Such sources may disagree with each other on some aspects of the schema. To provide a uniform interface to the downstream receiver, the stream server may pre-define an output XML schema. Such schema must be “coarse” enough so that all XML messages in the stream do conform to it. This universal schema is likely to be rather coarse, if the diversity of sources is large, as it must be the lowest common denominator of the features shared across all sources. The schema will contain huge optional elements and alternative subtypes, thus becoming less amendable for schema based optimization.