> And what does it have anything to do with re-sending the commands when they didn't reach their target machine?

It has to do with whether its at all safe to run the command twice. If you don't hear back from a machine that your command ran, you don't know if the command reached the machine or not, in general. If you decide to not re-run the command, it may be that it never made it, and you never run the command. If you decide to re-run the command, it may have reached the machine, and you've now run the command twice. Consider:

  1. You send command $X to a remote machine.
  2. The remote receives and executes $X successfully.
  3. Just prior to sending back that $X was successful, the remote dies.

From the point of view of the deployment machine, you never hear back from the remote. You don't even need two machines to get into this state: even on a single command, if you run $X and the machine crashes just as $X is about to return whether or not it succeeded or failed, you're out of luck.

For specific commands, such as ones that log their progress and can recover from crashes, you can determine after a crash the correct procedure automatically. But not every arbitrary Unix administration command falls into that bucket, especially in the weird and wacky things that need to be done to keep production machines alive.

> Uhm... What? Where did you get the idea that upgrading database is managing configuration? It's a deployment operation. A different category.

We used our deployment infra for all sorts of fun stuff.