, pluginDO.getName(), conditionDataList))));    }

Change data released by eventPublisher.PublishEvent() is complete, the eventPublisher object is a ApplicationEventPublisher class, The fully qualified class name is org.springframework.context.ApplicationEventPublisher. Here we see that publishing data is done through Spring related functionality.

ApplicationEventPublisher：

When a state change, the publisher calls ApplicationEventPublisher of publishEvent method to release an event, Spring container broadcast event for all observers, The observer's onApplicationEvent method is called to pass the event object to the observer. There are two ways to call publishEvent method, one is to implement the interface by the container injection ApplicationEventPublisher object and then call the method, the other is a direct call container, the method of two methods of publishing events not too big difference.

• ApplicationEventPublisher: publish event;
• ApplicationEvent: Spring event, record the event source, time, and data;
• ApplicationListener: event listener, observer.

In Spring event publishing mechanism, there are three objects,

An object is a publish event ApplicationEventPublisher, in ShenYu through the constructor in the injected a eventPublisher.

The other object is ApplicationEvent , inherited from ShenYu through DataChangedEvent, representing the event object.

public class DataChangedEvent extends ApplicationEvent {//......}

The last object is ApplicationListener in ShenYu in through DataChangedEventDispatcher class implements this interface, as the event listener, responsible for handling the event object.

@Componentpublic class DataChangedEventDispatcher implements ApplicationListener<DataChangedEvent>, InitializingBean {
    //......    }

2.3 Dispatch Data

• DataChangedEventDispatcher.onApplicationEvent()

Released when the event is completed, will automatically enter the DataChangedEventDispatcher class onApplicationEvent() method of handling events.

@Componentpublic class DataChangedEventDispatcher implements ApplicationListener<DataChangedEvent>, InitializingBean {
  /**     * This method is called when there are data changes   * @param event     */    @Override    @SuppressWarnings("unchecked")    public void onApplicationEvent(final DataChangedEvent event) {        // Iterate through the data change listener (usually using a data synchronization approach is fine)      for (DataChangedListener listener : listeners) {            // What kind of data has changed        switch (event.getGroupKey()) {                case APP_AUTH: // app auth data                    listener.onAppAuthChanged((List<AppAuthData>) event.getSource(), event.getEventType());                    break;                case PLUGIN:  // plugin data                    listener.onPluginChanged((List<PluginData>) event.getSource(), event.getEventType());                    break;                case RULE:    // rule data                    listener.onRuleChanged((List<RuleData>) event.getSource(), event.getEventType());                    break;                case SELECTOR:   // selector data                    listener.onSelectorChanged((List<SelectorData>) event.getSource(), event.getEventType());                    break;                case META_DATA:  // metadata                    listener.onMetaDataChanged((List<MetaData>) event.getSource(), event.getEventType());                    break;                default:  // other types throw exception                  throw new IllegalStateException("Unexpected value: " + event.getGroupKey());            }        }    }    }

When there is a data change, the onApplicationEvent method is called and all the data change listeners are iterated to determine the data type and handed over to the appropriate data listener for processing.

ShenYu groups all the data into five categories: APP_AUTH, PLUGIN, RULE, SELECTOR and META_DATA.

Here the data change listener (DataChangedListener) is an abstraction of the data synchronization policy. Its concrete implementation is:

These implementation classes are the synchronization strategies currently supported by ShenYu:

• WebsocketDataChangedListener: data synchronization based on Websocket;
• ZookeeperDataChangedListener:data synchronization based on Zookeeper;
• ConsulDataChangedListener: data synchronization based on Consul;
• EtcdDataDataChangedListener：data synchronization based on etcd;
• HttpLongPollingDataChangedListener：data synchronization based on http long polling;
• NacosDataChangedListener：data synchronization based on nacos;

Given that there are so many implementation strategies, how do you decide which to use?

Because this paper is based on Etcd data synchronization source code analysis, so here to EtcdDataDataChangedListener as an example, the analysis of how it is loaded and implemented.

A global search in the source code project shows that its implementation is done in the DataSyncConfiguration class.

/** * Data Sync Configuration * By springboot conditional assembly * The type Data sync configuration. */@Configurationpublic class DataSyncConfiguration {            /**     * The type Etcd listener.     */    @Configuration    @ConditionalOnProperty(prefix = "shenyu.sync.etcd", name = "url")    @EnableConfigurationProperties(EtcdProperties.class)    static class EtcdListener {
        @Bean        public EtcdClient etcdClient(final EtcdProperties etcdProperties) {            Client client = Client.builder()                    .endpoints(etcdProperties.getUrl())                    .build();            return new EtcdClient(client);        }
        /**         * Config event listener data changed listener.         *         * @param etcdClient the etcd client         * @return the data changed listener         */        @Bean        @ConditionalOnMissingBean(EtcdDataDataChangedListener.class)        public DataChangedListener etcdDataChangedListener(final EtcdClient etcdClient) {            return new EtcdDataDataChangedListener(etcdClient);        }
        /**         * data init.         *         * @param etcdClient        the etcd client         * @param syncDataService the sync data service         * @return the etcd data init         */        @Bean        @ConditionalOnMissingBean(EtcdDataInit.class)        public EtcdDataInit etcdDataInit(final EtcdClient etcdClient, final SyncDataService syncDataService) {            return new EtcdDataInit(etcdClient, syncDataService);        }    }        // other code is omitted......}

This configuration class is implemented through the SpringBoot conditional assembly class. The EtcdListener class has several annotations:

• @Configuration: Configuration file, application context;

• @ConditionalOnProperty(prefix = "shenyu.sync.etcd", name = "url"): attribute condition. The configuration class takes effect only when the condition is met. That is, when we have the following configuration, etcd is used for data synchronization.

  shenyu:    sync:     etcd:          url: localhost:2181

• @EnableConfigurationProperties(EtcdProperties.class)：import EtcdProperties; The properties in the class EtcdProperties is relative to the properties which is with shenyu.sync.etcd as prefix in the configuration file.

 @Data@ConfigurationProperties(prefix = "shenyu.sync.etcd")public class EtcdProperties {
  private String url;
  private Integer sessionTimeout;
  private Integer connectionTimeout;
  private String serializer;}

When the shenyu.sync.etcd.url property is set in the configuration file, Admin would use the etcd data synchronization, EtcdListener is generated and the beans with type EtcdClient, EtcdDataDataChangedListener and EtcdDataInit would also be generated.

• The bean with the type EtcdClient would be generated, named etcdClient. This bean configues the connection properties of the etcd server based on the configuration file and can operate the etcdnodes directly.
• The bean with the type EtcdDataDataChangedListener would be generated, named etcdDataDataChangedListener. This bean use the bean etcdClient as a member variable and so when the event is listened, etcdDataDataChangedListener would call the callback method and use the etcdClient to operate the etcd nodes.
• The bean with the type EtcdDataInit would be generated, named etcdDataInit. This bean use the bean etcdClient and syncDataService as member variables, and use etcdClient to judge whether the data are initialized, if not, would use syncDataService to refresh data. We would dive into the details later.

So in the event handler onApplicationEvent(), it goes to the corresponding listener. In our case, it is a selector data update, data synchronization is etcd, so, the code will enter the EtcdDataDataChangedListener selector data change process.

    @Override    @SuppressWarnings("unchecked")    public void onApplicationEvent(final DataChangedEvent event) {        // Iterate through the data change listener (usually using a data synchronization approach is fine)        for (DataChangedListener listener : listeners) {            // what kind of data has changed         switch (event.getGroupKey()) {                                    // other code logic is omitted                                    case SELECTOR:   // selector data                    listener.onSelectorChanged((List<SelectorData>) event.getSource(), event.getEventType());   // In our case, will enter the EtcdDataDataChangedListener selector data change process                    break;         }    }

2.4 Etcd Data Changed Listener

• EtcdDataDataChangedListener.onSelectorChanged()

In the onSelectorChanged() method, determine the type of action, whether to refresh synchronization or update or create synchronization. Determine whether the node is in etcd based on the current selector data.

/** * EtcdDataDataChangedListener. */@Slf4jpublic class EtcdDataDataChangedListener implements DataChangedListener {    @Override    public void onSelectorChanged(final List<SelectorData> changed, final DataEventTypeEnum eventType) {        if (eventType == DataEventTypeEnum.REFRESH && !changed.isEmpty()) {            String selectorParentPath = DefaultPathConstants.buildSelectorParentPath(changed.get(0).getPluginName());            etcdClient.deleteEtcdPathRecursive(selectorParentPath);        }        for (SelectorData data : changed) {            String selectorRealPath = DefaultPathConstants.buildSelectorRealPath(data.getPluginName(), data.getId());            if (eventType == DataEventTypeEnum.DELETE) {                etcdClient.delete(selectorRealPath);                continue;            }            //create or update            updateNode(selectorRealPath, data);        }    }  }

This part is very important. The variable changed represents the SelectorData list, the variable eventType reprents the event type. When the event type is REFRESH and the SelectorData has changed, all the selector nodes under this plugin would be deleted in etcd. We should notice that the condition that the SelectorData has changed is necessary, otherwise a bug would appear that all the selector nodes would be deleted when no SelectorData data has changed.

As long as the changed data is correctly written to the etcd node, the admin side of the operation is complete.

In our current case, updating one of the selector data in the Divide plugin with a weight of 90 updates specific nodes in the graph.

We series the above update flow with a sequence diagram.

3. Gateway Data Sync

Assume that the ShenYu gateway is already running properly, and the data synchronization mode is also etcd. How does the gateway receive and process the selector data after updating it on the admin side and sending the changed data to etcd? Let's continue our source code analysis to find out.

3.1 EtcdClient Accept Data

• EtcdClient.watchDataChange()

There is a EtcdSyncDataService class on the gateway, which subscribing to the data node through etcdClient and can sense when the data changes.

/** * Data synchronize of etcd. */@Slf4jpublic class EtcdSyncDataService implements SyncDataService, AutoCloseable {    private void subscribeSelectorDataChanges(final String path) {      etcdClient.watchDataChange(path, (updateNode, updateValue) -> cacheSelectorData(updateValue),              this::unCacheSelectorData);    }  //other codes omitted}

Etcd's Watch mechanism notifies subscribing clients of node changes. In our case, updating the selector information goes to the watchDataChange() method. cacheSelectorData() is used to process data.

3.2 Handle Data

• EtcdSyncDataService.cacheSelectorData()

The data is not null, and caching the selector data is again handled by PluginDataSubscriber.

    private void cacheSelectorData(final SelectorData selectorData) {        Optional.ofNullable(selectorData)                .ifPresent(data -> Optional.ofNullable(pluginDataSubscriber).ifPresent(e -> e.onSelectorSubscribe(data)));    }

PluginDataSubscriber is an interface, it is only a CommonPluginDataSubscriber implementation class, responsible for data processing plugin, selector and rules.

3.3 Common Plugin Data Subscriber

• PluginDataSubscriber.onSelectorSubscribe()

It has no additional logic and calls the subscribeDataHandler() method directly. Within methods, there are data types (plugins, selectors, or rules) and action types (update or delete) to perform different logic.

/** * The common plugin data subscriber, responsible for handling all plug-in, selector, and rule information */public class CommonPluginDataSubscriber implements PluginDataSubscriber {    //......     // handle selector data    @Override    public void onSelectorSubscribe(final SelectoData selectorData) {        subscribeDataHandler(selectorData, DataEventTypeEnum.UPDATE);    }            // A subscription data handler that handles updates or deletions of data    private <T> void subscribeDataHandler(final T classData, final DataEventTypeEnum dataType) {        Optional.ofNullable(classData).ifPresent(data -> {            // plugin data            if (data instanceof PluginData) {                PluginData pluginData = (PluginData) data;                if (dataType == DataEventTypeEnum.UPDATE) { // update                    // save the data to gateway memory                     BaseDataCache.getInstance().cachePluginData(pluginData);                    // If each plugin has its own processing logic, then do it                    Optional.ofNullable(handlerMap.get(pluginData.getName())).ifPresent(handler -> handler.handlerPlugin(pluginData));                } else if (dataType == DataEventTypeEnum.DELETE) {  // delete                    // delete the data from gateway memory                    BaseDataCache.getInstance().removePluginData(pluginData);                    // If each plugin has its own processing logic, then do it                    Optional.ofNullable(handlerMap.get(pluginData.getName())).ifPresent(handler -> handler.removePlugin(pluginData));                }            } else if (data instanceof SelectorData) {  // selector data                SelectorData selectorData = (SelectorData) data;                if (dataType == DataEventTypeEnum.UPDATE) { // update                    // save the data to gateway memory                    BaseDataCache.getInstance().cacheSelectData(selectorData);                    // If each plugin has its own processing logic, then do it                     Optional.ofNullable(handlerMap.get(selectorData.getPluginName())).ifPresent(handler -> handler.handlerSelector(selectorData));                } else if (dataType == DataEventTypeEnum.DELETE) {  // delete                    // delete the data from gateway memory                    BaseDataCache.getInstance().removeSelectData(selectorData);                    // If each plugin has its own processing logic, then do it                    Optional.ofNullable(handlerMap.get(selectorData.getPluginName())).ifPresent(handler -> handler.removeSelector(selectorData));                }            } else if (data instanceof RuleData) {  // rule data                RuleData ruleData = (RuleData) data;                if (dataType == DataEventTypeEnum.UPDATE) { // update                    // save the data to gateway memory                    BaseDataCache.getInstance().cacheRuleData(ruleData);                    // If each plugin has its own processing logic, then do it                    Optional.ofNullable(handlerMap.get(ruleData.getPluginName())).ifPresent(handler -> handler.handlerRule(ruleData));                } else if (dataType == DataEventTypeEnum.DELETE) { // delete                    // delete the data from gateway memory                    BaseDataCache.getInstance().removeRuleData(ruleData);                    // If each plugin has its own processing logic, then do it                    Optional.ofNullable(handlerMap.get(ruleData.getPluginName())).ifPresent(handler -> handler.removeRule(ruleData));                }            }        });    }    }

3.4 Data cached to Memory

Adding a selector will enter the following logic:

// save the data to gateway memoryBaseDataCache.getInstance().cacheSelectData(selectorData);// If each plugin has its own processing logic, then do itOptional.ofNullable(handlerMap.get(selectorData.getPluginName())).ifPresent(handler -> handler.handlerSelector(selectorData));

One is to save the data to the gateway's memory. BaseDataCache is the class that ultimately caches data, implemented in a singleton pattern. The selector data is stored in the SELECTOR_MAP Map. In the subsequent use, also from this data.

public final class BaseDataCache {    // private instance    private static final BaseDataCache INSTANCE = new BaseDataCache();    // private constructor    private BaseDataCache() {    }        /**     * Gets instance.     *  public method     * @return the instance     */    public static BaseDataCache getInstance() {        return INSTANCE;    }        /**      * A Map of the cache selector data     * pluginName -> SelectorData.     */    private static final ConcurrentMap<String, List<SelectorData>> SELECTOR_MAP = Maps.newConcurrentMap();        public void cacheSelectData(final SelectorData selectorData) {        Optional.ofNullable(selectorData).ifPresent(this::selectorAccept);    }           /**     * cache selector data.     * @param data the selector data     */    private void selectorAccept(final SelectorData data) {        String key = data.getPluginName();        if (SELECTOR_MAP.containsKey(key)) { // Update operation, delete before insert            List<SelectorData> existList = SELECTOR_MAP.get(key);            final List<SelectorData> resultList = existList.stream().filter(r -> !r.getId().equals(data.getId())).collect(Collectors.toList());            resultList.add(data);            final List<SelectorData> collect = resultList.stream().sorted(Comparator.comparing(SelectorData::getSort)).collect(Collectors.toList());            SELECTOR_MAP.put(key, collect);        } else {  // Add new operations directly to Map            SELECTOR_MAP.put(key, Lists.newArrayList(data));        }    }    }

Second, if each plugin has its own processing logic, then do it. Through the IDEA editor, you can see that after adding a selector, there are the following plugins and processing. We're not going to expand it here.

After the above source tracking, and through a practical case, in the admin end to update a selector data, the ZooKeeper data synchronization process analysis is clear.

Let's series the data synchronization process on the gateway side through the sequence diagram:

The data synchronization process has been analyzed. In order to prevent the synchronization process from being interrupted, other logic is ignored during the analysis. We also need to analyze the process of Admin synchronization data initialization and gateway synchronization operation initialization.

4. Admin Data Sync initialization

When admin starts, the current data will be fully synchronized to etcd, the implementation logic is as follows:

/** * EtcdDataInit. */@Slf4jpublic class EtcdDataInit implements CommandLineRunner {
  private final EtcdClient etcdClient;
  private final SyncDataService syncDataService;
  public EtcdDataInit(final EtcdClient client, final SyncDataService syncDataService) {    this.etcdClient = client;    this.syncDataService = syncDataService;  }
  @Override  public void run(final String... args) throws Exception {    final String pluginPath = DefaultPathConstants.PLUGIN_PARENT;    final String authPath = DefaultPathConstants.APP_AUTH_PARENT;    final String metaDataPath = DefaultPathConstants.META_DATA;    if (!etcdClient.exists(pluginPath) && !etcdClient.exists(authPath) && !etcdClient.exists(metaDataPath)) {      log.info("Init all data from database");      syncDataService.syncAll(DataEventTypeEnum.REFRESH);    }  }}

Check whether there is data in etcd, if not, then synchronize.

EtcdDataInit implements the CommandLineRunner interface. It is an interface provided by SpringBoot that executes the run() method after all Spring Beans initializations and is often used for initialization operations in a project.

• SyncDataService.syncAll()

Query data from the database, and then perform full data synchronization, all authentication information, plugin information, selector information, rule information, and metadata information. Synchronous events are published primarily through eventPublisher. After publishing the event via publishEvent(), the ApplicationListener performs the event change operation. In ShenYu is mentioned in DataChangedEventDispatcher.

@Servicepublic class SyncDataServiceImpl implements SyncDataService {    // eventPublisher    private final ApplicationEventPublisher eventPublisher;         /***     * sync all data     * @param type the type     * @return     */    @Override    public boolean syncAll(final DataEventTypeEnum type) {        // app auth data        appAuthService.syncData();        // plugin data        List<PluginData> pluginDataList = pluginService.listAll();        eventPublisher.publishEvent(new DataChangedEvent(ConfigGroupEnum.PLUGIN, type, pluginDataList));        // selector data        List<SelectorData> selectorDataList = selectorService.listAll();        eventPublisher.publishEvent(new DataChangedEvent(ConfigGroupEnum.SELECTOR, type, selectorDataList));        // rule data        List<RuleData> ruleDataList = ruleService.listAll();        eventPublisher.publishEvent(new DataChangedEvent(ConfigGroupEnum.RULE, type, ruleDataList));        // metadata        metaDataService.syncData();        return true;    }    }

5. Gateway Data Sync Init

The initial operation of data synchronization on the gateway side is mainly the node in the subscription etcd. When there is a data change, the changed data will be received. This relies on the Watch mechanism of etcd. In ShenYu, the one responsible for etcd data synchronization is EtcdSyncDataService, also mentioned earlier.

The function logic of EtcdSyncDataService is completed in the process of instantiation: the subscription to Shenyu data synchronization node in etcd is completed. Subscription here is divided into two kinds, one kind is existing node data updated above, through this etcdClient.subscribeDataChanges() method; Another kind is under the current node, add or delete nodes change namely child nodes, it through etcdClient.subscribeChildChanges() method.

EtcdSyncDataService code is a bit too much, here we use plugin data read and subscribe to track, other types of data operation principle is the same.

/** * Data synchronize of etcd. */@Slf4jpublic class EtcdSyncDataService implements SyncDataService, AutoCloseable {    /**     * Instantiates a new Zookeeper cache manager.     *     * @param etcdClient             the etcd client     * @param pluginDataSubscriber the plugin data subscriber     * @param metaDataSubscribers  the meta data subscribers     * @param authDataSubscribers  the auth data subscribers     */    public EtcdSyncDataService(final EtcdClient etcdClient, final PluginDataSubscriber pluginDataSubscriber,                                    final List<MetaDataSubscriber> metaDataSubscribers, final List<AuthDataSubscriber> authDataSubscribers) {        this.etcdClient = etcdClient;        this.pluginDataSubscriber = pluginDataSubscriber;        this.metaDataSubscribers = metaDataSubscribers;        this.authDataSubscribers = authDataSubscribers;        watcherData();        watchAppAuth();        watchMetaData();    }
    private void watcherData() {        final String pluginParent = DefaultPathConstants.PLUGIN_PARENT;        List<String> pluginZKs = etcdClientGetChildren(pluginParent);        for (String pluginName : pluginZKs) {            watcherAll(pluginName);        }
        etcdClient.watchChildChange(pluginParent, (updateNode, updateValue) -> {            if (!updateNode.isEmpty()) {                watcherAll(updateNode);            }        }, null);    }
    private void watcherAll(final String pluginName) {        watcherPlugin(pluginName);        watcherSelector(pluginName);        watcherRule(pluginName);    }
    private void watcherPlugin(final String pluginName) {        String pluginPath = DefaultPathConstants.buildPluginPath(pluginName);        cachePluginData(etcdClient.get(pluginPath));        subscribePluginDataChanges(pluginPath, pluginName);    }
    private void cachePluginData(final String dataString) {        final PluginData pluginData = GsonUtils.getInstance().fromJson(dataString, PluginData.class);        Optional.ofNullable(pluginData)                .flatMap(data -> Optional.ofNullable(pluginDataSubscriber)).ifPresent(e -> e.onSubscribe(pluginData));    }
    private void subscribePluginDataChanges(final String pluginPath, final String pluginName) {    etcdClient.watchDataChange(pluginPath, (updatePath, updateValue) -> {      final String dataPath = buildRealPath(pluginPath, updatePath);      final String dataStr = etcdClient.get(dataPath);      final PluginData data = GsonUtils.getInstance().fromJson(dataStr, PluginData.class);      Optional.ofNullable(data)              .ifPresent(d -> Optional.ofNullable(pluginDataSubscriber).ifPresent(e -> e.onSubscribe(d)));    }, deleteNode -> deletePlugin(pluginName));  }  }

The above source code is given comments, I believe you can understand. The main logic for subscribing to plug-in data is as follows:

1. Create the current plugin path
2. Read the current node data on etcd and deserialize it
3. The plugin data is cached in the gateway memory
4. Subscribe to the plug-in node

6. Summary

This paper through a practical case, etcd data synchronization principle source code analysis. The main knowledge points involved are as follows:

• Data synchronization based on etcd is mainly implemented through watch mechanism;

• Complete event publishing and listening via Spring;

• Support multiple synchronization strategies through abstract DataChangedListener interface, interface oriented programming;

• Use singleton design pattern to cache data class BaseDataCache;

• Loading of configuration classes via conditional assembly of SpringBoot and starter loading mechanism.