The Runtime Lifecycle, LvlChk and LwtChk

The IBM i Display File Lifecycle

The IBM i Display File, as explored elsewhere, is systematically created from a DDS. The DDS defines formats, fields, subfiles and the like. Each format is assigned a Format ID during *DSPF generation. For example, if a simple DDS with two record formats (FM1 and FM2) each of which has two fields (CREDIT and DEBIT for FM1, HOME and CELL for FM2), was used to create a *DSPF, each record format would be assigned a Format ID (FMTID1 and FMTID2).

When an RPG program that uses the *DSPF file (based on a workstation F-spec in the source code) is created, the compiler embeds the Format IDs of the file into the program. At runtime, when the workstation file is opened, the system checks the Format IDs embedded in the RPG program against the ones in the display file *DSPF object. If the *DSPF has been altered without recompiling the program, the Format IDs won't match during the Format Level Check and a LVLCHK exception will be thrown, stopping the program.

One way around this safeguard is to override the level check using CHGDSPF LVKCHK(*NO), however this opens your program up to possibly corrupt data or other errors arising if incorrect or mismatched instructions are present in the *DSPF.

The Mobile Display File Lifecycle

MobileRPG is designed to work with RPG programs, and therefore it uses several parts of the IBM i lifecycle. Specifically, the RPG program still checks for Format IDs; however, in a mobile environment there is no *DSPF available at runtime to obtain the Format IDs of the display file from, instead it is the responsibilty of the Mobile RPG Handler to provide these Format IDs.

Mobile Display Files are .aspx pages: they are the equivalent of both the DDS source where the display file is described and the *DSPF object used at runtime to render the display on a screen. In order for the Handler to interpret the data buffers presented to it by the RPG program accurately, it must know the layout of the fields in the buffer; basically, it must know the format of the record, and there must be a perfect match between the layout according to the RPG program and Mobile.

MobileRPG stores the essence of the record format layout in a file parallel to the .aspx with the extension of .wrf (Wings Record Format, a name taken from ASNA Wings). In a way, the .aspx and WRF pair takes the place of the IBM i *DSPF object at run time. The WRF file contains an entry for each display record format with the ordered list of the field's attributes composing the record. The entry for each record format also contains its IBM i Format ID. This Format ID can then be used by the Handler at open time to ensure that its mapping of the record layout matches what was embedded by the RPG compiler when the program was created.

The WRF file is created by Mobile RPG when an initial .aspx page created, and then it gets updated everytime that the .aspx is Exported to the IBM i. In order to guarantee that the mapping of the formats in the .aspx and the WRF definitions correspond to each other, Mobile RPG embeds the timestamp (kept by Windows) of the last write operation done to the .aspx page into the WRF. This timestamp is known as the 'LastWrite Time' or LWT.

When the Handler receives a request to open a display file, it looks for the .aspx and WRF pair and checks that the .aspx LWT timestamp matches the one embeded in the WRF file.

If all goes smoothly, the RPG program runs against the .aspx page just as if it were a perfectly formatted *DSPF.

Mismatch Scenarios

If there is a mismatch in the LWTs, it means that the .aspx file was updated after the last Export operation. It is possible, in the course of development or deployment, that the .aspx is changed without Exporting it back to the IBM i. Naturally, this creates a failure-point, as a WRF that doesn't match up with its .aspx could cause data corruption or other errors. To prevent this, when performing an open operation the Handler runs a process called LwtCheck (Last Write Time Check) and checks for the last time the .aspx was saved and the LWT embedded in the WRF. If they match up, the Handler concludes that all is well and moves on.

However, the LWTs failing to match isn't, in itself, sufficient to throw an exception. Many alterations to the .aspx do not impact the formats and fields that concern the RPG program. For example, changing a background color or a border width. Therefore, if the LwtCheck fails, the Handler recomputes the the WRF from the .aspx: if no RPG-impacting changes have been made, the two WRFs will be identical (except the timestamp), and the open operation will be completed successfully.

In Debug mode, the WRF will also be automatically updated with the new LWT to prevent having to recompute it every time. Deployment environments (where the Handler may not have permission to update the WRF) merely pass the .aspx on while leaving the WRF unchanged.

If the recomputed WRF definition does not match the existing WRF, due to a signficant modification of the .aspx, for example a field name or type change, or a reording of the fields in the format, a 'Level Check' exception is thrown and the open operation halts on the IBM i.

Published Websites

.aspx pages are not accessed directly by a browser, they are instead instructions that allow the ASPX engine to compile the page into HTML that a browser can read directly. It does this automatically the first time it is loaded by a web browser, creating a DLL (dynamic link library) reflecting the page or pages.

This one-time compilation can cause a slowdown for the first user accessing the page. One way around this is to publish the .aspx as non-updatable. This means that ASPX creates all of the required DLLs well before the first use, consolidating all of the data into a single resource which cannot easily be edited. However, this process replaces the original .aspx page with a placeholder, an effectively blank document that points toward the DLL but contains almost nothing itself. This scenario interferes with MobileRPG's default operations, as the LWT of the WRF (created on Export) will necessarily be different from the LWT on the .aspx placeholder (set on publish), and leaving MobileRPG with no way of generating a new LWT because the .aspx page is empty. This will always cause a LwtCheck exception.

This scenario can be resolved by disabling LwtCheck; if the Handler doesn't check for the LWT, it will simply pass in the data from the WRF and follow the placeholder's pointer without issue. This can be set either within the WRF parameters on a page-by-page basis, or starting with Mobile RPG 6.2, globally in the web.config file.

Web pages can also be slowed down if there is a mismatch between the LWTs of the .aspx page and the WRF. The process of validating the .aspx, while not especially heavy, can cause a noticable delay (particularly if there are many pages with mismatched WRFs) as the Handler constructs a new WRF.

Therefore it's a best practice to always ensure that your .aspx pages and WRFs are in sync, but development teams must weigh the benefits of stability against the convenience of maintaining an updatable .aspx.

Disabling LwtCheck

On a single Mobile Display File:

The LwtChk property in the MobileRPGRecordFormat element of the WRF must be set to "*NO".

<MobileRecordFormat LvlChk="*YES" CcsId="37" LwtChk="*NO">

Globally

Starting with Mobile RPG 6.2, the Web.Config file's AppSettings AsnaDisplayFileLwtChk key's value can be set to "*NO".

<AppSettings>
	<add key="AsnaDisplayFileLwtChk" value="*NO" />
</AppSettings>

See Also

• Wings Record Format
• Optimizing Speed