序

本文主要研究一下cache2k这款新型缓存

示例

Cache<String,String> cache = new Cache2kBuilder<String, String>() {}
                .eternal(true)
                .expireAfterWrite(5, TimeUnit.MINUTES)    // expire/refresh after 5 minutes
                .setupWith(UniversalResiliencePolicy::enable, b -> b // enable resilience policy
                                .resilienceDuration(30, TimeUnit.SECONDS)          // cope with at most 30 seconds
                        // outage before propagating
                        // exceptions
                )
                .refreshAhead(true)                       // keep fresh when expiring
                .loader(k -> expensiveOperation(k))         // auto populating function
                .build();

常见问题的解决方案

空值问题

JCache规范不支持null，所以cache2k默认也不支持，不过可以通过permitNullValues(true)来开启，这样子缓存就可以存储null值

cache stampede问题

又称作cache miss storm，指的是高并发场景缓存同时失效导致大面积回源，cache2k采用的是block的请求方式，避免对同一个key并发回源

org/cache2k/core/HeapCache.java

protected Entry<K, V> getEntryInternal(K key, int hc, int val) {
    if (loader == null) {
      return peekEntryInternal(key, hc, val);
    }
    Entry<K, V> e;
    for (;;) {
      e = lookupOrNewEntry(key, hc, val);
      if (e.hasFreshData(clock)) {
        return e;
      }
      synchronized (e) {
        e.waitForProcessing();
        if (e.hasFreshData(clock)) {
          return e;
        }
        if (e.isGone()) {
          metrics.heapHitButNoRead();
          metrics.goneSpin();
          continue;
        }
        e.startProcessing(Entry.ProcessingState.LOAD, null);
        break;
      }
    }
    boolean finished = false;
    try {
      load(e);
      finished = true;
    } finally {
      e.ensureAbort(finished);
    }
    if (e.getValueOrException() == null && isRejectNullValues()) {
      return null;
    }
    return e;
  }

同步回源造成的接口稳定性问题

cache2k提供了refreshAhead参数，在新数据没有拉取成功之前，过期数据仍然可以访问，避免请求到来时发现数据过期触发同步回源造成接口延时增大问题。不过具体底层还依赖prefetchExecutor，如果refresh的时候没有足够的线程可以使用则会立马过期，等待下次get出发同步回源

org/cache2k/core/HeapCache.java

public void timerEventRefresh(Entry<K, V> e, Object task) {
    metrics.timerEvent();
    synchronized (e) {
      if (e.getTask() != task) { return; }
      try {
        refreshExecutor.execute(createFireAndForgetAction(e, Operations.SINGLETON.refresh));
      } catch (RejectedExecutionException ex) {
        metrics.refreshRejected();
        expireOrScheduleFinalExpireEvent(e);
      }
    }
  }

默认的executor如下，采用的是SynchronousQueue队列，可以通过builder自己去设置refreshExecutor

  Executor provideDefaultLoaderExecutor(int threadCount) {
    int corePoolThreadSize = 0;
    return new ThreadPoolExecutor(corePoolThreadSize, threadCount,
      21, TimeUnit.SECONDS,
      new SynchronousQueue<>(),
      threadFactoryProvider.newThreadFactory(getThreadNamePrefix()),
      new ThreadPoolExecutor.AbortPolicy());
  }

回源故障问题

针对回源的下游出现故障的问题，cache2k提供了ResiliencePolicy策略，其实现类为UniversalResiliencePolicy 当load方法抛出异常且cache里头还有数据的时候，异常不会抛给client，用当前的数据返回，这里有个resilienceDuration时间，如果超过这个时间load方法还继续抛出异常则异常会抛给client。如果没有单独设置resilienceDuration，则默认取的是expiryAfterWrite时间

org/cache2k/core/HeapCache.java

private Object loadGotException(Entry<K, V> e, long t0, long t, Throwable wrappedException) {
    ExceptionWrapper<K, V> exceptionWrapper =
      new ExceptionWrapper(keyObjFromEntry(e), wrappedException, t0, e, exceptionPropagator);
    long expiry = 0;
    long refreshTime = 0;
    boolean suppressException = false;
    RefreshAheadPolicy.Context<Object> refreshCtx;
    try {
      if (e.isValidOrExpiredAndNoException()) {
        expiry = timing.suppressExceptionUntil(e, exceptionWrapper);
      }
      if (expiry > t0) {
        suppressException = true;
      } else {
        expiry = timing.cacheExceptionUntil(e, exceptionWrapper);
      }
      refreshCtx = getContext(e, t0, t, true, true, false, expiry);
      refreshTime = timing.calculateRefreshTime(refreshCtx);
    } catch (Exception ex) {
      return resiliencePolicyException(e, t0, t, new ResiliencePolicyException(ex), null);
    }
    exceptionWrapper = new ExceptionWrapper<>(exceptionWrapper, Math.abs(expiry));
    Object wrappedValue = exceptionWrapper;
    if (expiry != 0) {
      wrappedValue = timing.wrapLoadValueForRefresh(refreshCtx, e, exceptionWrapper);
    }
    Object loadResult;
    synchronized (e) {
      insertUpdateStats(e, (V) wrappedValue, t0, t, true, expiry, suppressException);
      if (suppressException) {
        e.setSuppressedLoadExceptionInformation(exceptionWrapper);
        loadResult = e.getValueOrException();
      } else {
        if (isRecordModificationTime()) {
          e.setModificationTime(t0);
        }
        e.setValueOrWrapper(exceptionWrapper);
        loadResult = exceptionWrapper;
      }
      finishLoadOrEviction(e, expiry, refreshTime);
    }
    return loadResult;
  }

这里timing.suppressExceptionUntil是委托给了ResiliencePolicy#suppressExceptionUntil

cache2k-addon/src/main/java/org/cache2k/addon/UniversalResiliencePolicy.java

public long suppressExceptionUntil(K key,
                                     LoadExceptionInfo<K, V> loadExceptionInfo,
                                     CacheEntry<K, V> cachedEntry) {
    if (resilienceDuration == 0 || resilienceDuration == Long.MAX_VALUE) {
      return resilienceDuration;
    }
    long maxSuppressUntil = loadExceptionInfo.getSinceTime() + resilienceDuration;
    long deltaMs = calculateRetryDelta(loadExceptionInfo);
    return Math.min(loadExceptionInfo.getLoadTime() + deltaMs, maxSuppressUntil);
  }

UniversalResiliencePolicy还提供了异常重试的功能，重试间隔为retryInterval，如果没有配置则为resilienceDuration的5%，采取的是指数退避的模式，factor为1.5

小结

cache2k提供了Guava Cache及Caffeine没有的ResiliencePolicy，针对C端高并发场景提供了容错的功能，值得借鉴一下。

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doc

• cache2k
• cache2k User Guide
• Introduction to cache2k
• caffeine
• guava cache