Cachix Deploy: auto-rollback support

[domenkozar]

In case the activation of the deployment is not successful, the agent should be able to recover.

[domenkozar]

The only generally applicable assumption is that the agent should still be able to connect to the websocket after the deployment is finished. If not, the deployment should rollback.

Besides that, each agent could have an option to specify what script to run after deployment to check if everything is fine and rollback if not.

I suggest the following deploy specification to support auto-rollback:

{
  "agents": {
    "<agent-name>":  { 
        "deploy": "<top-level-store-path>",
        "rollbackChecks": "<store-path>"
    }
  }
}

I was thinking to make the rollbackChecks script a global option, but it's not really applicable when you run different OSes.

cc @sandydoo, @zupo & @dhess

[zupo]

I don't foresee using Cachix Deploy for anything other NixOS, so I don't understand the argument against a global rollbackChecks script.

The rest is great, can I haz it yesterday ser plz?

[domenkozar]

If you deploy the wrong script to the wrong OS, you'll get a weird error. But that can be solved by inspecting .drv file and checking the system 👍

[domenkozar]

The rest is great, can I haz it yesterday ser plz?

Next week 🚀

[dhess]

I don't foresee using Cachix Deploy for anything other NixOS, so I don't understand the argument against a global rollbackChecks script.

We use it on macOS, where it’s an even more valuable service than on NixOS, as there are no comparable alternatives such as NixOps or deploy-rs.

[domenkozar]

I went for a bit different approach how to specify the rollback script, rollbacks scripts are now keyed by the system:

"rollbackChecks": 
   { "x86_64-linux": "<store-path>",
   , "aarch64-darwin": "<store-path>"
   }

That way the can be specified globally and per system and the backend will dispatch the right one per agent.

WIP at #442

[domenkozar]

Deployed! Last bit missing are the docs: cachix/docs.cachix.org#86

[domenkozar]

Done: https://docs.cachix.org/deploy/reference#deploy-json

Will be release as part of Cachix 1.0.0 tomorrow!