View Source Config.Provider behaviour (Elixir v1.14.2)
Specifies a provider API that loads configuration during boot.
Config providers are typically used during releases to load
external configuration while the system boots. This is done
by starting the VM with the minimum amount of applications
running, then invoking all of the providers, and then
restarting the system. This requires a mutable configuration
file on disk, as the results of the providers are written to
the file system. For more information on runtime configuration,
see mix release
.
Multiple config files
One common use of config providers is to specify multiple
configuration files in a release. Elixir ships with one provider,
called Config.Reader
, which is capable of handling Elixir's
built-in config files.
For example, imagine you want to list some basic configuration
on Mix's built-in config/runtime.exs
file, but you also want
to support additional configuration files. To do so, you can add
this inside the def project
portion of your mix.exs
:
releases: [
demo: [
config_providers: [
{Config.Reader, {:system, "RELEASE_ROOT", "/extra_config.exs"}}
]
]
]
You can place this extra_config.exs
file in your release in
multiple ways:
If it is available on the host when assembling the release, you can place it on "rel/overlays/extra_config.exs" and it will be automatically copied to the release root
If it is available on the target during deployment, you can simply copy it to the release root as a step in your deployment
Now once the system boots, it will load both config/runtime.exs
and extra_config.exs
early in the boot process. You can learn
more options on Config.Reader
.
Custom config provider
You can also implement custom config providers, similar to how
Config.Reader
works. For example, imagine you need to load
some configuration from a JSON file and load that into the system.
Said configuration provider would look like:
defmodule JSONConfigProvider do
@behaviour Config.Provider
# Let's pass the path to the JSON file as config
@impl true
def init(path) when is_binary(path), do: path
@impl true
def load(config, path) do
# We need to start any app we may depend on.
{:ok, _} = Application.ensure_all_started(:jason)
json = path |> File.read!() |> Jason.decode!()
Config.Reader.merge(
config,
my_app: [
some_value: json["my_app_some_value"],
another_value: json["my_app_another_value"],
]
)
end
end
Then, when specifying your release, you can specify the provider in the release configuration:
releases: [
demo: [
config_providers: [
{JSONConfigProvider, "/etc/config.json"}
]
]
]
Link to this section Summary
Callbacks
Invoked when initializing a config provider.
Loads configuration (typically during system boot).
Link to this section Types
@type config() :: keyword()
A path pointing to a configuration file.
Since configuration files are often accessed on target machines, it can be expressed either as:
a binary representing an absolute path
a
{:system, system_var, path}
tuple where the config is the concatenation of the environment variablesystem_var
with the givenpath
@type state() :: term()
Link to this section Callbacks
Invoked when initializing a config provider.
A config provider is typically initialized on the machine
where the system is assembled and not on the target machine.
The init/1
callback is useful to verify the arguments
given to the provider and prepare the state that will be
given to load/2
.
Furthermore, because the state returned by init/1
can
be written to text-based config files, it should be
restricted only to simple data types, such as integers,
strings, atoms, tuples, maps, and lists. Entries such as
PIDs, references, and functions cannot be serialized.
Loads configuration (typically during system boot).
It receives the current config
and the state
returned by
init/1
. Then, you typically read the extra configuration
from an external source and merge it into the received config
.
Merging should be done with Config.Reader.merge/2
, as it
performs deep merge. It should return the updated config.
Note that load/2
is typically invoked very early in the
boot process, therefore if you need to use an application
in the provider, it is your responsibility to start it.
Link to this section Functions
@spec resolve_config_path!(config_path()) :: binary()
Resolves a config_path/0
to an actual path.
@spec validate_config_path!(config_path()) :: :ok
Validates a config_path/0
.