ALE - Process

ALE has three prime entities, which participate in the message exchange process:
SAP R/3 system
ALE converter (inclusive of its sub-components)
External non SAP R/3 system
The following diagram describes how these three work together in an ALE scenario.
Figure 2: ALE FUNCTIONALITY
2.1 Concept of Logical Systems (LS):
Logical Systems represent R/3 or external systems in the SAP R/3 environment for the distribution of data. The existence of LS is mandatory for the corresponding R/3 client (which uses ALE or EDI). This LS acts as the “sender” for outbound messages and a “receiver” for inbound messages.
In addition to the base LS, another LS must be present within that R/3 system for each R/3 or external system used for ALE interfaces. In an inbound ALE interface, second LS represent the sender (another R/3 or external system) with respect to the base LS (receiver). In an outbound ALE interface, this second LS is the receiver on behalf of the R/3 or external system with respect to the base LS (sender).
2.2 Message Type:
Message types (required for sending the data across participating systems) relate to the structure of the data called an IDOC (Intermediate Document). Message types provide the format of the data, which can be exchanged between SAP and non-SAP systems. ALE supports over 200 message types in R/3.
2.3 IDocs:
IDocs (Intermediate Documents) are the crux of ALE. These are the documents with the help of which data is exchanged between SAP R/3 and non-R/3 systems. As the name suggests, these documents act as intermediate storage of information, which can be sent bi-directionally. Message types are responsible for creation of Idocs.
IDocs Components
Data on Internet is exchanged using data formats like IPV6 – and SAP uses a similar approach. Idocs are a well-structured medium through which data transfer takes place. Figure 3 explains the structure of Idocs.
Figure 3: IDoc COMPONENTS

An IDoc is made up of the following parts:
Control Record: This section contains control information regarding the Idoc. Its constituents are Sender’s name, Receiver name, Message type and Idoc type (illustrated in figure 3). The format of the control record is similar for all IDoc types.
Data Segment: It consists of a header that contains the identity of the Idoc. Its constituents include, a sequential segment number, a segment type description and field containing the actual data of the segment.
Status records: The status record shows the information regarding the already processed stages and remaining processing stages of the Idoc. It has an identical format for each IDoc type.
Idocs Structure:
Idocs support a hierarchical structure. Figure 4 shows an Idoc, starting with the header. The header has items, which in turn have sub-items. The lowest level in the hierarchy contains the text. The end of the Idoc is represented with the help of ACCUM (means accumulate) segment. Remember that an IDoc can only contain character fields.
2.4 ALE Converters

ALE converters need to be compatible with SAP R/3 system and therefore chosen carefully. SAP offer a certification program for ALE converters – and there are a large number available.
The minimum requirements of the ALE converter are:
Ability to accept and understand participating IDocs in message exchange.
Capability of transaction id management and status tracking.
Ability of mapping IDoc fields to and from external systems.
Independency from R/3 application server.