What is pipeline and how do messages flow in XI.
Pipeline is the term used to refer to all steps that are performed during the processing of an XML message.
A pipeline represent a fixed processing run for particular XML messages and differs from other pipelines depending on the configuration of the Integration Engine. There are pipelines for a local Integration Engine in the sender or receiver system, or for a central Integration Engine that is configured as the Integration Server, for example.
The individual processing steps that an XML message must pass through when it is processed are called pipeline elements. Each pipeline element calls exactly one pipeline service.
Pipeline services provide functions for processing XML messages. They read and can change inbound XML messages, for example, the mapping or routing. They can also call processing processes, for example functions to create business data in an SAP component. The service type, number, and sequence are specified for each pipeline and cannot be changed.
The pipeline services are either part of the Integration Engine itself or originate from other areas of the SAP Exchange Infrastructure. A pipeline service can be used by more than one pipeline element.
When a source message reaches the Integration server
the messages under goes this
a) Receiver Determination:
This steps determines the system that participates in the exchange of the message.
b) Interface Detremination:
For each receiver determine which interface will should receieve the message.
c) Message Split:
If more than one receievers are found, XI will instantiate new message for each receiver.
d) Message Mapping:
Mapping to transform the source message to destination message format.
e) Technical Routing:
Bind a specific destination and protocol to the message.
f) Call outbound Adapter:
Send the transformed message to the adapter or a proxy.
Refer to this link for more details;