１、概览
    通过LDB_PROCESS函数可以允许任何程序访问逻辑数据库，允许一个程序访问多个逻辑数据库，当然也允许多次连续访问访问同个逻辑数据库。当使用LDB_PROCESS函数来访问逻辑数据库时，选择屏幕将不显示，其选择参数由FIELD_SELECTION参数传入。

2、LDB_PROCESS参数说明
LDBNAME
Name of the logical database you want to call.

VARIANT
Name of a variant to fill the selection screen of the logical database.
The variant must already be assigned to the database program of the
logical database. The data is passed in the same way as when you use the
WITH SELECTION-TABLE addition in a SUBMIT statement.

EXPRESSIONS
In this parameter, you can pass extra selections for the nodes of the
logical database for which dynamic selections are allowed. The data type
of the parameter RSDS_TEXPR is defined in the type group RSDS. The data
is passed in the same way as when you use
the WITH FREE SELECTION addition in a SUBMIT statement.

FIELD_SELECTION
You can use this parameter to pass a list of the required fields for the
nodes of the logical database for which dynamic selections are allowed.
The data type of the parameter is the deep internal table RSFS_FIELDS,
defined in the type group RSFS. The component
TABLENAME contains the name of the node and the deep component FIELDS
contains the names of the fields that you want to read.

The function module has the following tables parameters:

CALLBACK
You use this parameter to assign callback routines to the names of nodes
and events. The parameter determines the nodes of the logical database
for which data is read, and when the data is passed back to the program
and in which callback routine.

SELECTIONS
You can use this parameter to pass input values for the fields of the
selection screen of the logical database. The data type of the parameter
corresponds to the structure RSPARAMS in the ABAP Dictionary. The data
is passed in the same way as when you use the
WITH SELECTION-TABLE addition in a SUBMIT statement.

3、LDB_PROCESS的CALLBACK回调参数的具体字段的说明
LDBNODE
Name of the node of the logical database to be read.

GET
A flag (contents X or SPACE), to call the corresponding callback routine at the GET event.

GET_LATE
A flag (contents X or SPACE), to call the corresponding callback routine at the GET LATE event.

CB_PROG
Name of the ABAP program in which the callback routine is defined.

CB_FORM
Name of the callback routine.

４、回调函数的编写
回调子程序的标准形式
FORM <subr> USING <node> LIKE LDBCB-LDBNODE
                  <wa>   [TYPE <t>]
                  <evt>
                  <check>.
......
ENDFORM.
其中参数说明作用:
<node> contains the name of the node.
<wa> is the work area of the data read for the node. The program
that calls the function module LDB_PROCESS and the program containing
the callback routine do not have to declare interface work areas using
NODES or TABLES. If the callback routine is only used
for one node, you can use a TYPE reference to refer to the data type of
the node in the ABAP Dictionary. Only then can you address the
individual components of structured nodes directly in the subroutine. If
you use the callback routine for more than one node,
you cannot use a TYPE reference. In this case, you would have to
address the components of structured nodes by assigning them one by one
to a field symbol.

<evt> contains G or L, for GET or GET LATE respectively. This
means that the subroutine can direct the program flow using the contents
of <evt>.

<check> allows the callback routine to influence how the program
is processed further (but only if <evt> contains the value G). The
value X is assigned to the parameter when the subroutine is called. If
it has the value SPACE when the subroutine ends, this
flags that the subordinate nodes of the logical database should not be
processed in the function module LDB_PROCESS. This is the same as
leaving a GET event block using CHECK in an executable program. If this
prevents unnecessary data from being read, it will
improve the performance of your program.

５、样例代码及说明

 TABLES spfli.

 SELECT-OPTIONS s_carr FOR spfli-carrid.

 TYPE-POOLS: rsds, rsfs.

 DATA: callback TYPE TABLE OF ldbcb,

       callback_wa LIKE LINE OF callback.

 DATA: seltab TYPE TABLE OF rsparams,

       seltab_wa LIKE LINE OF seltab.

 DATA: texpr TYPE rsds_texpr,

       fsel  TYPE rsfs_fields.

 *设置需要回调的数据节点和回调对应的子程序

 callback_wa-ldbnode     = 'SPFLI'.

 callback_wa-get         = 'X'.

 callback_wa-get_late    = 'X'.

 callback_wa-cb_prog     = sy-repid.

 callback_wa-cb_form     = 'CALLBACK_SPFLI'.

 APPEND callback_wa TO callback.

 CLEAR callback_wa.

 callback_wa-ldbnode     = 'SFLIGHT'.

 callback_wa-get         = 'X'.

 callback_wa-cb_prog     = sy-repid.

 callback_wa-cb_form     = 'CALLBACK_SFLIGHT'.

 APPEND callback_wa TO callback.

 *设置对应的选择屏幕的参数的传入值

 seltab_wa-kind = 'S'.

 seltab_wa-selname = 'CARRID'.

 LOOP AT s_carr.

   MOVE-CORRESPONDING s_carr TO seltab_wa.

   APPEND seltab_wa TO seltab.

 ENDLOOP.

 *调用函数

 CALL FUNCTION 'LDB_PROCESS'

   EXPORTING

     ldbname                     = 'F1S'

     variant                     = ' '

     expressions                 = texpr

     field_selection             = fsel

   TABLES

     callback                    = callback

     selections                  = seltab

   EXCEPTIONS

     ldb_not_reentrant           =

     ldb_incorrect               =

     ldb_already_running         =

     ldb_error                   =

     ldb_selections_error        =

     ldb_selections_not_accepted =

     variant_not_existent        =

     variant_obsolete            =

     variant_error               =

     free_selections_error       =

     callback_no_event           =

     callback_node_duplicate     =

     OTHERS                      = .

 IF sy-subrc <> .

   WRITE: 'Exception with SY-SUBRC', sy-subrc.

 ENDIF.

 *SPFLI节点对应的回调处理函数

 FORM callback_spfli USING name  TYPE ldbn-ldbnode

                           wa    TYPE spfli

                           evt   TYPE c

                           check TYPE c.

   CASE evt.

     WHEN 'G'.

       WRITE: / wa-carrid, wa-connid, wa-cityfrom, wa-cityto.

       ULINE.

     WHEN 'L'.

       ULINE.

   ENDCASE.

 ENDFORM.                    "CALLBACK_SPFLI

 *SFIGHT节点对应的回调处理函数

 FORM callback_sflight USING name  TYPE ldbn-ldbnode

                             wa    TYPE sflight

                             evt   TYPE c

                             check TYPE c.

   WRITE: / wa-fldate, wa-seatsocc, wa-seatsmax.

 ENDFORM.                    "CALLBACK_SFLIGHT

注意：通过'LDB_PROCESS'函数访问逻辑数据库时，请不要在程序属性里绑定逻辑数据库，否则会出LDB_ALREADY_RUNNING错误。

资料来源sap library.