[DUG] Validating CDS files

[Jolyon Smith]

> What I'm really after is a file level test to check that file should even
> be attempted. e.g. open file stream seek start and seek end and check a
> couple of bytes... that type of thing.

It seems to me that such a check would be incomplete and therefore
unreliable and that you may introduce this additional check in front of an
attempt to load a CDS files which is still never-the-less corrupt and so
still have to accommodate that potential.

So instead of:

  - attempt to load
    - abend if corrupt

You would have:

  - check validity (however you choose to do this)
  - if OK then
    - attempt to load
      - abend if corrupt
  - else
      - abend (due to detected corruption)


Notice how the flow simply and completely encapsulates the existing flow, it
doesn't avoid or simplify any part of it.  It may simplify and streamline
the abend flow however - but if your application is required to deal with
failures efficiently because you expect more failure than success then you
should perhaps reconsider your approach entirely.

:)


The overhead of "additional" loading is only necessary if you deliberately
perform a full load into some temporary object simply to ensure that you
will successfully be able to load it into the actual desired target object.
But if you can simply load it into the target object then you don't need to
duplicate the load attempt - just go ahead and try to load it and deal with
any failure as required.


Introducing pre-load validity checks is a waste of time if an attempt to
load an invalid file causes nothing other than a "failed" load attempt that
can be safely detected and handled.

Pre-load validity checks would have value if the attempt to load an invalid
file would cause some otherwise undetectable and undesirable side effect.


But it is hard to say for sure what is the correct/most appropriate approach
without more detailed knowledge of the application and its operating
conditions.

+0.02