|By Bill Beale||
|December 1, 2012 12:00 PM EST||
I recently read Berndt Hamboeck's article, ‘NetStart' Your PowerBuilder Application after being kindly pointed to it by Bruce Armstrong. It made perfect sense to me to be able to distribute .pbd patches to client machines by having the client-application automatically install them from the application's database. In the lead-up in Berndt's article, he touched on a similar problem - the difficulties of distributing database modification scripts to DBAs prior to the installation of a new executable. In this respect, I had coded functionality for the application I develop to process database modifications from within the application without the need for the DBA's intervention. I tentatively mentioned this to Bruce. (I say ‘tentatively' because I'm a relatively inexperienced programmer). He asked me to write this article.
By way of background, I'm a self-taught PowerBuilder programmer, having been introduced to programming in an ancillary subject at university back in the days of mark-sense cards, Fortran, and Cobol. I am a law enforcement investigator by trade, and designed an application about 15 years ago to process telephone call charge records and financial statements for an Australian law enforcement agency at which I worked. I later engaged a skilled PowerBuilder programmer, Sue Stagg, to set up a commercial version as the New South Wales Police, the fifth-largest police force in the world, wanted to use the application. Unfortunately, Sue had to move on because she started a family, so I took over the programming. I mention this background for two reasons.
The first is as a caveat. Experienced programmers may be able further enhance or refine my methods, which I'll set out below.
The second is that the application I develop (CallRunner) has to be continually enhanced, which invariably includes database changes. The need for ongoing enhancement is because of rapid changes with the technology of mobile phone communications - such as the introduction of Internet access, email, SMS, video, photography, and GPS.
Cellphones have become a valuable source of intelligence for law enforcement. As a demonstration for a new extension to my application, the NSW Police extracted a file of 4400 pages of data from my cellphone. It included historical SMS messages, emails, photographs, and a myriad of GPS locations I'd plotted from all over the county while riding my Harley, as well as the GPS coordinates of where I took my photographs. This data has to be read in and linked to existing data within the application.
In distributing new executables or patches, I'd regularly experience the issues mentioned by Berndt, and I'd often have to spend time at locations where there was no luxury of a DBA.
To save my clients and myself this grief, I created a library entry - call it, say, database.pbl. It included an nvo called nvo_database. A function, of_initialise(), returned the build date and version number as instance variables from the Application Manager.
The fact that modifications were effected from a .pbd meant that if I needed to send out a .pbd patch that relied on database modifications, I could include a database.pbd as well.
Code in the pfc_open event of the Application Manager instantiates unvo_database and calls of_initialise(). Calls are then made from the pfc_open event to at least two like functions in the nvo to effect the database changes.
The first function called from pfc_open manages database changes for the preceding build of the application - just in case the user hasn't installed that version and has jumped to the current release. The second function call is relevant to database changes for the current build. There is an overlap - a ‘boots and braces' approach. You can see from the code below that, as the builds progress, obsolete function calls are remmed. The two valid functions do the work to effect the more recent database changes.
The database modifications typically include:
- Creating a table
- Adding a column to an existing table
- Altering a column in an existing table
- Adding data to an existing table, or
- Adding an index
The functionality varies slightly for each but an underlying theme is to trap error codes in the first instance to confirm that a table or column does not exist. If they exist, as they would after the first run of the application, then the change routine is skipped.
The first job of this functionality was to create a table to hold a record of the SQL code used to modify the database. This table held the SQL code, the build date, and the version number of the application, and the date and time the changes were effected. The following is the code initiated from of_database_changes() function of unvo_database ...
The code in the Create Table function is:
The first process in this function is to determine whether the table exists; this is a key function. If the table doesn't exist and returns FALSE, a message is given asking the user whether they want to exit to backup the database.
If the table does not exist, dynamic SQL is used to create the table.
If an error is encountered at any point "Failed on:" is prefixed to SQL code which is saved, whether the changes were successful or not, by way of a call to the function of_save_database_changes(..) in the table just created.
If there is an error at any point, an instance variable, ib_fail, is set to TRUE and, after all subsequent modifications have been processed, a message is given to the user. The DBA can print a report from the database_changes table that will indicate exactly where the error occurred.
The following is the code in the key function of_check_for_existing_table():
A bogus SELECT COUNT call is made on the table. If the table doesn't exist, an error code will be returned. The error code for a non-existing table is 208 and this is compared to an instance constant of that value. The error is trapped and a Boolean value of FALSE is returned to indicate that the table does not exist. If the table exists, TRUE is returned - nothing else needs to be done.
The following is similar code used when adding a column to a table - in this case, column cue_word_sample_text:
Checking and implementing changes for adding indexes or entering data into a table are effected in much the same way. This involves either checking for an error code for the existence of a column or an index, or using SELECT to determine whether data already exists in a table. When using SELECT I found that I had to use SQL Code such that I did not get an error for returning more than one row.
An exception where SQL code could not be used involved the altering of a column width from, say, varchar(100) to, say, varchar(200). With my limited experience I could find no way to use SQL code to determine the attributes of a database column (‘describe' and ‘ColType' clearly only work on the DataWindow object, which is independent to, and may not always comply with, the column description in the table). In such cases, I simply checked the database_changes table to ensure that the SQL has not already been executed.
To guarantee that I was distributing the correct changes and that they will all work as expected, I test the SQL code in the database painter of an auxiliary application. I then copy/paste the tested SQL code into my PowerBuilder development application functions as described. Because my ongoing coding and testing will be contingent on the changes working correctly, I'll quickly establish if there is an error.
I have only had one hiccup and that was where a user had added an index to a column the application was attempting to alter. The DBA was able to print the Database Changes Report and we quickly established where the error occurred from the ‘Failed on' prefix. He dropped the index, re-booted the application to automate the change, and then reinstated the index. All was well.
Following from this, if you wish to alter a column that has an index, you need to code the dropping and reinstating of the index either side of the column alteration.
In this regard, there may be the prospect beyond my knowledge of using SQL to check first and get a handle on an unknown index before altering a column.
This process has been a tremendous time-saver for me. But more than anything, I can distribute my application with a great deal more confidence. Users are not frustrated or lose confidence in the application because of installation errors. Nor has there been any noticeable deterioration in the start-up time of the application on the first run when the changes are effected.
The sample code can be downloaded here.
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