4 ms·

In Ansible, you declare a set of actions, that are then performed, one by one. Occasionally being skipped, if a certain condition holds true. So, you are basic

In Ansible, you declare a set of actions, that are then performed, one by one. Occasionally being skipped, if a certain condition holds true. So, you are basically saying "do this, do that, then do that". In a declarative model, you would say "this is how the end result should look" and the tool would then go off and make that happen, in whatever order its scheduling tools would say. Sort of the difference between Rust on one side, and Prolog on the other (yes, it is possible to get a specific flow of instructions in Prolog, but it is much easier to let the prolog interpreter/compiler to Just Make It Happen Somehow). FWIW, Puppet gets closer to a declarative model, but unfortunately, the last version I played around with seriously was actually quite bad at inferring ordering on its own, so a LOT of work ended up going into "well, A has to happen before B, so let us string a dependency here".

tehbeard 5y ago
I guess I need an example of the declarative model, as I can see the Ansible model in my head and it still looks declarative to me atleast at the singular module level. in Ansible, you say "make sure these packages are installed" and they'll be installed as needed, to match that state, or ignored if already there. Even the file level stuff you can say "make sure this line is in the file" and it either adds it or says "nope, that's already in there". Is it that there's modules that aren't declarative? Sort of the esoteric ones to poke specific cloud infrastructure (though even the few of those I looked at seemed to be declarative if needed).
- randomswede 5y ago
  It is not declarative. The simple fact that the playbook will ALWAYS be run in the order you specify, even if a later step is (technically) a prerequisite of a previous step, means that you are in an imperative mode. Puppet is declarative, you simply say "these things must, or must not, hold" and a combination of user-declared and inferred dependencies arrange the sequencing, which can be different in each run (as long as the before/after dependencies hold).
  - tehbeard 5y ago
    Ah so it's that the playbooks are imperative and "dumb" (does exactly what you tell it, rather than inferring "these actions must happen, do them in a sensible order"). That makes sense. Dove into the puppet docs/wiki article, I guess part of the difference as well is that puppet considers each "unit" a resource, vs. ansible being a "module/action". It does seem like ansible roles have a dependency mechanism, I guess that might be the intended level for a "declarative" approach in ansible, to encapsulate the playbooks/modules underneath that are more of an implementation detail at that point.

tehbeard 5y ago

I guess I need an example of the declarative model, as I can see the Ansible model in my head and it still looks declarative to me atleast at the singular module level. in Ansible, you say "make sure these packages are installed" and they'll be installed as needed, to match that state, or ignored if already there. Even the file level stuff you can say "make sure this line is in the file" and it either adds it or says "nope, that's already in there". Is it that there's modules that aren't declarative? Sort of the esoteric ones to poke specific cloud infrastructure (though even the few of those I looked at seemed to be declarative if needed).

randomswede 5y ago
It is not declarative. The simple fact that the playbook will ALWAYS be run in the order you specify, even if a later step is (technically) a prerequisite of a previous step, means that you are in an imperative mode. Puppet is declarative, you simply say "these things must, or must not, hold" and a combination of user-declared and inferred dependencies arrange the sequencing, which can be different in each run (as long as the before/after dependencies hold).
- tehbeard 5y ago
  Ah so it's that the playbooks are imperative and "dumb" (does exactly what you tell it, rather than inferring "these actions must happen, do them in a sensible order"). That makes sense. Dove into the puppet docs/wiki article, I guess part of the difference as well is that puppet considers each "unit" a resource, vs. ansible being a "module/action". It does seem like ansible roles have a dependency mechanism, I guess that might be the intended level for a "declarative" approach in ansible, to encapsulate the playbooks/modules underneath that are more of an implementation detail at that point.