Why we implement methods in DPC_EXT/MPC_EXT and not in DPC/MPC?

When we change the Model or implement the Service in GW Service builder, every time run time object gets generated, hence DPC and MPC existing code gets replaced with new sets of code. If we implement any code in DPC/MPC while generating runtime object the code gets deleted.

Tcode: SEGW

Created new project z_demo_dpc_exte.

Importing DDIC structure zemployee

Key is employee id.

Generation of run time object.

we are writing code in ZCL_Z_DEMO_DPC_EXTE_DPC (DPC class)

We will regenerate the run time object.

The code gets deleted.  The reason is whenever u change the Model or Service implementation in your GW Service builder, again run-time object should be regenerated in-order to incorporate the latest changes. During this time your MPC & DPC will be loaded with the new code.