RFC: Pack Build Default Process Flag

[hone]

readable

[sclevine]

Would we prefer the --process flag set CMD or set CNB_PROCESS_TYPE?
(The spec suggests using CNB_PROCESS_TYPE.)

[hone]

@sclevine that would just set the env var?

[sclevine]

Yep, on the image so that launcher can read it.

[jkutner]

I prefer setting CMD as it aligns better with standard OCI image stuff (which seems like something we should try to do when possible). For example, a consumer of the image would be able to easily understand why it's running the worker process instead of the web process without knowing anything about CNB_ vars.

[nebhale]

Happy with the CMD strategy. Expecting another RFC to determine precedence between CMD, $CNB_PROCESS_TYPE, and other possibilities.

[ekcasey]

I think CNB_PROCESS_TYPE is a better path forward. That means we are changing the default process type, but a user can still override by explicitly overriding CNB_PROCESS_TYPE. That override wouldn't work if we set CMD instead, given the current precedence. I don't see a compelling reason to change the precedence.

[ekcasey]

I wonder if we could have two flags, --default-process <process-type> which sets CNB_PROCESS_TYPE and another --process <type>="<command>" which creates an entry in metadata.toml that looks like: (assume direct false, and no args)

[[processes]]
type = "<type>"
command = "<command>"
args = []
direct = false

You could also support --process=</path/to/process.toml> where process.toml supports all process options.

To add a process and make it the default you would need both --process and --default-process flags. Or you could use --process to override web and it would be the default.

[hone]

@nebhale @sclevine @jkutner updated the PR based off of @ekcasey's comments on our discussion. Can you please re-review.

[hone]

With @jkutner's vote this has entered Final Comment Period. This will be open until next Thurs on which it will be merged and accepted at the WG meeting.