Event Replay Transmission Pattern
The Transmission Sender (an HL7 Application) sends the initial Event Replay Transmission. The Transmission Receiver (another HL7 Application) performs an accept-level validation, and sends an accept-level acknowledgement transmission if the Sender had requested such an acknowledgement.
The Response Transmission contains a root or “outer” Control Act that holds the administrative and high-level information common to all replayed events in the interaction (this is effectively the controlAct as present in the current ControlAct wrapper) and one or more “inner” or payload Control Acts (apart from the payload, detectedIssueEvent and queryAck the inner Control Acts mostly have no associations, but inherit them from the "outer" controlAct). As a result the model factors out the commonly repeating transmission control and administrative elements required when items are batched at the transmission-wrapper.
An event replay query uses the Query Transmission Pattern. This means that the outer Control Act has a queryAck class associated with it which indicates the number of results (i.e. replayed events) in the outer control act, as well as the number of remaining results which can be queried by a Query Continuation interaction. The queryAck class also contains the queryId, a business process level ID of the Interaction Pattern, which links queries, responses as well as query continuation transmissions.
Event replay isn't supported in HL7 v3 - nobody has come forward with a use-case yet.
Event Replay in v2
From HL7 v2 (RQQ^Q09, Ch.5): The Event Replay Query under version 2.x provides a way for the querying system to request data formatted very similar to the format that would have been used were this data to be sent as an update in response to a trigger event.. The response message (ERP^R09) consists of the MSH, MSA, QAK, ERQ segments; the remainder of this message is defined by the contents of the corresponding segment-oriented record-oriented unsolicited update message, excluding the MSH, as defined by the function-specific chapters of this specification. The input parameter list may be satisfied by more than one record-oriented unsolicited update message: in this case, the segment group after the ERQ segment may repeat.