Friday, November 03, 2006

QA - Obscure Behavior simultaneously across Multiple Machines?

Question:

On our IIS 5 (w2k server etc) using perl 5, our bulletin board has suddenly stopped adding new files. After some testing, its fine updating existing ones, but seems unable to add new - writing them via code. Since I havne't changed anything in the code, is it possibly some obscure piece of system-wide policy has caused this?

The exact same symptoms are happening on another server, and the problem is not restricted to one folder either.

Answer:

Come now... baffling events can be viewed as logical events whose logic is not immediately clear to the observing party. It does not mean toss logic aside in favor of random fantasies. ;-) I mean, computers do not yet dream/fantasize... they are stubbornly logical and require strict instructions in the form of binary code.

Put another way (proof through contradiction) - suppose the issue is some obscure piece of system-wide policy - how likely is the same SYSTEM-WIDE policy simultaneously applied on two DIFFERENT servers and multiple folders?

Since you observe the same behavior from multiple servers, and it is highly unlikely for two random system-wide configuration changes to generate matching behaviors, I do not believe in merely blaming "obscurities in Windows". Instead, I would look for what resources are COMMON to both servers and look for limits in them - such as:

  • The Bulletin Board software itself
  • Commonly shared UNC fileshares used by the BBS (Are you sure "adding new files to the BBS" actually creates files in the FileSystem? Having URLs do not mean there are real files because URLs are merely resource descriptors and not strictly mapped to filenames on a FileSystem)
  • Commonly shared SQL database between the servers
  • etc.

I mean, here are some suggested thoughts:

  • Maybe the Bulletin Board software has a built-in limit on the number of posts for a given forum, so as soon as you reached it from one server, the other server will also fail for that forum
  • Or the NTFS Disk Quota on a commonly shared UNC fileshare for a commonly shared access-user is reached by one server and seen from the other
  • Or the number of rows in your SQL database table is capped and you cannot add any more files/posts

In general, I do not believe in "obscure behavior" on a Computer because it usually means I have not tried hard enough to think outside the box, to gather information and knowledge, to make sense of the situation. I mean, it is tempting to hypothesize from one's knowledge of the system even if it is better to logically deduce from facts.

//David

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