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Comment by argd678 | Hacker News Reader

The problem with the idempotent trend in configuration management is that it’s all based on not tracking or knowing what the current state of something is. So reasoning about how these systems work is fundamentally impossible. It would be better to focus on systems where by we can always know the state and improve the tooling there.

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On the contrary. With idempotence you know the state after an action. rm -f will always delete the file (if possible). ln -sfn will always work, even for a directory.

With the default behaviour of rm, ln -s, etc you know neither the state before, nor after.

The option -f is making the script more heavy handed and less likely to fail. But this is not necessary for the script to be idempotent and sometimes is not desirable. You can deal with errors in a more safer way and still be idempotent.

I don't see any conflict here. Idempotency is not a silver bullet and cannot fully replace state tracking, but it adds simplicity where appropriate.

It's a pragmatic approach to this problem. Usually tracking state means that you write some version number or similar to a file, but than if for some reason that goes out of sync with the actual state, for example because you put it in /etc and someone accidentally restores an older version of it, hilarity ensues, because it's guaranteed the devs never expected this to happen and don't handle it in any way.

For the case of a setup script you'd need to update your state file after every step and in turn when running it again see what the file says and resume at whatever point it says. But then you risk running into above problem.

I try to write idempotent scripts whenever possible, combined with general sanity checks specific to whatever environment the script expects.

The reason it’s pragmatic is due to how systems are designed to begin with, it’s not what you want.

We know what the state is right after the command for an undetermined amount of time, but we have no knowledge of what’s changing it. Why not cover that? There’s no reason not to other than it hasn’t been worked on enough. Consider a database schema migration, there’s a reason those are not idempotent, you know each state. Seems quite solvable.

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