code: func (s *ChainService) putFilterToCache(blockHash *chainhash.Hash,
  	filterType filterdb.FilterType, filter *gcs.Filter) (bool, error) {
  
  	cacheKey := cache.FilterCacheKey{*blockHash, filterType}
  	return s.FilterCache.Put(cacheKey, &cache.CacheableFilter{Filter: filter})
  }
docs: // putFilterToCache inserts a given filter in ChainService's FilterCache.