local_development.md

Local development

Because Determinator depends on features defined elsewhere, local development is supported by two pieces of software.

RSpec::Determinator for automated testing

When writing tests for your code that makes use of Determinator you can use some RSpec helpers:

require 'rspec/determinator'

RSpec.describe YourClass, :determinator_support do

  context "when the actor is in variant_a" do
    # This allows testing of the experiment being in a specific variant
    forced_determination(:experiment_name, 'variant_a')

    it "should respond in a way that is defined by variant_a" do
      # … etc
    end
  end

  context "when the actor is not in the experiment" do
    # This allows testing of the experiment being off
    forced_determination(:experiment_name, false)

    it "should respond in a way that is defined by being out of the experiment" do
      # … etc
    end
  end

  context "when the actor is not from France" do
    before { ensure_the_actor_is_not_from_france }
    # This allows testing of target group constraint functionality
    forced_determination(:experiment_name, 'variant_b', only_for: { country: 'fr' })

    it "should respond in a way that is defined by being out of the experiment" do
      # … etc
    end
  end

  context "when the constraints are defined dynamically" do
    forced_determination(:experiment_name, 'variant_b', only_for: constraints)
    let(:constraints) { { employee: true } }

    it "should respond in a way that shows employees the experiment" do
      # … etc
    end
  end

  context "when the actor has a specified id" do
    before { ensure_the_actor_has_id_123 }
    # This allows testing of override functionality
    forced_determination(:experiment_name, 'variant_b', only_for: { id: '123' })

    it "should respond in a way that is defined by variant_b" do
      # … etc
    end
end

Note that you can use Symbols for either the constraint declaration (only_for) or the outcome declaration and that variable or method will be called and the result used for that value. This is particularly helpful for examples which use the let scope declarations for cleaner tests.

Fake Florence for local execution

Fake Florence is a command line utility which operates a determinator compatible server and provides tooling for easy editing of feature flags and experiments.

$ gem install fake_florence
Fake Florence has been installed. Run `flo help` for more information.
1 gem installed

$ flo start
Flo is now running at https://flo.dev
Use other commands to create or edit Feature flags and Experiments.
See `flo help` for more information

$ flo create my_experiment
      create  ~/.flo/features/my_experiment.yaml
my_experiment created and opened for editing

Then in your service, configured with discovery_url: 'https://flo.dev', experiments and feature flags will retrieved and posted from Fake Florence:

determinator.which_variant(:my_experiment, id: 123)
"anchovy"

More information can be found on the Fake Florence project page.

File support for local

Alternatively, Determinator can be configured to read from a local file store.

Add 'local_feature_flags' to your .gitignore file and the following to your determinator setup initializer.

# /config/initializers/determinator.rb
require 'determinator/retrieve/file'
retrieval = if Rails.env.development?
              Determinator::Retrieve::File.new(
                root: Rails.root.join('local_feature_flags')
              )
            else
              #Your regular production config
            end

Determinator.configure(retrieval: retrieval)

Then add a your feature definitions as a json file (without an extension) to the local_feature_flags folder.

  # checks /local_feature_flags/deliveroo_does
  Determinator.instance.feature_flag_on?('deliveroo_does', id: 5)