Method and apparatus to efficiently maintain cache coherency by reading/writing a domain state field associated with a tag entry within a cache tag directory. A value may be assigned to a domain state field of a tag entry in a cache tag directory. The cache tag directory may belong to a hierarchy of cache tag directories. Each tag entry may be associated with a cache line from a cache belonging to a first domain. The first domain may contain multiple caches. The value of the domain state field may indicate whether its associated cache line can be read or changed.