Preferred method for adding a new ecs-agent configuration parameter

[mello7tre]

Hi, looking at bottlerocket sources i have seen that a new ecs-agent configuration parameter, assuming that can be defined using json configuration, can by added changing the code in sources/api/ecs-settings-applier/src/ecs.rs (struct ECSConfig) or by using the environment variable file packages/ecs-agent/ecs.config.
(for both method other files need to be changed too, but i want to focus on the method specific one)

Which is the preferred one ? Thanks.

[zmrow]

Just to clarify, are you thinking of adding a new setting for the ECS agent that can be configured via user data? Something like settings.ecs.<your setting>?

[mello7tre]

i would like to have the settings ECS_IMAGE_PULL_BEHAVIOR.
And if it is not already scheduled to be added, maybe i can do it.

[tjkirch]

If you're talking about adding a setting that's available to any Bottlerocket user, not just to your custom builds of Bottlerocket, then you'd change the ECS data model. There's ECSConfig, as you saw, for settings that wind up in the ECS Agent config file. Then, the ECSSettings structure in sources/models/src/lib.rs, which is where ecs.rs gets the data from, and is what allows users to specify a setting like settings.ecs.image-pull-behavior. You'd also need to add a migration, which allows existing users to upgrade and downgrade Bottlerocket and have the new setting handled automatically during the transition.

If you're interested in implementing that yourself, we have a lot of examples in pull requests where we've added the settings we have today. A recent example is for the hostname setting. There were commits to handle the modeled type, which ensures the value of the setting is valid; you'd do something similar for the valid options of image pull behavior, though perhaps with an enum like this change since there are only a few valid options. The migration would look something like this. And then a README update. :)

If you're just talking about adding that setting to your own custom build, then you can change the ecs.config file however you like, and not worry about any of the above.

[tjkirch]

(Updated the link to a migration example; you'd be adding a single setting, not a whole prefix, so #1659 is simpler than #1664.)

[mello7tre]

Thanks @tjkirch, i have already figured out all this, because i had a deep look at the sources to add settings (not ecs) required by a new program (#1728).
But your clear explanation is welcome.

Coming back to my first answer, assuming that i want to create a PR to add a settings for any Bottlerocket users, which method do your prefer:

• sources/api/ecs-settings-applier/src/ecs.rs
  adding a key to the struct ECSConfig (using proper parameter validation)
• packages/ecs-agent/ecs.config [environment]
  adding something like:
  ECS_IMAGE_PULL_BEHAVIOR="{{settings.ecs.image-pull-behavior}}"

[tjkirch]

Ah, I'm sorry I misunderstood the question. The short answer is that anything that can go into ECSConfig in ecs.rs should go there. The available attributes are defined in this schema for the ecs-agent, which does include pull behavior. Values in this file are passed directly to the agent. If a setting isn't available in the schema, it goes in ecs.config and is passed to the agent using an environment variable. (Thanks to @samuelkarp for background here!)