The diagram shows a top-level coordinator and an interposed
coordinator. The top-level coordinator is responsible for driving the original, top-level transaction
to completion or rollback. The interposed coordinator manages its participants in a subordinate transaction, but
it cannot act autonomously. From the point of view of the parent coordinator, it appears to be another
participant in the top-level transaction. The interposed coordinator operates as an intermediary. It forwards
incoming prepare
and commit
/rollback
messages to its participants, combining their responses and returning them back to its parent coordinator.