2.9. Interposition

Interposition is a scoping mechanism which allows coordination of a transaction to be delegated across a hierarchy of coordinators. See Figure 2.3, “Interpositions” for a graphical representation of this concept.

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.

Figure 2.3. Interpositions

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.