External configuration beside app.conf & environment variables for revel go framework

I have read revel app.conf manual for custom configuration and environment variables. however I couldn't find way to use additional external configuration along with app.conf.

My goal is to achieve external configuration file in addition to internal app.conf. Let's say creating a product called example and example product maintains it's sensible defaults with app.conf (not exposing to end user) instead product exposes config attributes via example.conf (default location could be /etc/example/example.conf) for product users.

For example: http config field from app.conf

http.addr =
http.port = 9000

extend it to example.conf

http.addr =
http.port = 9000

[database]
host = "localhost"
port = 8080
user = "username"
password = "password"
# etc...

Then I read example.conf during an application start use values also apply values on top of app.conf (overriding). Finally revel server starts!

How to achieve this goal with revel go framework?

It appears you are working against the design of the app.conf. It is already setup to be sectioned, for example all this is in a single app.conf file

[dev]
results.pretty = true
watch = true
http.addr = 192.168.1.2

[test]
results.pretty = true
watch = true
http.addr = 192.168.1.22

[prod]
results.pretty = false
watch = false
http.addr = 192.168.1.100

you can launch 3 different scenarios by using three different command line options

revel run bitbucket.org/mycorp/my-app dev

revel run bitbucket.org/mycorp/my-app test

revel run bitbucket.org/mycorp/my-app prod

I know this is not exactly what your goal is but you can acheive a similar result.

In github.com/revel/revel/revel.go around line 152 you have something like

Config, err = LoadConfig("app.conf").

Maybe you can try and modify that with this

if len(os.Getenv("SOME ENV VAR")) > 0 {
    Config, err = LoadConfig("path/to/your/example.conf")
} else {
    Config, err = LoadConfig("app.conf")
}

You just need to set env var on your prod server.

That way you will not be using app.conf but your example.conf.