Message Services 
Message Services enables you to keep a copy of data that is sent to and from your plant systems, which allows plant operations data to be independent of the ERP system.
Message services allow you to do the following:
Enable communication between systems
Transfer data using IDocs and remote function calls (RFC)
Provides Quality of Service (QoS) support
Exactly Once (EO) for all message types like RFC, Idoc, and HTTP
Exactly Once in Order (EOIO) for HTTP messages
Prerequisites
You can use the Message Services actions in the SAP Manufacturing Integration and Intelligence (SAP MII) Workbench. For more information, see Message Services.
Process
The received message is processed regardless of the sources system by the Message Listener.
Based on the message name, the message is processed either by a transaction or put into a category (queue) by the Message Processing Rule.
The transaction processing uses a new thread per message (JMX Thread Pool).
In category processing, single scheduled transaction thread processes message from the queue.
The cleanup rules for the processed messages can be configured using Message Cleanup Rules.
The message details for the processed message can be displayed using Message Monitor.
Process Flow of Message Services in SAP MII