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this adds support for specifying runtime config options via cli args. cli args take precedence over the any values in config files. note that we don't support the entire nerve config; service level/check config can still only be done via the config file
use a json schema to encode the docs for nerve config and improve docs
Rob-Johnson
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EvanKrall,
giuli007,
ilkinmammadzada and
nemacysts
ilkinmammadzada
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giuli007
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I haven't read it all in superdetail but looks reasonable to me!
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Background: We use nerve-tools to restart nerve at Yelp. Notably, rather than doing a hard restart, we launch a second copy of nerve, replace the 'main' nerve process, and then terminate the backup. This prevents us from deregistering all the backends running on a host every time we restart nerve. We run the backup nerve instance using the same config file that we run the main instance with.
In #16 we added support for exporting prometheus metrics from nerve. this requires binding an http server to the configured port. However, this doesn't play nicely with our graceful restart procedure; we can't have 2 nerve processes bind to the same prometheus port.
To overcome this, we've chosen to disable prometheus on the backup nerve instance. Which raises the question: how should we tell the backup nerve instance to not enable prom metrics? We could tell the backup nerve instance to use a backup config file, but that introduces the suprise factor of having 2 config files on disk.
Instead, this PR introduces the ability to set the runtime nerve config (not the service/healthcheck config) via cli args. cli args take precedence over the config file. That'll allow us to just run the backup nerve instance with --prometheus-enabled=false, but still use the same config file for all other options.
I've tried to improve the docs for the config while I'm here, encoding the config in a json schema and using it to auto generate
docs/configuration.md. I've included a make target for generating this automatically.