Eureka系列(四) 获取服务Server端具体实现

获取服务 Server端流程

  我们先看下面这张图片，这张图片简单描述了下我们EurekaClient在调用EurekaServer 提供的获取服务Http接口，Server端实现接口执行的大致流程，图中还包含了服务注册的大致流程，因为服务注册和获取服务有关联的部分，因此两个流程合到了一起

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Eureka 二级缓存

  我们先看看我们Eureka二级缓存的结构：

 // 一级缓存 只读缓存
 private final ConcurrentMap<Key, Value> readOnlyCacheMap = new ConcurrentHashMap<Key, Value>();
 // 二级缓存 读写缓存
 private final LoadingCache<Key, Value> readWriteCacheMap; 

  上面我们看到一级缓存是由我们jdk自带的ConcurrentHashMap实现，而我们的二级缓存却是有google提供的guava包中LoadingCache实现。我们接着看下Eureka二级缓存readWriteCacheMap的初始化：

this.readWriteCacheMap =
          CacheBuilder.newBuilder().initialCapacity(1000)
                        .expireAfterWrite(serverConfig.getResponseCacheAutoExpirationInSeconds(), TimeUnit.SECONDS)
                        .removalListener(new RemovalListener<Key, Value>() {
                            @Override
                            public void onRemoval(RemovalNotification<Key, Value> notification) {
                                Key removedKey = notification.getKey();
                                if (removedKey.hasRegions()) {
                                    Key cloneWithNoRegions = removedKey.cloneWithoutRegions();
                                    regionSpecificKeys.remove(cloneWithNoRegions, removedKey);
                                }
                            }
                        })
                        .build(new CacheLoader<Key, Value>() {
                            @Override
                            public Value load(Key key) throws Exception {
                                if (key.hasRegions()) {
                                    Key cloneWithNoRegions = key.cloneWithoutRegions();
                                    regionSpecificKeys.put(cloneWithNoRegions, key);
                                }
                                Value value = generatePayload(key);
                                return value;
                            }
                        });

   不太了解LoadingCache类的小伙伴可以自行百度了解下，上面的代码简单描述就是设置了缓存180s会自行过期以及如果我们调用get()方法获取数据，查询不到对应的缓存则会执行load方法，从而得到key对应的数据。

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获取服务Server端实现源码分析

  获取服务的流程就想文章顶部文章描述的，首先会去查只读(一级)缓存(前提是没有配置只读缓存为false)，如果只读缓存没有对应数据，则去查读写缓存(二级)缓存，如果读写缓存也没有，则会触发LoadingCache()的load方法，从内存中读取存取的实例信息。
  下面我们通过源码再来看看整个流程：

private final ResponseCache responseCache;
@GET
public Response getContainers(@PathParam("version") String version,
                              @HeaderParam(HEADER_ACCEPT) String acceptHeader,
                              @HeaderParam(HEADER_ACCEPT_ENCODING) String acceptEncoding,
                              @HeaderParam(EurekaAccept.HTTP_X_EUREKA_ACCEPT) String eurekaAccept,
                              @Context UriInfo uriInfo,
                              @Nullable @QueryParam("regions") String regionsStr) {

    boolean isRemoteRegionRequested = null != regionsStr && !regionsStr.isEmpty();
    String[] regions = null;
    if (!isRemoteRegionRequested) {
        EurekaMonitors.GET_ALL.increment();
    } else {
        regions = regionsStr.toLowerCase().split(",");
        Arrays.sort(regions); // So we don't have different caches for same regions queried in different order.
        EurekaMonitors.GET_ALL_WITH_REMOTE_REGIONS.increment();
    }
    // Check if the server allows the access to the registry. The server can
    // restrict access if it is not
    // ready to serve traffic depending on various reasons.
    if (!registry.shouldAllowAccess(isRemoteRegionRequested)) {
        return Response.status(Status.FORBIDDEN).build();
    }
    CurrentRequestVersion.set(Version.toEnum(version));
    KeyType keyType = Key.KeyType.JSON;
    String returnMediaType = MediaType.APPLICATION_JSON;
    if (acceptHeader == null || !acceptHeader.contains(HEADER_JSON_VALUE)) {
        keyType = Key.KeyType.XML;
        returnMediaType = MediaType.APPLICATION_XML;
    }
    Key cacheKey = new Key(Key.EntityType.Application,
            ResponseCacheImpl.ALL_APPS,
            keyType, CurrentRequestVersion.get(), EurekaAccept.fromString(eurekaAccept), regions
    );
    Response response;
    if (acceptEncoding != null && acceptEncoding.contains(HEADER_GZIP_VALUE)) {
        // 取压缩的缓存数据
        response = Response.ok(responseCache.getGZIP(cacheKey)) 
                .header(HEADER_CONTENT_ENCODING, HEADER_GZIP_VALUE)
                .header(HEADER_CONTENT_TYPE, returnMediaType)
                .build();
    } else {
        // 取缓存数据
        response = Response.ok(responseCache.get(cacheKey)) 
                .build();
    }
    return response;
}

   responseCache.getGZIP(cacheKey) 和 responseCache.get(cacheKey)方法的区别是getGZIP方法是压缩后的实例信息，但这两个方法最终都会调用getValue()方法，如下所示：

public byte[] getGZIP(Key key) {
    Value payload = getValue(key, shouldUseReadOnlyResponseCache);
    if (payload == null) {
        return null;
    }
    return payload.getGzipped();
}

public String get(final Key key) {
    return get(key, shouldUseReadOnlyResponseCache);
}

@VisibleForTesting
String get(final Key key, boolean useReadOnlyCache) {
    Value payload = getValue(key, useReadOnlyCache);
    if (payload == null || payload.getPayload().equals(EMPTY_PAYLOAD)) {
        return null;
    } else {
        return payload.getPayload();
    }
}

 &emps;我们接着看getValue这个方法，代码如下：

// 只读缓存，使用concurrentHashMap实现
private final ConcurrentMap<Key, Value> readOnlyCacheMap = new ConcurrentHashMap<Key, Value>(); 
// 读写缓存，使用google提供的LoadingCache实现
private final LoadingCache<Key, Value> readWriteCacheMap; 
@VisibleForTesting
Value getValue(final Key key, boolean useReadOnlyCache) {
    Value payload = null;
    try {
        if (useReadOnlyCache) {
            final Value currentPayload = readOnlyCacheMap.get(key);
            if (currentPayload != null) {
                payload = currentPayload;
            } else {
                payload = readWriteCacheMap.get(key);
                readOnlyCacheMap.put(key, payload);
            }
        } else {
            payload = readWriteCacheMap.get(key);
        }
    } catch (Throwable t) {
        logger.error("Cannot get value for key : {}", key, t);
    }
    return payload;
}

   由上可见，我们获取实例信息，会先进行2次判断，判断如果是否启用了只读缓存，如果没有启用则直接从读写缓存中读取，启用了读写缓存，则我们会先尝试从读写缓存中读取数据，如果为空则从读写缓存中读取，然后再把数据put进只读缓存。
   注意：readWriteCacheMap.get(key)这个方法如果在原本的LoadingCache中查询不到数据，则会调用load方法取key对应的数据，最终返回给我们对应的数据。

   接下来我们来看看我们的只读缓存和读写缓存之间是咋进行更新的：

ResponseCacheImpl(EurekaServerConfig serverConfig, ServerCodecs serverCodecs, AbstractInstanceRegistry registry) {
    this.serverConfig = serverConfig;
    this.serverCodecs = serverCodecs;
    // 是否使用只读缓存
    this.shouldUseReadOnlyResponseCache = serverConfig.shouldUseReadOnlyResponseCache();
    this.registry = registry;

    // 缓存更新的时间间隔(用定时器更新,定时器的时间默认30秒执行一次)
    long responseCacheUpdateIntervalMs = serverConfig.getResponseCacheUpdateIntervalMs();
    // 构建读写缓存 默认缓存时间180秒
    this.readWriteCacheMap =
            CacheBuilder.newBuilder().initialCapacity(1000)
                    .expireAfterWrite(serverConfig.getResponseCacheAutoExpirationInSeconds(), TimeUnit.SECONDS)
                    .removalListener(new RemovalListener<Key, Value>() {
                        @Override
                        public void onRemoval(RemovalNotification<Key, Value> notification) {
                            Key removedKey = notification.getKey();
                            if (removedKey.hasRegions()) {
                                Key cloneWithNoRegions = removedKey.cloneWithoutRegions();
                                regionSpecificKeys.remove(cloneWithNoRegions, removedKey);
                            }
                        }
                    })
                    // 缓存加载器，当缓存不存在时，会自动执行load方法，进行缓存加载。同时返回缓存数据
                    .build(new CacheLoader<Key, Value>() {
                        @Override
                        public Value load(Key key) throws Exception {
                            if (key.hasRegions()) {
                                Key cloneWithNoRegions = key.cloneWithoutRegions();
                                regionSpecificKeys.put(cloneWithNoRegions, key);
                            }
                            Value value = generatePayload(key);
                            return value;
                        }
                    });

    // 使用只读缓存，如果使用，此处则启动一个定时器，用来复制readWriteCacheMap 的数据至readOnlyCacheMap
    if (shouldUseReadOnlyResponseCache) {
        timer.schedule(getCacheUpdateTask(),
                new Date(((System.currentTimeMillis() / responseCacheUpdateIntervalMs) * responseCacheUpdateIntervalMs)
                        + responseCacheUpdateIntervalMs),
                responseCacheUpdateIntervalMs);
    }
    try {
        Monitors.registerObject(this);
    } catch (Throwable e) {
        logger.warn("Cannot register the JMX monitor for the InstanceRegistry", e);
    }
}
// 复制readWriteCacheMap 的数据至readOnlyCacheMap
private TimerTask getCacheUpdateTask() {
    return new TimerTask() {
        @Override
        public void run() {
            logger.debug("Updating the client cache from response cache");
            for (Key key : readOnlyCacheMap.keySet()) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Updating the client cache from response cache for key : {} {} {} {}",
                            key.getEntityType(), key.getName(), key.getVersion(), key.getType());
                }
                try {
                    CurrentRequestVersion.set(key.getVersion());
                    Value cacheValue = readWriteCacheMap.get(key);
                    Value currentCacheValue = readOnlyCacheMap.get(key);
                    if (cacheValue != currentCacheValue) {
                        readOnlyCacheMap.put(key, cacheValue);
                    }
                } catch (Throwable th) {
                    logger.error("Error while updating the client cache from response cache for key {}", key.toStringCompact(), th);
                }
            }
        }
    };
}

  由上可知，我们Server启动了两个定时任务，只读缓存定时任务使用java.util.Timer timer类实现，读写缓存是由geegle guava LoadingCache实现。只读缓存定时设置30s，即每隔30s则会读取读写缓存中数据用来更新只读缓存中的数据。而我们的读写缓存则是设置的180s过期。