Multi-instance custom service cannot output services

[srid]

This restriction is so because by design we can only output processes (via outputs.settings) not further services.

services-flake/nix/lib.nix

Lines 43 to 54 in 7200b45

	settings = lib.mkOption {
	type = lib.types.lazyAttrsOf lib.types.raw;
	internal = true;
	description = ''
	process-compose settings for the processes under the ${service} service
	'';
	apply = v: v // {
	processes = lib.flip lib.mapAttrs v.processes (_: cfg:
	{ imports = [ config.outputs.defaultProcessSettings cfg ]; }
	);
	};
	};

https://community.flake.parts/services-flake/custom-service#multi-instance

--

Whereas, from user standpoint, it makes sense to have a hierarchy of services. Say, a infra-switch service expands to both a postgres process and a process-tracker service, which then further expands to a Haskell process and its own postgres process. Here, we can define a custom service for postgres-tracker service, but not the infra-switch service, unless we give up on it being a multi-instance service.

graph TD
    A[infra-switch service] --> B[postgres process]
    A --> C[process-tracker service]
    C --> D[Haskell process]
    C --> E[postgres process]

    style A fill:#f9f,stroke:#333,stroke-width:2px
    style C fill:#ccf,stroke:#333,stroke-width:2px
    style B fill:#cfc,stroke:#333,stroke-width:2px
    style D fill:#cfc,stroke:#333,stroke-width:2px
    style E fill:#cfc,stroke:#333,stroke-width:2px

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In principle, we don't intend to run multiple instances of infra-switch ... however, making it multi-instance means we can leverage name in certain places, like namespace (cf. #258) and generally provide a consistent modular interface.