HL7 version 2 has a very powerful event model, and it has certainly stood the test of time. But it suffers from a major flaw: in order to design interfaces using HL7 version 2, it is necessary to know the event model ahead of time, and all participating systems have to agree on what the appropriate response to each trigger event should be. Of course, there are other issues: developing a common content model with agreed upon semantics, for example. But for now, we’ll focus exclusively on the dynamic model. In practice, this means HL7 messaging systems need to be centrally designed. Or, perhaps more to the point, system integration requires a lot of coordinated effort to be expended on each of the participating systems. This is precisely the problem Roy Fielding sought to address in his doctoral dissertation.