聊聊eureka server的response cache

    /**
     * Gets information about a particular {@link com.netflix.discovery.shared.Application}.
     *
     * @param version
     *            the version of the request.
     * @param acceptHeader
     *            the accept header of the request to indicate whether to serve
     *            JSON or XML data.
     * @return the response containing information about a particular
     *         application.
     */
    @GET
    public Response getApplication(@PathParam("version") String version,
                                   @HeaderParam("Accept") final String acceptHeader,
                                   @HeaderParam(EurekaAccept.HTTP_X_EUREKA_ACCEPT) String eurekaAccept) {
        if (!registry.shouldAllowAccess(false)) {
            return Response.status(Status.FORBIDDEN).build();
        }

        EurekaMonitors.GET_APPLICATION.increment();

        CurrentRequestVersion.set(Version.toEnum(version));
        KeyType keyType = Key.KeyType.JSON;
        if (acceptHeader == null || !acceptHeader.contains("json")) {
            keyType = Key.KeyType.XML;
        }

        Key cacheKey = new Key(
                Key.EntityType.Application,
                appName,
                keyType,
                CurrentRequestVersion.get(),
                EurekaAccept.fromString(eurekaAccept)
        );

        String payLoad = responseCache.get(cacheKey);

        if (payLoad != null) {
            logger.debug("Found: {}", appName);
            return Response.ok(payLoad).build();
        } else {
            logger.debug("Not Found: {}", appName);
            return Response.status(Status.NOT_FOUND).build();
        }
    }

    /**
     * Get the cached information about applications.
     *
     * <p>
     * If the cached information is not available it is generated on the first
     * request. After the first request, the information is then updated
     * periodically by a background thread.
     * </p>
     *
     * @param key the key for which the cached information needs to be obtained.
     * @return payload which contains information about the applications.
     */
    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();
        }
    }

    @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;
    }

/**
 * The class that is responsible for caching registry information that will be
 * queried by the clients.
 *
 * <p>
 * The cache is maintained in compressed and non-compressed form for three
 * categories of requests - all applications, delta changes and for individual
 * applications. The compressed form is probably the most efficient in terms of
 * network traffic especially when querying all applications.
 *
 * The cache also maintains separate pay load for <em>JSON</em> and <em>XML</em>
 * formats and for multiple versions too.
 * </p>
 *
 * @author Karthik Ranganathan, Greg Kim
 */
public class ResponseCacheImpl implements ResponseCache {
    //......

    private final ConcurrentMap<Key, Value> readOnlyCacheMap = new ConcurrentHashMap<Key, Value>();

    private final LoadingCache<Key, Value> readWriteCacheMap;

    ResponseCacheImpl(EurekaServerConfig serverConfig, ServerCodecs serverCodecs, AbstractInstanceRegistry registry) {
        this.serverConfig = serverConfig;
        this.serverCodecs = serverCodecs;
        this.shouldUseReadOnlyResponseCache = serverConfig.shouldUseReadOnlyResponseCache();
        this.registry = registry;

        long responseCacheUpdateIntervalMs = serverConfig.getResponseCacheUpdateIntervalMs();
        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;
                            }
                        });

        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);
        }
    }
}

    /*
     * Generate pay load for the given key.
     */
    private Value generatePayload(Key key) {
        Stopwatch tracer = null;
        try {
            String payload;
            switch (key.getEntityType()) {
                case Application:
                    boolean isRemoteRegionRequested = key.hasRegions();

                    if (ALL_APPS.equals(key.getName())) {
                        if (isRemoteRegionRequested) {
                            tracer = serializeAllAppsWithRemoteRegionTimer.start();
                            payload = getPayLoad(key, registry.getApplicationsFromMultipleRegions(key.getRegions()));
                        } else {
                            tracer = serializeAllAppsTimer.start();
                            payload = getPayLoad(key, registry.getApplications());
                        }
                    } else if (ALL_APPS_DELTA.equals(key.getName())) {
                        if (isRemoteRegionRequested) {
                            tracer = serializeDeltaAppsWithRemoteRegionTimer.start();
                            versionDeltaWithRegions.incrementAndGet();
                            versionDeltaWithRegionsLegacy.incrementAndGet();
                            payload = getPayLoad(key,
                                    registry.getApplicationDeltasFromMultipleRegions(key.getRegions()));
                        } else {
                            tracer = serializeDeltaAppsTimer.start();
                            versionDelta.incrementAndGet();
                            versionDeltaLegacy.incrementAndGet();
                            payload = getPayLoad(key, registry.getApplicationDeltas());
                        }
                    } else {
                        tracer = serializeOneApptimer.start();
                        payload = getPayLoad(key, registry.getApplication(key.getName()));
                    }
                    break;
                case VIP:
                case SVIP:
                    tracer = serializeViptimer.start();
                    payload = getPayLoad(key, getApplicationsForVip(key, registry));
                    break;
                default:
                    logger.error("Unidentified entity type: {} found in the cache key.", key.getEntityType());
                    payload = "";
                    break;
            }
            return new Value(payload);
        } finally {
            if (tracer != null) {
                tracer.stop();
            }
        }
    }

    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);
                    }
                }
            }
        };
    }