code: private ValueCacheKey getRequestCacheKey(String cacheKey) {
          if (cacheKey != null) {
              /* create the cache key we will use to retrieve/store that include the type key prefix */
              return new ValueCacheKey(rcKey, cacheKey);
          }
          return null;
      }
docs: Request CacheKey: HystrixRequestCache.prefix + concurrencyStrategy + HystrixCommand.getCacheKey (as injected via get/put to this class)
  <p>
  We prefix with {@link HystrixCommandKey} or {@link HystrixCollapserKey} since the cache is heterogeneous and we don't want to accidentally return cached Futures from different
  types.
  
  @return ValueCacheKey