Application Engine Commit/Controlling flow of program.

One common issue faces by developers as they create an application engine program is committing of data.

Unlike Sqr, where you can fire the commit yourself, it is also possible to define commit strategy in an App engine program and let that do the work for you. In fact, in app engine, to take benefit of restartability, that is what you should do.


CONTROLLIING COMMITS
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First of by default, ( if you do not explicity change the settings and simply create your section, steps and actions ), the default behaviour is to commit at the completion of the AE program.

While this may seem to be the best alternative. Consider a very long running program which runs for 5 hours. If the program aborts everything is rolled back and you have to run again after rectifying your problem.

In this respect, it makes sense to understand how you can control commits in an AE progam.


Here are the different places where you can configure commit:

At Section: You can specify that the commit will happen after every step.

This will control the behaviour of commits after each step in that section is executed.
However, since each step can have its own unique requirement, you can overide this behaviour at a specific step.



At Step:

Various options include:
1. After Step: ( After the step completes. )
2. Later: Do not commit now. If you have flagged the Section to commit after step, use this to override and not commit after this specific step.
3. Default: use the value supplied at the section level. ( If at section not checked. Commit will occur for the AE at the end of the progam. If checked, commit will be after the step completes.
You can set the frquency option which is handy if you had do select, do while or do until action.


Exceptions
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When the program is restartable ( AE program is flagged as restartable ), the commits defined within a DO Select of type Select/Fetch are ignored.

For e.g. if there is an action wich calls a section with commit after step defined, the commit is ignored.




CONTROLLING FLOW ( when errors are encountered )


1. Step's On error property:

Depending on whether the above property is set to Abort, Ignore or suppress, the AE program will behave:
If Abort is selected, AE program will terminate. It will also rollback uncommitted changes.
If Ignore: Error will be logged but program will continue. No rollback issued.
If suppress: AE will not log and will continue. No rollback issued.

There is no property at the Section level that can control program flow when errror are encountered.

2. SQL Action:

You can control behaviour when No Rows are affected:
1. Set to Abort to rollback and terminate the AE progam.
2. Section Break: Break execution of the existing section.l Any subsequent steps in the section that sql action is part of are not executed. Returns control to the calling section.
3. Continue: Continue processing.
4. Skip Step: Seusequent actions in th step are skipped and control goes back to the next step.


Finally, if the application engine program is called synchronously on an event during component rocessing, all commits are ignored and the commit occurs after save psot change as part of the component commit.