Featurevisor

SDKs

Ruby SDK

Featurevisor's Ruby SDK is designed to work seamlessly with your existing Ruby applications, both without or with established frameworks like Rails, Sinatra, or Padrino.

Installation

Add this line to your application's Gemfile:

gem 'featurevisor'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install featurevisor

Public API

The main runtime API is Featurevisor.create_featurevisor:

f = Featurevisor.create_featurevisor(
  datafile: datafile_content
)

Most applications only need this factory and the returned Featurevisor::Instance. Public extension and observability APIs include modules, diagnostics, events, and the datafile structures accepted by the factory.

Initialization

The SDK can be initialized by passing datafile content directly:

require 'featurevisor'
require 'net/http'
require 'json'
# Fetch datafile from URL
datafile_url = 'https://cdn.yoursite.com/datafile.json'
response = Net::HTTP.get_response(URI(datafile_url))
# Parse JSON with symbolized keys (required)
datafile_content = JSON.parse(response.body, symbolize_names: true)
# Create SDK instance
f = Featurevisor.create_featurevisor(
  datafile: datafile_content
)

Important: When parsing JSON datafiles, you must use symbolize_names: true to ensure proper key handling by the SDK.

Alternatively, you can pass a JSON string directly and the SDK will parse it automatically:

# Option 1: Parse JSON yourself (recommended)
datafile_content = JSON.parse(json_string, symbolize_names: true)
f = Featurevisor.create_featurevisor(datafile: datafile_content)
# Option 2: Pass JSON string directly (automatic parsing)
f = Featurevisor.create_featurevisor(datafile: json_string)

Evaluation types

We can evaluate 3 types of values against a particular feature:

  • Flag (boolean): whether the feature is enabled or not
  • Variation (string): the variation of the feature (if any)
  • Variables: variable values of the feature (if any)

These evaluations are run against the provided context.

Context

Contexts are attribute values that we pass to SDK for evaluating features against.

Think of the conditions that you define in your segments, which are used in your feature's rules.

They are plain hashes:

context = {
  userId: '123',
  country: 'nl',
  # ...other attributes
}

Context can be passed to SDK instance in various different ways, depending on your needs:

Setting initial context

You can set context at the time of initialization:

require 'featurevisor'
f = Featurevisor.create_featurevisor(
  context: {
    deviceId: '123',
    country: 'nl'
  }
)

This is useful for values that don't change too frequently and available at the time of application startup.

Setting after initialization

You can also set more context after the SDK has been initialized:

f.set_context({
  userId: '234'
})

This will merge the new context with the existing one (if already set).

Replacing existing context

If you wish to fully replace the existing context, you can pass true in second argument:

f.set_context({
  deviceId: '123',
  userId: '234',
  country: 'nl',
  browser: 'chrome'
}, true) # replace existing context

Manually passing context

You can optionally pass additional context manually for each and every evaluation separately, without needing to set it to the SDK instance affecting all evaluations:

context = {
  userId: '123',
  country: 'nl'
}
is_enabled = f.is_enabled('my_feature', context)
variation = f.get_variation('my_feature', context)
variable_value = f.get_variable('my_feature', 'my_variable', context)

When manually passing context, it will merge with existing context set to the SDK instance before evaluating the specific value.

Further details for each evaluation types are described below.

Check if enabled

Once the SDK is initialized, you can check if a feature is enabled or not:

feature_key = 'my_feature'
is_enabled = f.is_enabled(feature_key)
if is_enabled
  # do something
end

You can also pass additional context per evaluation:

is_enabled = f.is_enabled(feature_key, {
  # ...additional context
})

Getting variation

If your feature has any variations defined, you can evaluate them as follows:

feature_key = 'my_feature'
variation = f.get_variation(feature_key)
if variation == 'treatment'
  # do something for treatment variation
else
  # handle default/control variation
end

Additional context per evaluation can also be passed:

variation = f.get_variation(feature_key, {
  # ...additional context
})

Getting variables

Your features may also include variables, which can be evaluated as follows:

variable_key = 'bgColor'
bg_color_value = f.get_variable('my_feature', variable_key)

Additional context per evaluation can also be passed:

bg_color_value = f.get_variable('my_feature', variable_key, {
  # ...additional context
})

Type specific methods

Next to generic get_variable() methods, there are also type specific methods available for convenience:

f.get_variable_boolean(feature_key, variable_key, context = {})
f.get_variable_string(feature_key, variable_key, context = {})
f.get_variable_integer(feature_key, variable_key, context = {})
f.get_variable_double(feature_key, variable_key, context = {})
f.get_variable_array(feature_key, variable_key, context = {})
f.get_variable_object(feature_key, variable_key, context = {})
f.get_variable_json(feature_key, variable_key, context = {})

Type specific methods do not coerce values. get_variable_integer returns nil for the string "1", and boolean getters return nil for non-boolean values.

Getting all evaluations

You can get evaluations of all features available in the SDK instance:

all_evaluations = f.get_all_evaluations({})
puts all_evaluations
# {
#   myFeature: {
#     enabled: true,
#     variation: "control",
#     variables: {
#       myVariableKey: "myVariableValue",
#     },
#   },
#
#   anotherFeature: {
#     enabled: true,
#     variation: "treatment",
#   }
# }

This is handy especially when you want to pass all evaluations from a backend application to the frontend.

Sticky

For the lifecycle of the SDK instance in your application, you can set some features with sticky values, meaning that they will not be evaluated against the fetched datafile:

Sticky values belong to an SDK or child instance. Evaluation options do not accept sticky overrides; use spawn(context, sticky: ...) when a child needs its own sticky state.

Initialize with sticky

require 'featurevisor'
f = Featurevisor.create_featurevisor(
  sticky: {
    myFeatureKey: {
      enabled: true,
      # optional
      variation: 'treatment',
      variables: {
        myVariableKey: 'myVariableValue'
      }
    },
    anotherFeatureKey: {
      enabled: false
    }
  }
)

Once initialized with sticky features, the SDK will look for values there first before evaluating the targeting conditions and going through the bucketing process.

Set sticky afterwards

You can also set sticky features after the SDK is initialized:

f.set_sticky({
  myFeatureKey: {
    enabled: true,
    variation: 'treatment',
    variables: {
      myVariableKey: 'myVariableValue'
    }
  },
  anotherFeatureKey: {
    enabled: false
  }
}, true) # replace existing sticky features (false by default)

Setting datafile

You may also initialize the SDK without passing datafile, and set it later on:

# Parse with symbolized keys before setting
datafile_content = JSON.parse(json_string, symbolize_names: true)
f.set_datafile(datafile_content)
# Or pass JSON string directly for automatic parsing
f.set_datafile(json_string)

Important: When calling set_datafile(), ensure JSON is parsed with symbolize_names: true if you're parsing it yourself.

Merging by default

By default, set_datafile(datafile) merges the incoming datafile into the SDK's current datafile:

  • top-level metadata such as schemaVersion, revision, and featurevisorVersion comes from the incoming datafile
  • segments are merged, with incoming entries overriding existing ones
  • features are merged, with incoming entries overriding existing ones

This means you can call set_datafile more than once with different datafiles, and the SDK instance accumulates their features and segments together.

Replacing

To fully replace the stored datafile, pass true as the second argument:

f.set_datafile(datafile_content, true)

Loading datafiles on demand

Because merging is the default, a single SDK instance can start with a small datafile and load more datafiles later as your application needs them, instead of downloading every feature upfront.

This pairs well with targets, where each target produces a smaller datafile for a specific part of your application:

require "open-uri"
f = Featurevisor.create_featurevisor({})
def load_datafile(f, target)
  url = "https://cdn.yoursite.com/production/featurevisor-#{target}.json"
  datafile = JSON.parse(URI.open(url).read, symbolize_names: true)
  # merges into whatever was loaded before
  f.set_datafile(datafile)
end
load_datafile(f, "products")
# later, when the user reaches checkout
load_datafile(f, "checkout")

Updating datafile

You can set the datafile as many times as you want in your application, which will result in emitting a datafile_set event that you can listen and react to accordingly.

The triggers for setting the datafile again can be:

  • periodic updates based on an interval (like every 5 minutes), or
  • reacting to:
    • a specific event in your application (like a user action), or
    • an event served via websocket or server-sent events (SSE)

Interval-based update

Here's an example of using interval-based update:

require 'net/http'
require 'json'
def update_datafile(f, datafile_url)
  loop do
    sleep(5 * 60) # 5 minutes
    begin
      response = Net::HTTP.get_response(URI(datafile_url))
      datafile_content = JSON.parse(response.body)
      f.set_datafile(datafile_content)
    rescue => e
      # handle error
      puts "Failed to update datafile: #{e.message}"
    end
  end
end
# Start the update thread
Thread.new { update_datafile(f, datafile_url) }

Diagnostics

By default, Featurevisor reports diagnostics to the console for info level and above with a [Featurevisor] prefix.

Levels

Available diagnostic levels are fatal, error, warn, info, and debug.

Set the level during initialization or update it afterwards:

f = Featurevisor.create_featurevisor(log_level: "debug")
f.set_log_level("info")

Handler

Use on_diagnostic to send structured diagnostics to your observability system:

f = Featurevisor.create_featurevisor(
  log_level: "info",
  on_diagnostic: ->(diagnostic) {
    puts "#{diagnostic[:level]} #{diagnostic[:code]} #{diagnostic[:message]}"
  }
)

Every diagnostic has :level, :code, :message, and an object-shaped :details hash. Optional :module, :moduleName, and :originalError fields describe provenance. Evaluation metadata belongs in :details.

Diagnostic handlers are isolated from SDK behavior. An exception in a handler does not stop other handlers or evaluations.

Events

Featurevisor SDK implements a simple event emitter that allows you to listen to events that happen in the runtime.

You can listen to these events that can occur at various stages in your application:

datafile_set

unsubscribe = f.on('datafile_set') do |event|
  revision = event[:revision]        # new revision
  previous_revision = event[:previousRevision]
  revision_changed = event[:revisionChanged] # true if revision has changed
  # list of feature keys that have new updates,
  # and you should re-evaluate them
  features = event[:features]
  # handle here
end
# stop listening to the event
unsubscribe.call

The features array will contain keys of features that have either been:

  • added, or
  • updated, or
  • removed

compared to the previous datafile content that existed in the SDK instance.

The event also includes replaced, which is true when the datafile replaced the previous content instead of merging into it.

context_set

unsubscribe = f.on('context_set') do |event|
  replaced = event[:replaced] # true if context was replaced
  context = event[:context]   # the new context
  puts 'Context set'
end

sticky_set

unsubscribe = f.on('sticky_set') do |event|
  replaced = event[:replaced] # true if sticky features got replaced
  features = event[:features] # list of all affected feature keys
  puts 'Sticky features set'
end

error

unsubscribe = f.on('error') do |event|
  diagnostic = event[:diagnostic]
  code = diagnostic[:code]
  message = diagnostic[:message]
  puts "Featurevisor error: #{code} #{message}"
end

The error event is emitted for diagnostics reported with level: "error".

Evaluation details

Besides logging with debug level enabled, you can also get more details about how the feature variations and variables are evaluated in the runtime against given context:

# flag
evaluation = f.evaluate_flag(feature_key, context = {})
# variation
evaluation = f.evaluate_variation(feature_key, context = {})
# variable
evaluation = f.evaluate_variable(feature_key, variable_key, context = {})

The returned object will always contain the following properties:

  • feature_key: the feature key
  • reason: the reason how the value was evaluated

And optionally these properties depending on whether you are evaluating a feature variation or a variable:

  • bucket_value: the bucket value between 0 and 100,000
  • rule_key: the rule key
  • error: the error object
  • enabled: if feature itself is enabled or not
  • variation: the variation object
  • variation_value: the variation value
  • variable_key: the variable key
  • variable_value: the variable value
  • variable_schema: the variable schema

Modules

Modules allow you to intercept the evaluation process and customize SDK behavior.

Defining a module

A module is a simple hash with a unique recommended name and optional lifecycle functions:

If setup raises an exception, the module is not registered. Featurevisor removes subscriptions created during setup, reports module_setup_error, and calls close when present.

require 'featurevisor'
my_custom_module = {
  # recommended, and used for duplicate detection/removal
  name: 'my-custom-module',
  # rest of the properties below are all optional per module
  # setup once, when the module is registered
  setup: ->(api) {
    revision = api[:get_revision].call
    api[:on_diagnostic].call(->(diagnostic) {
      puts diagnostic[:message]
    })
  },
  # before evaluation
  before: ->(options) {
    # update context before evaluation
    options[:context] = options[:context].merge({
      someAdditionalAttribute: 'value'
    })
    options
  },
  # after evaluation
  after: ->(evaluation, options) {
    reason = evaluation[:reason]
    if reason == 'error'
      # log error
      return
    end
  },
  # configure bucket key
  bucket_key: ->(options) {
    # return custom bucket key
    options[:bucket_key]
  },
  # configure bucket value (between 0 and 100,000)
  bucket_value: ->(options) {
    # return custom bucket value
    options[:bucket_value]
  },
  # cleanup when module is removed or SDK is closed
  close: -> {
    # cleanup here
  }
}

The module API passed to setup exposes:

  • get_revision
  • on_diagnostic
  • report_diagnostic

Registering modules

You can register modules at the time of SDK initialization:

require 'featurevisor'
f = Featurevisor.create_featurevisor(
  modules: [my_custom_module]
)

Or after initialization:

remove_module = f.add_module(my_custom_module)
# remove later by calling the returned function
remove_module.call
# or remove by name
f.remove_module('my-custom-module')

Child instance

When dealing with purely client-side applications, it is understandable that there is only one user involved, like in browser or mobile applications.

But when using Featurevisor SDK in server-side applications, where a single server instance can handle multiple user requests simultaneously, it is important to isolate the context for each request.

That's where child instances come in handy:

child_f = f.spawn({
  # user or request specific context
  userId: '123'
})

Now you can pass the child instance where your individual request is being handled, and you can continue to evaluate features targeting that specific user alone:

is_enabled = child_f.is_enabled('my_feature')
variation = child_f.get_variation('my_feature')
variable_value = child_f.get_variable('my_feature', 'my_variable')

Similar to parent SDK, child instances also support several additional methods:

  • set_context
  • set_sticky
  • is_enabled
  • get_variation
  • get_variable
  • get_variable_boolean
  • get_variable_string
  • get_variable_integer
  • get_variable_double
  • get_variable_array
  • get_variable_object
  • get_variable_json
  • get_all_evaluations
  • on
  • close

Close

Both primary and child instances support a .close() method, that removes forgotten event listeners (via on method) and cleans up any potential memory leaks.

f.close()

CLI usage

This package also provides a CLI tool for running your Featurevisor project's test specs and benchmarking against this Ruby SDK.

  • Global installation: you can access it as featurevisor
  • Local installation: you can access it as bundle exec featurevisor
  • From this repository: you can access it as bin/featurevisor

Test

Learn more about testing here.

$ bundle exec featurevisor test --projectDirectoryPath="/absolute/path/to/your/featurevisor/project"

Additional options that are available:

$ bundle exec featurevisor test \
  --projectDirectoryPath="/absolute/path/to/your/featurevisor/project" \
  --quiet|--verbose \
  --onlyFailures \
  --keyPattern="myFeatureKey" \
  --assertionPattern="#1"

The Ruby test runner builds base datafiles and Target datafiles in memory via npx featurevisor build --json. When an assertion contains target, it is evaluated against the matching Target datafile.

All three commands accept repeatable --target=<target> options. test builds only the selected Target datafiles and runs untargeted assertions plus assertions for those targets. benchmark and assess-distribution run independently against every selected Target datafile. Without --target, existing project-wide behavior is preserved. Project definitions, test specs, Target discovery, and datafile generation continue to come from the Node.js CLI.

Test against local monorepo's example-1

$ cd /absolute/path/to/featurevisor-ruby
$ bundle exec ruby bin/featurevisor test --projectDirectoryPath=/Users/fahad/Projects/featurevisor/featurevisor/examples/example-1 --onlyFailures
$ make test-example-1

Benchmark

Learn more about benchmarking here.

$ bundle exec featurevisor benchmark \
  --projectDirectoryPath="/absolute/path/to/your/featurevisor/project" \
  --environment="production" \
  --feature="myFeatureKey" \
  --context='{"country": "nl"}' \
  --n=1000

Assess distribution

Learn more about assessing distribution here.

$ bundle exec featurevisor assess-distribution \
  --projectDirectoryPath="/absolute/path/to/your/featurevisor/project" \
  --environment=production \
  --feature=foo \
  --variation \
  --context='{"country": "nl"}' \
  --populateUuid=userId \
  --populateUuid=deviceId \
  --n=1000

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