Tell me diff between userexit and badi and give me MM flow

Question

frnds  answer to my questions kindly

Answer 1

Hi Basha,
What are user exits    
   1.   User defined functionality included to predefined SAP standards. User should find the customer enhancements belonging to particular development class.
   2.   USER EXITS are FORMS and are called by SAP standard programs using PERFORM.
   3.   Inside the form (user exit) you can read and change almost any global data from host program.
  4.    User exits are more flexible because you have more information to use in your code.
  5.     Basically designed For SD module.
   6.    User-exits can be written only using access-key
With user-exits the code , the developer has written will not be there for next version unless modification assistance tool is used
Definition:
                            There are some requirements in standard programs for which a generic solution cannot be provided, as it much dependent on how the user wants to do it.
For Example:  
                              PO number validation in sales order (standard order). This is not a necessity and is not done by SAP. But if the user wants to check for this then it has to be done by means of these exits. The user exit points are already defined by SAP and these exits will be called at these predefined points. All we have to do is put our code inside that exit.
What are BADI's?
                                 BADI - Business Add Ins are enhancements to the standard version of the code of SAP. They can be inserted into the SAP system to accommodate user requirements too specific to be included in the standard delivery. Customers can find the possible enhancements (BADIs) in their SAP system in the Implementation Guide (IMG) and in the component hierarchy. BADI definitions are included in the IMG so that the customer can create suitable, company-specific implementations. SAP guarantees the upward compatibility of all Business Add-In interfaces. Release upgrades do not affect enhancement calls from within the standard software nor do they affect the validity of call interfaces.
Two different views are available:
•     In the definition view, an application programmer defines exit points in a source that allow specific industry sectors, partners, and customers to attach additional coding to standard SAP source code without having to modify the original object.
•     In the implementation view, the users of Business Add-Ins can customize the logic they need or use a standard solution if one is available.
MM Flow:
The typical procurement cycle for a service or material consists of the following phases:
1. Determination of Requirements
Materials requirements are identified either in the user departments or via materials planning and control. (This can cover both MRP proper and the demand-based approach to inventory control. The regular checking of stock levels of materials defined by master records, use of the order-point method, and forecasting on the basis of past usage are important aspects of the latter.) You can enter purchase requisitions yourself, or they can be generated automatically by the materials planning and control system.
2. Source Determination
The Purchasing component helps you identify potential sources of supply based on past orders and existing longer-term purchase agreements. This speeds the process of creating requests for quotation (RFQs), which can be sent to vendors electronically via SAP EDI, if desired.
3. Vendor Selection and Comparison of Quotations
The system is capable of simulating pricing scenarios, allowing you to compare a number of different quotations. Rejection letters can be sent automatically.
4. Purchase Order Processing
The Purchasing system adopts information from the requisition and the quotation to help you create a purchase order. As with purchase requisitions, you can generate Pos yourself or have the system generate them automatically. Vendor scheduling agreements and contracts (in the SAP System, types of longer-term purchase agreement) are also supported.
5. Purchase Order Follow-Up
The system checks the reminder periods you have specified and - if necessary - automatically prints reminders or expediters at the predefined intervals. It also provides you with an up-to-date status of all purchase requisitions, quotations, and purchase orders.
6. Goods Receiving and Inventory Management
Goods Receiving personnel can confirm the receipt of goods simply by entering the Po number. By specifying permissible tolerances, buyers can limit over- and under deliveries of ordered goods.
7. Invoice Verification
The system supports the checking and matching of invoices. The accounts payable clerk is notified of quantity and price variances because the system has access to PO and goods receipt data. This speeds the process of auditing and clearing invoices for payment.
Also refer to this link for the tables linked with this module.
http://www.erpgenie.com/sap/abap/tables_mm.htm
I hope this will be helpful for you.
Reward points according to you.
Cheers,
Swamy Kunche