Create a data contract class  

[DataContract]
class AcxSysOperationContract
{ 
PurchId purchId; 
[DataMemberAttribute("PurchId")]
public PurchId parmpurchId(PurchId _purchId = purchId)
{
purchId = _purchId;
return _purchId;
}

}

Create a service class where you will place your custom business logic

class AcxSysOperationService
{
public void operation(AcxSysOperationContract _dataContract)
{
info(_dataContract.parmpurchId());
//your business logic
}

}

Create a controller class

class AcxSysOperationController extends SysOperationServiceController
{
public static void main(Args args)
{
AcxSysOperationController sysOperationController = new AcxSysOperationController();
sysOperationController.parmClassName(classStr(AcxSysOperationService));
sysOperationController.parmMethodName(methodStr(AcxSysOperationService, operation));
sysOperationController.parmExecutionMode(SysOperationExecutionMode::Synchronous);
sysOperationController.startOperation();
}

}