hystrix总结之缓存

　　通过实现HystrixCommand或者HystrixObservableCommand的getCacheKey方法，可以启动缓存。

public class CommandUsingRequestCache extends HystrixCommand<Boolean> {
    private final int value;
    protected CommandUsingRequestCache(int value) {
        super(HystrixCommandGroupKey.Factory.asKey("ExampleGroup"));
        this.value = value;
    }
    @Override
    protected Boolean run() {
        return value == 0 || value % 2 == 0;
    }
    @Override
    protected String getCacheKey() {
        return String.valueOf(value);
    }
}

执行命令

@Test
        public void testWithoutCacheHits() {
            HystrixRequestContext context = HystrixRequestContext.initializeContext();
            try {
                assertTrue(new CommandUsingRequestCache(2).execute());
                assertFalse(new CommandUsingRequestCache(1).execute());
                assertTrue(new CommandUsingRequestCache(0).execute());
                assertTrue(new CommandUsingRequestCache(58672).execute());
            } finally {
                context.shutdown();
            }
        }

　　Hystrix通过getCacheKey方法来获取缓存中的值，缓存值的生命周期为一个请求。

　　本质上，在toObservable方法中，在执行前添加了从缓存中获取缓存的逻辑，在执行后，将返回结果存入缓存的逻辑。

public Observable<R> toObservable() {
...
        return Observable.defer(new Func0<Observable<R>>() {
            @Override
            public Observable<R> call() {
               ...
                final boolean requestCacheEnabled = isRequestCachingEnabled();
                final String cacheKey = getCacheKey();
                if (requestCacheEnabled) {
                    HystrixCommandResponseFromCache<R> fromCache = (HystrixCommandResponseFromCache<R>) requestCache.get(cacheKey);
                    if (fromCache != null) {
                        isResponseFromCache = true;
                        return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
                    }
                }
                Observable<R> hystrixObservable =
                        Observable.defer(applyHystrixSemantics)
                                .map(wrapWithAllOnNextHooks);
                Observable<R> afterCache;
                if (requestCacheEnabled && cacheKey != null) {
                    HystrixCachedObservable<R> toCache = HystrixCachedObservable.from(hystrixObservable, _cmd);
                    HystrixCommandResponseFromCache<R> fromCache = (HystrixCommandResponseFromCache<R>) requestCache.putIfAbsent(cacheKey, toCache);
                    ...
                } else {
                    afterCache = hystrixObservable;
                }
...
            }
        });
    }

　　HystrixCachedObservable内部使用type（命令类型：HystrixCommandKey为1，HystrixCollapserKey为2），commandKey的name，HystrixConcurrencyStrategy作为缓存的key　　

private static class RequestCacheKey {
        private final short type;
        private final String key;
        private final HystrixConcurrencyStrategy concurrencyStrategy;
...
｝
 private static class ValueCacheKey {
        private final RequestCacheKey rvKey;
        private final String valueCacheKey;
...
}

　　HystrixCachedObservable的值存储一个Observable，并且通过HystrixCachedObservable进行封装。

protected HystrixCachedObservable(final Observable<R> originalObservable) {
        ReplaySubject<R> replaySubject = ReplaySubject.create();
        this.originalSubscription = originalObservable
                .subscribe(replaySubject);

        this.cachedObservable = replaySubject
                .doOnUnsubscribe(new Action0() {
                    @Override
                    public void call() {
                        outstandingSubscriptions--;
                        if (outstandingSubscriptions == 0) {
                            originalSubscription.unsubscribe();
                        }
                    }
                })
                .doOnSubscribe(new Action0() {
                    @Override
                    public void call() {
                        outstandingSubscriptions++;
                    }
                });
    }

　　HystrixCachedObservable内部使用一个Map作为缓存， 这个Map存储在一个HystrixRequestVariableHolder中，所以它的生命周期为一个请求内部。

private static final HystrixRequestVariableHolder<ConcurrentHashMap<ValueCacheKey, HystrixCachedObservable<?>>> requestVariableForCache = new HystrixRequestVariableHolder<ConcurrentHashMap<ValueCacheKey, HystrixCachedObservable<?>>>(new HystrixRequestVariableLifecycle<ConcurrentHashMap<ValueCacheKey, HystrixCachedObservable<?>>>(); 

<T> HystrixCachedObservable<T> get(String cacheKey) {
        ValueCacheKey key = getRequestCacheKey(cacheKey);
        if (key != null) {
            ConcurrentHashMap<ValueCacheKey, HystrixCachedObservable<?>> cacheInstance = requestVariableForCache.get(concurrencyStrategy);
            if (cacheInstance == null) {
                throw new IllegalStateException("Request caching is not available. Maybe you need to initialize the HystrixRequestContext?");
            }
            /* look for the stored value */
            return (HystrixCachedObservable<T>) cacheInstance.get(key);
        }
        return null;
    }

如果异常 是否也缓存？

　　是的，fallback的异常也会被缓存下来。

缓存的有效范围？

　　为同一个请求内。