A group communication service maintains synchronization limiting the group to the processing entities that receive a plurality of source message streams from sources outside the group and transmit delivery message streams to destinations that are outside the group. Synchronization of message ordering and other state synchronization directives are communicated from a leader exclusively to other group members. Once having declared themselves to be synchronized and active by a reply protocol message, which is an opaque object and is interpreted by an application layer, the processing entities independently process the source message streams in accordance with the order specified in the directives, without requiring additional sequencing information from another processing entity.