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I have a script that migrates data in the database.

It copies property X to property Y.
If I want the script to be idempotent, what should it do on subsequent call if X changed?

For example: 

X is 'a'   
==> I run the script, then Y is 'a'.    
now X is 'b'
  • What would subsequent call to the script do? set Y to be 'b' or leave it untouched?

For those who wanted me to clarify my question - I am trying to clarify the meaning of "idempotent" when a resource it references changes.

I was asked to write an idempotent script, and one of the comments I got on it were that I did not handle the scenario where Y changes. I had to know if I misunderstood something or if it was not specified in the requirement.

Please do not close this answer as I think future readers may benefit it.

guy mograbi
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    If you set X to 'b', then run a script that copies X to Y, then obviously Y is now 'b'. – user253751 Jan 17 '19 at 22:23
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    A function is idempotent if it results in the same output given the same input. Trying to determine if a function is idempotent by giving it two different inputs is not a meaningful exercise. – Robert Harvey Jan 17 '19 at 23:12
  • @RobertHarvey I thought that the meaning was - no matter how many times I run it, the end result will always be that Y = X. So the end result will always be the same. Do those kind of scripts have a different name? – guy mograbi Jan 18 '19 at 22:57

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You already have the correct answer to your question: idempotence is a meaningless concept in a situation where the inputs change.

But för a more helpful answer, imagine the following scenario: your colleague says, "I locked the database to give you and me exclusive access to write to it. I then may or may not have run your supposedly idempotent script on it and I won't tell you which. I also set up a hook in the database such that if I ran the script before, it will explode if it changes anything now, and if I didn't run it before, the end result should still be the expected after you run it or it will explode."

It's a contrived scenario, but it leaves only one valid behavior for your script.

kqr
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  • So how do you call a script that the end result is always the same. in my example - every time I run it, X will become Y. is there a name for this? – guy mograbi Jan 18 '19 at 22:59
  • also - if my script logs output. does it affect its idempotency? logs are side effects. – guy mograbi Jan 18 '19 at 23:29