flags

Go flags

Configuration package inspired by this talk / article by Peter Bourgon.

The guiding idea behind this package is that flags are the best way to configure your program and thus it provides a thin wrapper around go standard flag package while providig an extra degree of configurability via different extendable / composable mechanisms such as environment variables, config files etc... on an opt-in basis.

Two of these mechanisms are provided by default env for environment variables and json for flat json config giles. Others resolvers can be implemented additionaly.

Example Usage

As (almost) drop-in replacement for flag package:

var fs flags.FlagSet

var (
	host     = fs.String("host", "DB host", "localhost")
	username = fs.String("username", "DB username", "root")
	port     = fs.Int("port", "DB port", 3306)
)

fs.Parse(os.Args)

Though where it really shines is when you use it in combination with different config mechanisms such as env flags or json config files:

var fs flags.FlagSet

var (
	host     = fs.String("host", "DB host", "localhost")
	username = fs.String("username", "DB username", "root", json.ByName(), env.Named("UNAME"))
	port     = fs.Int("port", "DB port", 3306, json.ByName(), env.ByName())
	cfg      = fs.String("config", "JSON Config file", "config.json", env.ByName())
)

fs.Parse(
	os.Args,
	env.WithPrefix("MYAPP_"), // Optional prefix
	json.WithConfigFile(cfg), // Path to config file
)

As you can see, flag definitions accept additional arguments where you can set different mechanisms that will be evaluated in a sequence and the first one that yields a valid value will be used.

For example here:

username = fs.String("username", "DB username", "root", json.ByName(), env.Named("UNAME"))

If no -username flag is provided, the package would next try to find the value as a json key in the provided config file (see below), if the value is not found it will try to read the environment variable UNAME. You can put as many resolvers as you want (even of the same kind).

NOTE If flag is provided explicitly on the command line it wil always take a precedence regardless if value exists in environment or config file for example.