Understanding RICEFW - Interfaces

Interface programming is responsible of making SAP communicate with external systems. 
How does SAP do this?
1. File Interface for reading and writing files.
2. RFC
3. BAPI
4. ALE & EDI

File system:
This is a good way to exchange data between 2 systems. The legacy system writes data to a file, this file is transported into the target SAP system, the target system reads the data in the file and writes it to the DB.  SAP has many such importing programs like -
CALL TRANSACTION:
How do you use one?
CALL TRANSACTION <TCODE>
USING <BDC_TAB>
MODE <MOD>
UPDATE<UPDATE>
MESSAGES INTO <TABLE>

1. Prepare a DB table with bdcdata specific to the transaction
<bdc_tab>-prognam
<bdc_tab>-dynpro
<bdc_tab>-dynbegin
 append <bdc_dat>

2. MODE can be A, N, E. Where in you can mention how you want the data transfer process be displayed.
A - Display All (All screens and the data)
N - No display (Even if there are errors, the screens are processed invisibly)
E - Display only errors.

3. UPDATE
A - Asynchronous - COMMIT WORK
S - Synchronous - COMMIT WORK AND WAIT
L - Local - SET UPDATE TASK LOCAL

BATCH INPUT:
The main advantage of batch input is the controlling and correction of errors.
BDC Programming is the batch interface technique in sap. But the batch input is little slower compared to the call transaction. The batch input is especially advantageous when transferring large amounts of data.
SHDB is the transaction, to perform a session recording.

RFC:
The ABAP function modules can be called from external systems using the RFC function protocol.
All the parameters in an RFC must be referenced to the ABAP dictionary and all of them must be passed by value. From version 6.0 RFCs also include changing parameters. The destination parameter is the one which lets the rfc function modules be called from other external systems. To create a destination use the tcode SM59.

BAPI:
Business Object (BO) gives an object oriented view of the business data and transactions. A BOR consists of the object types of these BO. To maintain a BOR SW0 and SW01 are to be used. The methods of some object types are identified as BAPI, and BAPIs are commonly used for RFC purposes.
The reason why BAPIs amongst all the methods of BO types have become so famous is
BAPIs have stable interface, meaning the interface is not changed by any new release of SAP.
BAPIs do not trigger exceptions, they only give return parameters.
DBs are updated in an update task, meaning BAPIs perform updates by calling an update module  CALL FUNCTION IN BACKGROUND TASK. A commit work or a rollback work is not committed in a BAPI.

EDI:
EDI is used for exchanging data between 2 systems for which an IDOC structure is required. An IDOC consists of DATA, CONTROL and STATUS records.
ALE is also a similar technology, but it is used to transfer data internally, but if it is data transfer between SAP and an external system EDI is the one which is generally preferred.
IDOC:
To create segments - WE31
To create IDOC - WE30
Create or assign message type - WE81/WE82
Partner profile set up - WE20

RSNAST00 program is used when batch input process is set.
Ex: In a sales order output the processing mode should be set to 1 and not to 4 which is immediate processing. RSNAST00 program picks up all the outputs whose processing mode is set to 1 from the NAST table and executes the data and gives an outbound idoc.

CHANGE POINTERS:
It is the process in SAP which triggers every time there is a change in the master data. Change pointer is something similar to the CDPOS and CDHDR.
BD50- Activate change pointer in a message type
BD52- Create change document item
BD61- Activate change pointer globally
Change pointers are captured in table BDCPS and BDCP
BD20 - To create IDOC from change pointers. (Report RBDMIDOC)
BD20 - To delete change pointer entries.