所有文章
https://www.cnblogs.com/lay2017/p/11908715.html
正文
eureka服务端维护了一个服务信息的列表，服务端节点之间相互复制服务信息。而作为eureka的客户端将会从eureka服务端请求这个服务信息列表，选择对应的实例。本文就来看看eureka服务端对客户端提供的获取服务信息列表的http接口。
eureka服务端基于jersey来提供http服务调用，所以我们先找到它的Resource。
ApplicationsResource
@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) {
// ...
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)) {
response = Response.ok(responseCache.getGZIP(cacheKey))
.header(HEADER_CONTENT_ENCODING, HEADER_GZIP_VALUE)
.header(HEADER_CONTENT_TYPE, returnMediaType)
.build();
} else {
response = Response.ok(responseCache.get(cacheKey))
.build();
}
return response;
}
这里从responseCache当中获取了Applications的序列号结果直接返回了，所以我们先看看ResponseCache是从哪里来的
可以看到ResponseCache是再ApplicationsResource构造的时候从Registry中获取的
@Inject
ApplicationsResource(EurekaServerContext eurekaServer) {
this.serverConfig = eurekaServer.getServerConfig();
this.registry = eurekaServer.getRegistry();
this.responseCache = registry.getResponseCache();
}
我们再看看ResponseCache的get方法做了什么
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();
}
}
继续跟进getValue
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;
}
可以看到，其实只是从ReadWriteCacheMap当中获取对应的值，那么我们再看看ReadWriteCacheMap是怎么被构造的
this.readWriteCacheMap = CacheBuilder.newBuilder().initialCapacity(serverConfig.getInitialCapacityOfResponseCache())
.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;
}
});
从这里可以看出，调用ReadWriteCacheMap的get方法，将会触发这里的generatePayload方法
我们跟进generatePayload
private Value generatePayload(Key key) {
Stopwatch tracer = null;
try {
String payload;
switch (key.getEntityType()) {
case Application:
boolean isRemoteRegionRequested = key.hasRegions();
// 获取所有Application
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())) {
// ...
} else {
tracer = serializeOneApptimer.start();
// 获取某个Application
payload = getPayLoad(key, registry.getApplication(key.getName()));
}
break;
// ...
}
return new Value(payload);
} finally {
}
}
我们看到这里payload主要构成元素是Application，也就是我们需要的服务列表信息。
最后，我们跟进getPayLoad方法，看看这些服务列表信息是怎么被序列号成payload的
private String getPayLoad(Key key, Applications apps) {
EncoderWrapper encoderWrapper = serverCodecs.getEncoder(key.getType(), key.getEurekaAccept());
String result;
try {
result = encoderWrapper.encode(apps);
} catch (Exception e) {
return "";
}
return result;
}
编码器的实现比较多种，这里就不展开了
总结
获取服务信息列表其实就是从registry当中获取Applications，然后做一次序列化，最后通过http响应回去。总体来说还是比较简单的。