Interposition is particularly useful for Web Services transactions, as a way of limiting the amount of network traffic required for coordination. For example, if communications between the top-level coordinator and a web service are slow because of network traffic or distance, the web service might benefit from executing in a subordinate transaction which employs a local coordinator service. In
Figure 2.3, “Interpositions”,to
prepare
, the top-level coordinator only needs to send one
prepare
message to the subordinate coordinator, and receive one
prepared
or
aborted
reply. The subordinate coordinator forwards a
prepare
locally to each participant and combines the results to decide whether to send a single
prepared
or
aborted
reply.