Integration Server Engines 
The Integration Server is the runtime component of SAP NetWeaver PI that is used in an A2A or B2B scenario to receive messages and forward them in a controlled manner. When you want to configure a system landscape that is to be integrated using the Integration Server, you assign the role of the Integration Server to a client of the SAP NetWeaver AS. The following two engines work together in this client to control the message flow:
The Integration Engine is responsible for central Integration Server services, for example, routing and mapping.
The Business Process Engine is responsible for executing and persisting integration processes.
Furthermore, the majority of the adapters run on the central Adapter Engine, which is installed on the SAP NetWeaver AS (Java), and which can take over inbound and outbound processing on the Integration Server from the Integration Engine. In this way, the central services of the Adapter Engine (for example, persistence), can be used by all adapters.
Special Features of the Integration Engine
The Integration Engine can be deployed in various roles in different business systems. It comprises the following parts:
A messaging logic, which implements the XI message protocol. The messaging logic receives messages and forwards them on. In the process, it ensures that the services that are defined by the protocol, such as quality of service and status management, also apply to messages (including processing of acknowledgments).
For more information, see: XI message protocol
An integration logic, which is used to describe the central services of the Integration Engine: Logical routing, technical routing, mapping, and adapters.
The application logic in the application systems should be kept separate from these components of the Integration Engine. Application logic includes the selection of application-specific data or the updating of requests in a business system, for example. For example, the application program can use proxies to exchange messages by means of the Integration Server in a cross-system process. Proxy objects are include in the application logic.
You can only use the integration logic of the Integration Engine if you have assigned the role of central Integration Server to the Integration Engine in exactly one client. The following is an example of proxy communication by using the Integration Server:
Note
You do not have to configure the Integration Engine as the Integration Server in client 010. This is just an example.
Integration Engines
In other clients, you can configure the Integration Engine in such a way that its task is simply to send and receive messages. If no other client of the SAP NetWeaver AS (ABAP) sends or receives messages, the role of the Integration Server is assigned to the entire AS ABAP, although technically speaking only the engines of a client actually take on this role. You can also assign a client of SAP NetWeaver AS (ABAP) as the Integration Server while the other clients are used as application clients.
For more information, see: Communication with Embedded Process Integration
Note
The messaging logic for proxy communication is part of the SAP NetWeaver AS on either the sender or receiver business system. In adapter communication with the Integration Server, messaging is part of the Adapter Engine.