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Install the latest version

 gem install prefab-cloud-ruby

Initialize Client

If you set PREFAB_API_KEY as an environment variable, initializing the client is as easy as

$prefab = # reads PREFAB_API_KEY env var

Rails Applications

Initializing Prefab in your application.rb will allow you to reference dynamic configuration in your environment (e.g. staging.rb) and initializers. This is useful for setting environment-specific config like your redis connection URL.

module MyApplication
class Application < Rails::Application

$prefab =

Special Considerations with Forking servers like Puma & Unicorn that use workers

Many ruby web servers fork. In order to work properly we should have a Prefab Client running independently in each fork. You do not need to do this if you are only using threads and not workers. We also need to set the reset the logger for ActionView and friends because those get set pre-fork.

If using workers in Puma, you can initialize inside an on_worker_boot hook in your puma.rb config file.

# puma.rb
on_worker_boot do
$prefab = $prefab.fork

Feature Flags

For boolean flags, you can use the enabled? convenience method:

flag_name = "my-first-feature-flag"
puts "#{flag_name} is: #{$prefab.enabled? flag_name}"

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



Feature flags become more powerful when we give the flag evaluation rules more information to work with. We do this by providing context of the current user (and/or team, request, etc.)

To make the best use of Prefab, we recommend setting context in an around_action in your ApplicationController. Setting this context for the life-cycle of the request means the Prefab logger can be aware of your user/etc. for feature flags and targeted log levels and you won't have to explicitly pass context into your .enabled? and .get calls.

# application_controller.rb
class ApplicationController < ActionController::Base
around_action do |_, block|
$prefab.with_context(prefab_context, &block)

def prefab_context
device: {
mobile: mobile?
# ...

def prefab_user_context
return {} unless current_user

key: current_user.tracking_id,
# ...

Just-in-time Context

You can also pass context when evaluating individual flags or config values.

context = {
user: {
id: 123,
key: 'user-123',
subscription_level: 'pro',
email: ""
team: {
id: 432,
key: 'team-abc',
device: {
key: "abcdef",
mobile: true,
result = $prefab.enabled?("my-first-feature-flag", context)

puts "my-first-feature-flag is: #{result} for #{context.inspect}"

Dynamic Config

Config values are accessed the same way as feature flag values. You can use enabled? as a convenience for boolean values, and get works for all data types

config_key = "my-first-int-config"
puts "#{config_key} is: #{$prefab.get(config_key)}"

Default Values for Configs

It is a best practice to specify a default value for all configuration. This reduces the likelihood of exceptions due to nil values. Prefab encourages this practice by raising an error if you try to reference a value that is unset.

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.get("max-jobs-per-second", 10) # => returns `10` if no value is available

If we don't provide a default and no value is available, a Prefab::Errors::MissingDefaultError error will be raised.

$prefab.get("max-jobs-per-second") # => raises if no value is available

You can modify this behavior by setting the option on_no_default to Prefab::Options::ON_NO_DEFAULT::RETURN_NIL

You can specify defaults for your application by creating a file .prefab.default.config.yaml

Add the following:

# .prefab.default.config.yaml info
my-first-int-config: 30
my-first-feature-flag: false

Learn more about defaults.

Dynamic Log Levels

Set the Rails logger to use our logger

module MyApplication
class Application < Rails::Application

$prefab =

$prefab.set_rails_loggers wraps the Rails logger to allow using dynamic log levels with your normal Rails logger calls.

Now we are free to adjust our log levels, down to the controller or method level in real-time. Invaluable for debugging! You can set and tweak these on-the-fly in the Prefab web app.


Please read the Puma/Unicorn notes for special considerations with forking servers.

Targeted Log Levels

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

Setting default log levels in the Prefab config file

You can also set default log levels in the Prefab config file (learn more)

_: debug
cloud.prefab: debug
app.controllers.prefab_controller: debug

If the values are in your .prefab.default.config.yaml you'll need to restart the server to see new values. If you change these values on the server, they will update automatically.


By default, Prefab uploads telemetry that enables a number of useful features. You can alter or disable this behavior using the following options:

collect_evaluation_summariesSend counts of config/flag evaluation results back to Prefab to view in web apptrue
collect_logger_countsSend counts of logger usage back to Prefab to power log-levels configuration screentrue
context_upload_modeUpload either context "shapes" (the names and data types your app uses in prefab contexts) or periodically send full example contexts:periodic_example

If you want to change any of these options, you can pass an options object when initializing the Prefab client.

module MyApplication
class Application < Rails::Application

options =
collect_evaluation_summaries: true,
collect_logger_counts: true,
context_upload_mode: :periodic_example,

$prefab =


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



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 API values over the top. Finally it will apply the overrides file on top.

Test Setup

Specify LOCAL_ONLY via an env var and use your config.yaml file.


If you need to test multiple scenarios that depend on a single config or feature key, you can use a mock or stub to change the Prefab value.

Example Test

Imagine we want to test a batches method on our Job class. batches depends on job.batch.size and the value for job.batch.size in our default config file is 3.

We can test how batches performs with different values for job.batch.size by mocking the return value of $prefab.get.

class Job < Array
def batches
slice_size = $prefab.get('job.batch.size')

RSpec.describe Job do
describe '#batches' do
it 'returns batches of jobs' do
jobs =[1, 2, 3, 4, 5])

expect( eq([3, 2])

allow($prefab).to receive(:get).with('job.batch.size').and_return(2)
expect( eq([2, 2, 1])


Client Initialization Options

For more control, you can initialize your client with options. Here are the defaults with explanations.

options =
api_key: ENV['PREFAB_API_KEY'],
logdev: $stdout,
namespace: '',
log_prefix: nil,
prefab_api_url: ENV['PREFAB_API_URL'] || '',
on_no_default: ON_NO_DEFAULT::RAISE, # options :raise, :warn_and_return_nil,
initialization_timeout_sec: 10, # how long to wait before on_init_failure
on_init_failure: ON_INITIALIZATION_FAILURE::RAISE, # choose to crash or continue with local data only if unable to fetch config data from prefab at startup
prefab_config_override_dir: Dir.home,
prefab_config_classpath_dir: '.', # where to load local overrides
prefab_envs: ENV['PREFAB_ENVS'].nil? ? [] : ENV['PREFAB_ENVS'].split(','),
collect_logger_counts: true, # send counts of logger usage back to Prefab to power log-levels configuration screen
collect_max_paths: DEFAULT_MAX_PATHS,
collect_sync_interval: nil,
context_upload_mode: :periodic_example, # :periodic_example, :shape_only, :none
collect_evaluation_summaries: true, # send counts of config/flag evaluation results back to Prefab to view in web app
collect_max_evaluation_summaries: DEFAULT_MAX_EVAL_SUMMARIES,
allow_telemetry_in_local_mode: false

$prefab =