View source for Hello World (using database, display file and RPG)

{{FIXME|Rephrase, reformat, set links and wade out unneccessary stuff.}}
This project will show you how to create the minimal components of a runnable application:
* a database file,
* a display file and
* a program to show the content, in ILE RPG.

This could have been done more easily<ref>e.&thinsp;g. with [[Hello World (using C)]]</ref>, but [[User:Heiko|Heiko]] wanted to use these standard approach particular features to learn how to do this.

To start, a basic knowledge about AS/400 concepts and how to edit files is mandatory.

By the way: Don't bother about string cases. The AS/400 won't either. Using capital commands and names just makes it more readable in this article.

== Creating a physical file to hold the sources ==
On most systems, every user has a personal library, just like a home in a Linux environment. Multiple libraries coexist, the current (= default for all operations) is stored in the special value <tt>*CURLIB</tt><ref>About the Current Library: https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_71/rzarl/rzarlcurlib.htm</ref>, just like you are used to a cwd, current working directory in common operating systems with command lines.

Because the value of <tt>*CURLIB</tt> can be changed by the screen you are currently working in, all following commands will contain my personal library name <tt>HKRETSCHME</tt>. If you are sure <tt>*CURLIB</tt> is set correctly, you can usually omit the library.

This command creates a source physical file named <tt>HELLO01</tt> that is about to contain all the source "files" (called "members"):
 CRTSRCPF FILE(HKRETSCHME/HELLO01) RCDLEN(112) TEXT('Project Hello World')

I prefer to do this with prompting, that I'd recommend to every beginner:
 CRTSRCPF

If you don't supply any arguments for this command (or press <tt>F4</tt> instead of <tt>Enter</tt>), you will be presented with a form for helping you find the right parameters.
[[Image:HelloWorld01CRTSRCPF.png|666px]]

Using no parameters to bring up the prompt screen works great for <tt>CRTSRCPF</tt>, but in general, OS/400 will only ask for '''necessary''' arguments. Some commands, like <tt>CHGUSRPRF</tt> will just do nothing, because there was nothing to change.

==== A small detour: "Physical Files"? ====
[[User:Heiko|Heiko]] struggled a lot with this and thinks a small introduction is necessary:
* <tt>Libraries</tt>: Every user has his own personal library, think of it like a Linux home directory. [https://en.wikipedia.org/wiki/AS/400_library Details]
* <tt>Physical Files</tt>: Of course this is no physical file and you can't touch it. And don't think of if (like I as a german<ref>Yes, as a german IT guy you think of "file" as "Datei". But "file" in english can also mean "Akte" or "Karton mit Akten". Sorry for Kauderwelsch.</ref> do) as one file (f.e. <tt>/etc/passwd</tt>), it's more like an collection of informations, like you might know from MacOS resources.
* <tt>Members</tt>: A member is a file in an physical file, like a book in a filing box. Members of a physical file share the given structure of the PF. Record structure, field lengths, etc.

A more visal approach of explaining:<ref>I'm not good at drawing...</ref><br/> 
[[Image:PhysicalFileOrWhat.png|666px]]

The <tt>Library</tt> is the room you keep your <tt>Physical Files</tt> (like filing boxes) in. The <tt>Physical Files</tt> contain <tt>Members</tt> (like books) but must be the same size (okay, a crude explanation for the "must share structure" approach).

* You can have as much rooms, filing boxes and books as you want.
* But you can't put rooms in rooms, boxes in boxes or books in books<ref>And as you may have guessed, you can't put rooms in boxes, boxes in books&#x2026;</ref>.
* The AS/400 has no tree-like storage structure, it is just a list (of Libraries) with a limited depth (Physical Files and Members) to unfold:<br/>[[Image:PhysicalFileList.png]]

=== Creating the database ===
The procedure is as follows:
* Create a text file (called <tt>DDS</tt>) describing the database
* Compile it into a database physical file

==== Creating the DDS ====
The textual description of a database is called <tt>DDS</tt>. You can learn more [https://www.ibm.com/support/pages/data-transfer-creating-physical-file-using-dds here about DDS] and [https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_72/rzakc/rzakcdatel.htm here about the DDS format and database fields].

* Input this to work with our just created Source Physical File:<br/>[[Image:Hello01wrkmbrpdm01.png| 666px]]<br/><code>wrkmbrpdm file(hkretschme/hello01)</code>
* Hit <code>F6</code> to create a new member:<br/>[[Image:Hello01wrkmbrpdm01a.png|666px]]
* This will run STRSEU:<br/>(Input member name and type <code>PF</code>)<br/>[[Image:Hello01wrkmbrpdm02.png| 666px]]
* That brings you to SEU:<br/>(Enter text as shown)
         ***************** Datenanfang *******************************************************************************************
 0001.00      A          R MESSAGES                                                                               191001           means: <code>A</code>dd<ref>I know, <tt>A</tt> stands alphanumeric. But <tt>Add</tt> is no much better to remember. :)</ref> <code>R</code>ecord named <code>MESSAGES</code>
 0002.00      A            TEXT          42A                                                                      191001           means: <code>A</code>dd field named <code>TEXT</code> with <code>42</code> bytes of <code>A</code>lphabet characters.
 0003.00      A          K TEXT                                                                                   191010           means: Make the field <code>TEXT</code> the key.
         ******************Datenende *********************************************************************************************
* Enter <tt>Y</tt> (here: <tt>J</tt>) to save the member.<br/>[[Image:Hello01wrkmbrpdm04.png|666px]]<br/>After saving, check the lower lines for messages. If there is a synthax error, return to SEU and check every line.<br/>'''''Hint: Use <code>F4</code> and <code>F1</code> for assistance while coding!'''''

==== Creating the database physical file ====
* While working with PDM, enter <code>14</code> to compile the database.<br/>[[Image:Hello01wrkmbrpdm05.png| 666px]]
* The compiling will be queued&#x2026;<br/>[[Image:Hello01wrkmbrpdm06.png| 666px]]
* &#x2026;and you have to check with <code>DSPMSG</code> whether it was successful or not<ref>Can anyone tell me how this is done synchronously? Performance isn't an issue any more&#x2026;</ref>
Hint: Checking my messages with DSPMSG only showed me it wouldn't compile. No error message or any other useful information. If you run into trouble, double check your DDS in SEU, using <code>F4</code> on every single line. And remember the AS/400 is picky. Even an empty line is enough cause to abort compilation.
* If compilation worked out, you can see your compiled database physical file here:<br/><code>wrklib + <F4></code>
* Enter <code>5</code> to show contents of your library
* And here it is:<br/>[[Image:Hello01wrkmbrpdm10.png| 666px]]<br>It is named <code>HELLO</code>, named after the DDS Member we compiled<ref>If we had chosen the same name for the member as for the source physical file, the PDM would have asked, if I'd want to overwrite the source physical file with the compiled database. Naming the member differently avoided this. This has the advantage that the DDS member remains for later use.</ref>.

=== Insert content into the database ===
* <code>STRSQL</code> + <code>F4</code>
** Enter the library name, in this example <code>HKRETSCHME</code><ref>Shortcut: <code>STRSQL LIBOPT(HKRETSCHME)</code></ref>
* In the interactive SQL session, enter: <code>INSERT INTO  HELLO (TEXT) VALUES ('Hello World!')</code>
* There are many ways to display the result:
** If you're still in STRSQL, enter: <code>SELECT * FROM HELLO</code>
** On command line, try this: <code>DSPF FILE(HKRETSCHME/HELLO)</code>

=== Display the content ===
==== Create a display file ====
A Display File (device) is a template for displaying your content.
* Read [https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_73/rzakc/displ.htm this] and [https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_74/cl/crtdspf.htm this] about creating Display Files (DSPF)
* <code>CRTDSPF</code> followed by <code>F4</code>
** Enter target file and library (I used: <code>HELLODSP</code>, <code>HKRETSCHME</code>), that will result in a newly created Display File named <tt>HELLODSP</tt>)
** Enter source file, library and DDS member (in this example: <code>HELLO01</code>, <code>HKRETSCHME</code> and <code>HELLO</code>)
** Press &#x23CE;
* Then I ran into an problem: The DSPF wasn't created and I couldn't locate the error message. Depending on your system's configuration you will find them in your Print-Queue (use <code>WRKOUTQ</code>!), or even printed out on a printer.

After some hours, I got it right:
    Spalten . . . :    1 100                                     Editieren                                          HKRETSCHME/HELLO01 
    SEU==>                                                                                                                   HELLODSPF
    FMT DP .....AAN01N02N03A.Name++++++RLängeDDsFZeiPosFunktionen++++++++++++++++++++++++++ ...+... 9 ...+... 0                       
        ***************** Datenanfang *******************************************************************************************  
   0001.00                                             <font color="red">REF(HKRETSCHME/HELLO)</font>                                        191007             
   0002.00      A          R MESSAGES<font color="red">_</font>                                                                              191007             
   0003.00      A            TEXT      <font color="red">R</font>   42A  <font color="red">O  5  3</font>                                                             191007             
           ******************Datenende *********************************************************************************************   
I copied the DDS Member to a new Member named HELLODSPF, changed the type to DSPF, and added the red characters.
Explanation:
* the first row to indicate this refers to a database physical file<ref>Beginning with column 45</ref>
* the record is named slightly differently than in the DDS, using a trailing <code>_</code>.
* <code>R</code> to mark this field als reference<ref>Column 29</ref>
* <code>O</code> to mark this field als output<ref>Column 38</ref>
* <code>5  3</code> to locate the field in row 5, column 3 of the output screen<ref>Row: 39-41, column: 42-44</ref>
If you struggle with the positions of the keywords, have a look at IBM's <tt>DDS Reference</tt> (PDF) (I used Version 4), the 708 pages are packed with great screenshots and explanations.

Finally, <code>14</code> to compile XOR <code>CRTDSPF FILE(HKRETSCHME/HELLODSP) SRCFILE(HKRETSCHME/HELLO01) SRCMBR(HELLODSPF)</code> worked out:<br/>
[[Image:HelloWorldCRTDSPFOK.png|666px]]

==== Create the program ====
Now it's getting clear, that the whole design follows the [https://en.wikipedia.org/wiki/Model–view–controller MVC Paradigma]:
* Database Physical File => Model
* Display File => View
* Program<ref>Short: PGM</ref> => Controller

You can create programs in many languages: CL, REXX, C, COBOL and most commonly used, RPG.

Let's create a RPG:
* <code>WRKMBRPDM</code>
* Press <code>F6</code> to create a new <tt>RPGLE</tt> file called <code>HELLORPG</code>
* Enter this code:
         ***************** Datenanfang *******************************************************************************************
 0001.00 F* Program for reading one row from database and write it do display                                191008               
 0002.00 FHELLO     IF   E           K DISK                                         Def. DB PF               191010               
 0003.00 FHELLODSPF CF   E             WORKSTN                                      Def. DSPF                191010               
 0004.00 C                   READ      MESSAGES                                     Read 1 row               191010               
 0005.00 C                   WRITE     MESSAGES_                                    Print 1 row              191010               
 0006.00 C                   READ      MESSAGES_                                    Wait for input (key)     191010               
 0007.00 C                   RETURN                                                 End program              191008               
        ******************Datenende *********************************************************************************************
* Use <code>14</code> to compile><ref>If it doesn't check <code>DSPSMG</code> and, most importantly, your queues with <code>WRKOUTQ</code>. In my case, the compilation reports will be filed as PDF in my home.</ref>

== Run the program ==
* Enter <code>CALL HKRETSCHME/HELLORPG</code>:<br/>[[Image:HelloWorldRPGFinallyDone.png]]

We did it! \o/

== Footnotes ==
<references />