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SAP ABAP ALE and IDOC Explained

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SAP ABAP IDOC explained :

IDOC Programming
 There are two processes in IDOC processing one is INBOUND PROCESS( IDOC coming to the system and its handling at various stages)  and the other is OUTBOUND PROCESS( IDOC is send to other system  . Separate ABAP programs are written for these 2 processes and different configuration settings are required for each one .Depending upon the trigeering mechanism different programming approaches are used.OUTBOUND PROGRAMS handle the IDOC creation and INBOUND PROGRAMS handle the inbound IDOC and the data trasfer from the IDOC to the database tables.
1)New IDOC was created
   Depending upon the triggering mechanism i.e change pointer,message control
   etc the programming technique changes and the type of process.
2)Existing IDOC was extended
   User Exits are used to populate data in new segments added and to read them 
3)To enhance an exsiting process.
   User Exits are used . 

Basic structure of an OUTBOUND PROGRAM
The basic structure of all outbound programs is same. The CONTROL
record which is of TYPE EDIDC has to be filled. It contains important fields like
IDOCTP      IDOC type
MESTYP    Message Type
RCVPRN    Recieving Partner (Destination) 
RCVPRT    Partner Type (it is LS i.e Logical System) 
The DATA RECORD internal table which is of TYPE EDIDD has to be filled
in the same order as there are segments in the IDOC definition.
It has 2 important fields which are to be filled ,these are
SEGNAM  Name of the segment
SEGDATA Data in the segment and can be upto 1000 bytes.This field
                is corresponds to each data segment in the IDOC.
DATA W_SEGDATA LIKE zsegment "custom segment zsegment
       W_SEGDATA-field1 = value    "taken from some table
       W_SEGDATA-field2 = value    "taken from some table
This will fill the data record corresponding to the segment ZSEGMENT . For all
segments the sam steps will be repeated until all the data corresponding to all the
segments in the idoc is filled in the internal table of type EDIDD.
      There is only one control record and can be multile data records in the idoc
Which data is to be extracted from the tables to be filled in EDIDD type internal table is determined in standalone programs from the objects which are specified in the selection screen and from the object key passed to the function module in case of message control and is taken from BDCP table in case of change pointers using function modules "CHANGE_POINTERS_READ"
      In case of stand alone programs and function modules based on change pointers method the function module "MASTER_IDOC_DISTRIBUTE" is used to send the IDOC to ALE layer for distribution ,in case of message control it is handled automatically by the RSNASTED program invoked by the message control.                     

 Outbound programs are used in outbound process to genearte IDOCs . There
purpose is to extract the data from the tables and to fill the IDOC data records
and the control record in the IDOC and pass ito the ALE layer for distribution to the recieving system .
 Depending upon the triggering mechanism (how and when the outbound program will be started different techniques and interface is used for outbound program.
1)Message control
 Application like PO(purchase order) creation use this technique , where the output type is linked to the application(Message control is a configurable cross application component which can be set in IMG .Such programs are created in the form of function modules and linked to the output type which is linked to the apllication.These have predefined interfaces.

In outbound programs(function module) that use message control the key of the
application document is passed to the function module whcig will read the data from the table depending upon the key passed(NAST structure) and fill the IDOC segemnts with the application data.The control record is also passed to the function module but is not fully filled, the rmaining fields are filled and the control is passed back to the calling program which is RSNASTED and is invoked by the
message control to process ALE and EDI output types and this program on recieving the IDOC data will create the physical IDOC in the system.
The parameteers passed to the function module are
  It contains documents key.
  It contains the partailly filled control record
 The internal table used to fill the data records in the same order as they exists in 
 the IDOC.
 The important parameters that should be filled in the custom function module are
 CONTROL_RECORD_OUT and  INT_EDIDD .The OBJK field of the structue NAST contains the key of the apllication document.

2)Stand alone programs
  Generally used for Master Data transfer and as such depending upon the requirement can have different interfaces.The programs have a selection screen which has select options to select the objects, and parameters for specifying the recipient Logical System and the Message Type.
 Data is selected based on the specified objects from the tables . and control record of type EDIDC is created and filled with important values like DIRECTION ,IDOCTP,RCVPRT,RCVPRN and MESTYP .
 Data is filled in an internal table of TYPE EDIDD according to segment definitions
and in same order and the function module MASTER_IDOC_DISTRIBUTE is called passing the control record and the internal table containg IDOC data and importing an internal table consisting of control records for each communication IDOCS created.

3)Change pointer
Change Pointer table is read and checked if changes were made to important fields , if so the outbound program will be triggered automatically , these have predefined interfaces

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