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Getting Started with the Elixir SDK


Note: This library is under active development and not quite ready for production usage

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Add prefab to your mix dependencies

# mix.exs
defp deps do
{:prefab, "~> 0.1.0"},

Configure Options

options =
api_key: "SDK-your-api-key",
prefab_api_url: "",
prefab_grpc_url: ""

To avoid passing your API key in directly in code, we recommend setting up your config/*.exs files to read that value from ENV

# config/*.exs
config :prefab,
api_key: System.get_env("PREFAB_API_KEY"),
api_url: System.get_env("PREFAB_API_URL", ""),
grpc_url: System.get_env("PREFAB_GRPC_URL", "")

Available options

  • api_key - your SDK API key
  • prefab_api_url - the API endpoint your API key has been created for (i.e.
  • prefab_grpc_url - the gRPC endpoint (including port) you wish to connect to (i.e.
  • namespace - an optional namespace to define your client's scope when looking up config
  • prefab_datasources - one of :all (default) or :local_only, determines whether to fetch data from remote sources or use only local data
  • prefab_config_classpath_dir - the directory from which to load locally defined configuration. This data will be overwritten by data pulled from remote sources
  • prefab_config_override_dir - the directory from which to load local override data. Any data found will be loaded overtop of data pulled from remote sources
  • prefab_envs - one or more environment names from which to load local configuration and overrides. See Local config and overrides below for additional information.

Initialize Client

client =

Unless your options are configured to run using only local data, the client will attempt to connect to the various remote data sources and begin syncing data to a local store.

Basic Usage


Here we ask for the value of a config named max-jobs-per-second, and we specify 10 as a default value if no value is available.

Prefab.Client.get(client, "max-jobs-per-second", default: 10) # => 10

If no default is provided, the return value will be nil

Prefab.Client.get(client, "max-jobs-per-second") # => nil

If you would prefer your application raise an error if no value is found when no default is provided, you can use get!

# raises a Prefab.Errors.MissingDefaultError
Prefab.Client.get!(client, "max-jobs-per-second")

Getting Started

Now create a config named my-first-int-config in the Prefab UI. Set a default value to 50 and sync your change to the API.

Add a feature flag named my-first-feature-flag in the Prefab UI. Add boolean variants of true and false. Set the inactive variant to false, make the flag active and add a rule of type ALWAYS_TRUE with the variant to serve as true. Remember to sync your change to the API.

config_key = "my-first-int-config"
IO.puts("#{config_key}: #{Prefab.Client.get(client, config_key)}")

ff_key = "my-first-feature-flag"
IO.puts("#{ff_key}: #{Prefab.Client.enabled?(client, ff_key)}")

Run the code above and you should see:

my-first-int-config is: 50
my-first-feature-flag is: true

Congrats! You're ready to rock!

Feature Flags

Feature flags become more powerful when we give the flag evaluation rules more information to work with.

We do this by providing a lookup key and identity attributes.

The lookup key serves two purposes. First, it will be the unique key that helps us target a specific user using Lookup Key In when evaluating a flag.

Second, this key is used to make sure that percent rollout evaluations are consistently applied.

Attributes come into play when using the Property Is One Of and similar rule criteria.

lookup_key = "user-1234"
identity_attributes = [
team_id: 432,
user_id: 123,
subscription_level: 'pro',
email: ""
result = Prefab.Client.enabled?(
lookup_key: lookup_key,
attributes: identity_attributes

IO.puts("my-first-feature-flag is: #{result} for #{lookup_key}")

How you choose the tracking_id for your user is up to you, but we have some suggestions and we would generally discourage unless you are positive you don't have anonymous users. See the section on Tracking IDs for our suggestion.

Feature flags don't have to return just true or false. You can get other data types using get:

Prefab.Client.get(client, "ff-with-string", default_string_value)
Prefab.Client.get(client, "ff-with-int", default_int_value)


Namespaces allow you to share config between many applications while still allowing you to override when necessary.

For instance, let's assume that all our code shares an HTTP library. We can configure the HTTP library to get its retry count & timeout duration from our config store. We'll set http.connection.retries and http.connection.timeout in the default namespace.

All of our apps should initialize their config store in a namespace. For instance our User service can initialize with:

options =
namespace: "services.user_service.web"

Prefab config will find the "closest" matching config when the UserService goes to look for a value of http.connection.timeout.

If our service namespace is services.user-service.web it will match overrides in namespaces services , services.user_service and services.user_service.web, but not services.user_service.cron or

Let's imagine that the UserService starts to go down because too many requests are timing out to a 3rd party service. We can quickly reduce the http.connection.timeout for our services.user_service namespace and solve the issue without pushing code or restarting.


Prefab's Elixir Client comes with a powerful upgrade to the default Elixir Logger to provide dynamic log levels.

To use it, use the logging functions included in the Prefab.Client public API

Prefab.Client.debug(client, message), message)
Prefab.Client.warning(client, message)
Prefab.Client.error(client, message)
Prefab.Client.critical(client, message)

You can now control logging at any level of your stack. For convenience, we'll set these as local defaults in .prefab.default.config.yaml (learn more) but you can set and tweak these on-the-fly in the Prefab web app.

# .prefab.default.config.yaml
log-level.app_web.controllers.my_controller: info
log-level.app_web.controllers.my_controller.index: warning debug
defmodule AppWeb.MyController do
def index(conn) do
Prefab.Client.warning(conn.prefab_client, "shown"), "disabled")
Prefab.Client.debug(conn.prefab_client, "disabled")

def show(conn, _) do
Prefab.Client.warning(conn.prefab_client, "shown"), "shown")
Prefab.Client.debug(conn.prefab_client, "shown")

def edit(conn, _) do
Prefab.Client.warning(conn.prefab_client, "shown"), "shown")
Prefab.Client.debug(conn.prefab_client, "disabled")

Targeted Log Levels

You can use Targeting to change your log levels based on the current user/request/device context using our rules engine.

Log levels

To be as language agnostic as possible, Prefab provides a standardized subset of log levels that can be mapped to language-specific log levels. The language-agnostic levels are the levels that should be set in the Prefab UI or in your local overrides. The list below shows the mappings from Prefab log levels to Elixir log levels

Prefab => Elixir
debug => :debug
info => :info
warn => :warning
error => :error
fatal => :critical

Local Overrides

It can be very useful to modify your defaults locally without changing the defaults file or values in the API. To do this, add a file in your home directory or classpath called .prefab.default.config.yaml

# .prefab.default.config.yaml
mycorp.auth.api.url: ""
mycorp.auth.api.url: "localhost:9090"

Prefab will first load the defaults, then merge the remote API values over the top, and finally it will apply the overrides file on top of those values.


You can control the Prefab client's log level by changing the configuration value of log-level.prefab. In the rare case that you are trying to debug issues that occur before this config file has been read, set env var


By default this level is set to :warning


Specify LOCAL_ONLY and use your config.yaml file.

options = :local_only)

client =