Precedence of Configuration options: Environment, Registry, Configuration file and Commandline In What Order?

Question

So this is a funny little thing I thing Stack Overflow has not discussed before. Not really a life and death question, but something I'd like to hear your input.

In short: my program is a bit over engineered in this regard: it takes configuration options from four different places:

• Command line parameters
• Configuration file
• Environment variable
• Windows Registry
• (And hardcoded default values)

The question is, in what order should these be evaluated? I think it's clear that the command line options have the last say, but what about the three others? If the same option is set in both environment and ini-file, which should take precedence?

What about registry, does registry override the ini assuming I use that to change the runtime settings of the program, should I rewrite changes applied to registry into the configuration file as well? Should I set it so that registry settings can't override settings read from environment?

(If you wonder how on earth this is possible, one word: X-macro.)

Answer 1

Ordered by priority i would go this way: cli > envvars > registry > config > defaults

• cli: it takes the options that you surely want to use when you manually start the program
• envvar: generaly used to manually override configuration file options at runtime (often also used today to manage configuration options in containers environments, cf. 12factor app)
• registry: specific to Microsoft Windows world, but according to what you said it have almost the same purpose that environment variables
• config: defined at installation/configuration time so these are the "user wanted default options"
• defaults: minimal config to run securely (for example listen on localhost only) when nothing else is provided