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Create a Simple ABAP Daemon

Learn how to create ABAP Daemons and interact with them using PCP messages.
You will learn
  • How to create and run an ABAP Daemon
  • How to send PCP messages to an ABAP Daemon

Prerequisites

  • Your system needs to be based on SAP NetWeaver 7.52 or higher.
  • You need to use ABAP Development Tools.

Introduction to ABAP Daemons

ABAP Daemons are provided by the ABAP Daemon Framework (ADF). They are used to handle events in a reliable way by running in sessions with unlimited lifetime. ABAP programs can communicate with the daemons by sending messages to them using ABAP Messaging Channels and message type Push Channel Protocol.

You can make yourself more familiar with the ABAP Daemon Framework by reading the official documentation.

Push Channel Protocol (PCP) is a communications format similar to a simple HTTP message. It consists of header fields (name-value pairs) for metadata and a message body. For further information, have a look at the Specification of the Push Channel Protocol.

  • Step 1

    ABAP Daemons are instances of an ABAP Daemon class that extend the base class CL_ABAP_DAEMON_EXT_BASE.

    First, create a new ABAP class ZCL_TUTORIAL_SIMPLE_DAEMON and set CL_ABAP_DAEMON_EXT_BASE as its superclass.

    As you can see there is an error in line 1 since the necessary abstract methods have not been implemented yet. Click on the light bulb next to the line number and select Add 9 unimplemented methods to resolve this:

    Add unimplemented methods in ABAP Development Tools
  • Step 2

    Before a new instance of the ABAP Daemon class can be created, its ON_ACCEPT method is called to determine if the daemon should start.

    Replace your ON_ACCEPT method with the following code. It accepts all start requests from your own class and rejects all requests from any other program. This is achieved by checking the calling program that initiated the start.

    abap
    Copy
    METHOD if_abap_daemon_extension~on_accept.
      TRY.
          DATA lv_program_name TYPE program.
          lv_program_name = cl_oo_classname_service=>get_classpool_name( 'ZCL_TUTORIAL_SIMPLE_DAEMON' ).
    
          IF i_context_base->get_start_caller_info( )-program = lv_program_name.
            e_setup_mode = co_setup_mode-accept.
          ELSE.
            e_setup_mode = co_setup_mode-reject.
          ENDIF.
        CATCH cx_abap_daemon_error.
          " to do: error handling, e.g. write error log!
          e_setup_mode = co_setup_mode-reject.
      ENDTRY.
    ENDMETHOD.
    
  • Step 3

    To start the ABAP Daemon, the ON_START method is executed.

    You can include a PCP message containing arbitrary startup parameters which can be accessed in the ON_START method.

    In this simple scenario, you will pass a parameter timeout via PCP. The daemon will then set up a timer to display a popup message each time the timeout was reached. A counter will be used to stop the timer after five messages.

    At first, you need to create some private member variables to store the necessary data. Add these lines to the PRIVATE SECTION of your class:

    ABAP
    Copy
    DATA: mv_timeout TYPE i,
          mo_timer   TYPE REF TO if_abap_timer_manager,
          mv_counter TYPE i.
    

    In the ON_START method you can now retrieve the timeout value from PCP and initiate the timer afterwards. To do so, copy the following implementation:

    abap
    Copy
    METHOD if_abap_daemon_extension~on_start.
      TRY.
          " retrieve timeout from PCP start parameters
          mv_timeout = i_context->get_start_parameter( )->get_field( 'timeout' ).
    
          " start timer for displaying messages
          mo_timer = cl_abap_timer_manager=>get_timer_manager( ).
          mo_timer->start_timer( i_timeout = mv_timeout i_timer_handler = me ).
    
        CATCH cx_abap_daemon_error cx_ac_message_type_pcp_error cx_abap_timer_error.
          " to do: error handling, e.g. write error log!
      ENDTRY.
    ENDMETHOD.
    

    CAUTION! Please add your own exception handling for productive applications as this has not been included in this simple tutorial.

  • Step 4

    While your ABAP Daemon is running in the background, the ON_TIMEOUT method will be triggered when the timeout elapses. Therefore, you need to implement a handler interface. Add this code to the PUBLIC SECTION of your class:

    ABAP
    Copy
    INTERFACES if_abap_timer_handler.
    

    Now insert the following ON_TIMEOUT method into your class implementation. It will display a popup message using function module TH_POPUP and restart the timer five times:

    ABAP
    Copy
    METHOD if_abap_timer_handler~on_timeout.
      " increment the loop counter
      ADD 1 TO mv_counter.
    
      " display popup message
      CALL FUNCTION 'TH_POPUP'
        EXPORTING
          client  = sy-mandt
          user    = sy-uname
          message = CONV th_popup( |Timeout triggered. Number of loops: { mv_counter }| ).
    
      " restart the timer if any loops are remaining
      IF mv_counter < 5.
        TRY.
            mo_timer->start_timer( i_timeout = mv_timeout i_timer_handler = me ).
          CATCH cx_abap_timer_error.
            " to do: error handling, e.g. write error log!
        ENDTRY.
      ENDIF.
    ENDMETHOD.
    
  • Step 5

    You can also send PCP messages to your daemon while it is continuously running in the background. The ON_MESSAGE method will be called every time the daemon receives a PCP message.

    In this tutorial, you will send a plain text to the daemon, which then will be shown in a popup message. As a real life example, you could imagine the ABAP Daemon receiving live data from a sensor and sending a notification under certain conditions.

    Replace your ON_MESSAGE method with this code:

    abap
    Copy
    METHOD if_abap_daemon_extension~on_message.
      TRY.
          " get text from PCP message
          DATA(lv_text) = i_message->get_text( ).
    
          " display popup
          CALL FUNCTION 'TH_POPUP'
            EXPORTING
              client  = sy-mandt
              user    = sy-uname
              message = CONV th_popup( |Message received: { lv_text }| ).
        CATCH cx_ac_message_type_pcp_error.
          " to do: error handling, e.g. write error log!
      ENDTRY.
    ENDMETHOD.
    
  • Step 6

    Since the security concept of ABAP Daemons is based on “source code authorization” (see documentation), i.e. only the program (e.g. a class) which starts the daemon has the right to send messages to the daemon or to stop it. For simplicity reasons we use the daemon class to implement the start, stop and send functionalities for the daemon to access them from different programs.

    This simple ABAP Daemon will be started by a static method. Thus, you need to create a new method START in your daemon class with two import parameters:

    • IV_DAEMON_NAME: The name of the daemon
    • IV_TIMEOUT: The timeout between two successive timer events (milliseconds)

    To do so, paste the following code into the PUBLIC SECTION of the class definition:

    ABAP
    Copy
    CLASS-METHODS start
      IMPORTING
        iv_daemon_name TYPE string
        iv_timeout     TYPE i
      RAISING
        cx_abap_daemon_error
        cx_ac_message_type_pcp_error.
    

    Instantiate the ABAP Daemon using the ABAP Daemon Manager. Its static method CL_ABAP_DAEMON_CLIENT_MANAGER=>START requires the name and priority of the daemon as parameters. Additionally, you can pass the startup parameters as a PCP message.

    Copy these lines into your class implementation:

    ABAP
    Copy
    METHOD start.
      " set ABAP Daemon start parameters
      DATA(lo_pcp) = cl_ac_message_type_pcp=>create( ).
      lo_pcp->set_field( i_name = 'timeout' i_value = CONV #( iv_timeout ) ).
    
      " start the daemon application using the ABAP Daemon Manager
      cl_abap_daemon_client_manager=>start(
          i_class_name = 'ZCL_TUTORIAL_SIMPLE_DAEMON'
          i_name       = CONV #( iv_daemon_name )
          i_priority   = cl_abap_daemon_client_manager=>co_session_priority_low
          i_parameter  = lo_pcp ).
    ENDMETHOD.
    
  • Step 7

    Implement a static method SEND which will be used to send text to the daemon and therefore trigger its ON_MESSAGE method. The method will need the following importing parameters:

    • IV_DAEMON_NAME: The name of the daemon that will receive the message.
    • IV_TEXT: The text message that will be send to the daemon.

    You can have multiple instances of your daemon running independently at the same time as they can be distinguished by their name.

    Paste the code below into the PUBLIC SECTION of the class definition.

    ABAP
    Copy
    CLASS-METHODS send
      IMPORTING
        iv_daemon_name TYPE string
        iv_text        TYPE string
      RAISING
        cx_abap_daemon_error
        cx_ac_message_type_pcp_error.
    

    In order to send PCP messages to your daemon, you first have to retrieve a list of all running ABAP Daemon instances of your class ZCL_TUTORIAL_SIMPLE_DAEMON. Afterwards, you can compare the name of each instance to IV_DAEMON_NAME and send the PCP message accordingly.

    Insert the following code into the class implementation:

    ABAP
    Copy
    METHOD send.
      " retrieve the list of ABAP Daemon instances
      DATA(lt_ad_info) = cl_abap_daemon_client_manager=>get_daemon_info( i_class_name = 'ZCL_TUTORIAL_SIMPLE_DAEMON').
    
      " create PCP message with text
      DATA(lo_pcp) = cl_ac_message_type_pcp=>create( ).
      lo_pcp->set_text( iv_text ).
    
      " for each running daemon instance of this class
      LOOP AT lt_ad_info ASSIGNING FIELD-SYMBOL(<ls_info>).
    
        " send a message if the names match
        IF iv_daemon_name = <ls_info>-name.
          cl_abap_daemon_client_manager=>attach( <ls_info>-instance_id )->send( lo_pcp ).
        ENDIF.
    
      ENDLOOP.
    ENDMETHOD.
    
  • Step 8

    Congratulations, you have now created your first ABAP Daemon class!
    Activate it by pressing Ctrl+F3.

    You can run your daemon by simply calling the static START method created previously. Therefore, create a new ABAP Program Z_TUTORIAL_SIMPLE_DAEMON_START with only one line of code:

    abap
    Copy
    zcl_tutorial_simple_daemon=>start( iv_daemon_name = 'simple_daemon' iv_timeout = 10000 ).
    

    Activate the program and run it as ABAP Application:

    Running the ABAP Program

    You should see a popup notification every 10 seconds:

    Popup message from an ABAP Daemon
  • Step 9

    In addition to the timeout, you can trigger a popup message manually by calling the static SEND method. Create another ABAP Program Z_TUTORIAL_SIMPLE_DAEMON_SEND with the following content:

    abap
    Copy
    DATA(lv_text) = `This is a simple ABAP Daemon message sent via PCP.`.
    zcl_tutorial_simple_daemon=>send( iv_daemon_name = 'simple_daemon' iv_text = lv_text ).
    

    Feel free to change the message text in line 1. After activating and executing the program you should see a popup message similar to this:

    Popup message from an ABAP Daemon via ON_MESSAGE

    If you do not receive any message: Make sure your daemon has been started previously in step 8. The daemon will keep running even after the popup messages stop.

  • Step 10

    You can monitor all running ABAP Daemons using transaction SMDAEMON in SAPGUI. There you can see their state, check for errors, and also restart and terminate them.

    To stop your daemon, select it from the list and go to ABAP Daemon > Terminate Daemon.

    Alternatively, you can also create a static STOP method. Therefore, add this to the PUBLIC SECTION of your class definition:

    ABAP
    Copy
    CLASS-METHODS stop
      IMPORTING
        iv_daemon_name TYPE string
      RAISING
        cx_abap_daemon_error.
    

    Copy the following lines into the class implementation:

    ABAP
    Copy
    METHOD stop.
      " retrieve the list of ABAP Daemon instances
      DATA(lt_ad_info) = cl_abap_daemon_client_manager=>get_daemon_info( i_class_name = 'ZCL_TUTORIAL_SIMPLE_DAEMON').
    
      " for each running daemon instance of this class
      LOOP AT lt_ad_info ASSIGNING FIELD-SYMBOL(<ls_info>).
    
        " stop the daemon if the names match
        IF iv_daemon_name = <ls_info>-name.
            cl_abap_daemon_client_manager=>stop( i_instance_id = <ls_info>-instance_id ).
        ENDIF.
    
      ENDLOOP.
    ENDMETHOD.
    

    Now you can stop your daemon by executing

    ABAP
    Copy
    zcl_tutorial_simple_daemon=>stop( iv_daemon_name = 'simple_daemon' ).
    

    from any ABAP Program.

    For further information on ABAP Daemons, see the ABAP Daemons documentation.

  • Step 11

    Start your daemon using the following line of code. Note that the daemon name is now significantly longer and the timeout is negative.

    ABAP
    Copy
    zcl_tutorial_simple_daemon=>start( iv_daemon_name = 'does_the_daemon_start_up_if_you_use_this_as_your_daemon_name' iv_timeout = -10000 ).
    

    Which of these is correct?

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