Service coordination protocols

Abstract

The basic Web services infrastructure presented in the previous chapter suffices to implement simple interactions. In particular, it supports interactions where the client invokes a single operation on a Web service. When the interaction involves coordinated sequences of operations, additional abstractions and tools are needed to ensure the correctness and consistency of the interactions. This is no different from how conventional middleware evolved. RPCs support simple, one-call-at-a-time interactions between clients and servers. Adding guarantees to the interactions (e.g., transactions), requires additional protocols (e.g. two-phase commit) and an infrastructure that supports the necessary abstractions and the corresponding protocols (e.g., a TP monitor). The same applies to Web services. Once we go beyond simple, independent service invocations, new protocols, abstractions, and infrastructures are needed.