MLLP V2 Content Exchange Model

MLLP Release 2 is a reliable message transport protocol. It guarantees in order delivery and at least once delivery of HL7 content. HL7 content is framed in a block and sent to the destination system. The destination system acknowledges the receipt of the message by returning a commit acknowledgement message. The MLLP V2 acknowledgement protocol is synchronous: the source system will not send new HL7 content until an acknowledgement for the previous HL7 content has been received.

Figure 23 Interaction Diagram

If the HL7 content (a query in the example below) triggers the sending of HL7 content (a response) by the destination system, then this HL7 content is framed in a block and sent. MLLP has no knowledge of the HL7 content, nor does it base any part of its behavior on HL7 content. With the TCP/IP HL7 Adapter, a database is used to persist the message.

In inbound mode, once the message is received by the HL7 Adapter, it is persisted into the database and a commit acknowledgement is sent to the sender before the HL7 message is passed to the Collaboration level. If the persistence fails, a negative acknowledgement is sent to the sender and the Collaboration receives null as its message. Inside the Collaboration, the message received is null and the Collaboration returns without proceeding.

If the message received is a duplicate message and if an ACK/NAK is already available in the persistence database, the same is retrieved from the database and sent to the sender. The Collaboration receives null as the received message.

In outbound mode, the message is sent to the sender and waits for the commit acknowledgement or negative acknowledgement from the receiver. If the adapter receives a negative acknowledgement, the message is sent again until the configured maximum number of retries is made.