How did this happen?

Last time I showed two different implementations of the naive method for generating unique coupon codes. The traditional method does this:

    do
    {
        id = pick random number
    } until id not in used numbers
    code = generate code from id

The other is slightly different:

    do
    {
        id = pick random number
        code = generate code from id
    } until code not in used codes

Before we start, understand that I strongly discourage you from actually implementing such code. The naive selection algorithm performs very poorly as the number of items you choose increases. But the seemingly small difference in these two implementations allows me to illustrate why I think that the second version is broken in a fundamental way.

Think about what we’re doing here. We’re obtaining a number by which we are going to identify something. Then we’re generating a string to express that number. It just so happens that in the code I’ve been discussing these past few entries, the expression is a six-digit, base-31 value.

So why are we saving the display version of the number? If we were going to display the number in decimal, there’d be no question: we’d save the binary value. After all, the user interface already knows how to display a binary value in decimal. Even if we were going to display the number in hexadecimal, octal, or binary, we’d store the binary value. We certainly wouldn’t store the string “3735928559”, “11011110101011011011111011101111”, or “DEADBEEF”. So, again, why store the string “26BB62” instead of the the 32-bit value 33,554,432?

I can’t give you a good reason to do that. I can, however, give you reasons not to.

  1. Storing a six-character string in the database takes more space than storing a 32-bit integer.
  2. Everything you do with with a six-character code string takes longer than if you were working with an integer.
  3. Display logic should be part of the user interface rather than part of the database schema.
  4. Changing your coding scheme becomes much more difficult.

The only argument I can come up with for storing codes rather than integer identifiers is that with the former, there’s no conversion necessary once the code is generated. Whereas that’s true, it doesn’t hold up under scrutiny. Again, nobody would store a binary string just because the user interface wants to display the number in binary. It’s a simple number base conversion, for crying out loud!

If you’re generating unique integer values for database objects, then let the database treat them as integers. Let everybody treat them as integers. Except the users. “26BB62” is a whole lot easier to read and to type than is “33554432”.

I’ll be charitable and say that whoever came up with the idea of storing the display representation was inexperienced. I’m much less forgiving of the person who approved the design. The combination of the naive selection algorithm, storing the generated coupon code rather than the corresponding integer, and the broken base conversion function smacks of an inexperienced programmer working without adult supervision. Or perhaps an inexperienced programmer working with incompetent adult supervision, which amounts to the same thing.

The mere existence of the naive selection algorithm in this code is a sure sign that whoever wrote the code either never studied computer science, or slept through the part of his algorithms class where they studied random selection and the birthday paradox (also, birthday problem). The comment, “There is a tiny chance that a generated code may collide with an existing code,” tells us all we need to know of how much whoever implemented the code understood about the asymptotic behavior of this algorithm. Unless your definition of “tiny” includes a 10% chance after only one percent of the codes have been generated.

The decision to store the generated code string surprises me. You’d think that a DBA would want to conserve space, processor cycles, and data transfer size. Certainly every DBA I’ve ever worked with would prefer to store and index a 32-bit integer rather than a six-character string.

The way in which the base conversion function is broken is hard evidence that whoever modified it was definitely not an experienced programmer. I suspect strongly that the person who wrote the original function knew what he was doing, but whoever came along later and modified it was totally out of his depth. That’s the only way I can explain how the function ending up the way it did.

Finally, that all this was implemented on the database looks to me like a case of seeing the world through database-colored glasses. Sure, one can implement an obfuscated unique key generator in T-SQL. Whether one should is another matter entirely. Whoever did it in this case shouldn’t have tried, because he wasn’t up to the task.

However this design came to exist, it should not have been approved. If there was any oversight at all, it should have been rejected. That it was implemented in the first place is surprising. That it was approved and put into production borders on the criminal. Either there was no oversight, or the person reviewing the implementation was totally incompetent.

With the code buried three layers deep in a database stored procedure, it was pretty safe from prying eyes. So the bug remained hidden for a couple of years. Until a crisis occurred: a duplicate code was generated. Yes, I learned that the original implementation didn’t even take into account the chance of a duplicate. Apparently somebody figured that with 887 million possible codes, the chance of getting a duplicate in the first few million was so remote as to be ignored. Little did they know that the chance of getting a duplicate within the first 35,000 codes is 50%.

I also happen to know that at this point there was oversight by a senior programmer who did understand the asymptotic behavior of the naive algorithm, and yet approved “fixing” the problem by implementing the duplicate check. He selected that quick fix rather than replacing the naive algorithm with the more robust algorithm that was proposed: one that does not suffer from the same pathological behavior. The combination of arrogance and stupidity involved in that decision boggles the mind.

The history of this bug is rooted in company culture, where the database is considered sacrosanct: the domain of DBAs, no programmers allowed, thank you very much. And although programmers have access to all of the source code, they aren’t exactly encouraged to look at parts of the system outside of their own areas. Worse, they’re actively discouraged from making suggestions for improvement.

In such an environment, it’s little surprise that the horribly broken unique key generation algorithm survives.

So much for that. Next time we’ll start looking at good ways to generate unique, “random-looking” keys.

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