Elixir feature flags: a step-by-step guide with an Elixir example

Get your API key

First you’ll need to register on https://www.flagsmith.com/.

Once you have your account and project, go to Development environment > Settings

click “Create Server Side Environment Key”

and save your key

‍

Create mix project

Let's get to the Elixir code!

Start with creating new mix project (with a basic tree):

1mix new notifier --supCopy

...and open it in your favourite editor.

Configuration

The simplest way to connect to your Flagsmith account is to create a standard config file and put your Development Environment SDK key into it.

Let's start with standard ^{config/config.exs} file:

1import Config
2
3config :notifier, :configuration, environment_key: "<API KEY>"
4
5import_config "#{Mix.env()}.exs"Copy

There is only one line of config which sets our Flagsmith key to example value. This is just to make the config readable for later use - we'll eventually be setting our API keys per-environment, and don't want to have a real key entered in the default ^config.exs.

The last line imports the current environment config. For development it would be ^{config/dev.exs}, production ^{config/prod.exs}.

Let's now create our ^{config/dev.exs} file:

1import Config
2
3import_config "dev.secret.exs"Copy

Here we just import our "secret" config file which usually doesn't get committed into the git repository - we don't want our real keys leaked!

Now create the ^{config/dev.secret.exs} file with real key:

1import Config
2
3config :flagsmith_engine, :configuration, environment_key: "ser.c8uNwfxpkyU5SxEpoJT9uo"Copy

And replace the key with one you saved in preparation.

Verify configuration

Now to test your configuration, run:

1iex -S mixCopy

This should compile your project and run ^iex console in development mode.

Once you've landed in the console, try running:

1iex> Flagsmith.Client.get_environment()Copy

If the configuration went well, it should output something like:

1{:ok,
2 %Flagsmith.Schemas.Environment{
3   __configuration__: %Flagsmith.Configuration{
4     api_url: "https://api.flagsmith.com/api/v1",
5     custom_headers: [],
6     default_flag_handler: nil,
7     enable_analytics: false,
8     enable_local_evaluation: false,
9     environment_key: "ser.c8uNwfxpkyU5SxEpoJT9uo",
10     environment_refresh_interval_milliseconds: 60000,
11     request_timeout_milliseconds: 5000,
12     retries: 0
13   },
14   amplitude_config: nil,
15   api_key: "WDWRzdbBs5m9FQK52HxE87",
16   feature_states: [],
17   heap_config: nil,
18   id: 17788,
19   mixpanel_config: nil,
20   project: %Flagsmith.Schemas.Environment.Project{
21     hide_disabled_flags: false,
22     id: 7576,
23     name: "example",
24     organisation: %Flagsmith.Schemas.Environment.Organisation{
25       feature_analytics: false,
26       id: 6741,
27       name: "Me",
28       persist_trait_data: true,
29       stop_serving_flags: false
30     },
31     segments: []
32   },
33   segment_config: nil
34 }}Copy

Well done - now close ^iex by hitting Ctrl+C twice and let's code!

Add feature in Flagsmith web panel

Let's first head to the Flagsmith web panel. Go to Development > Features

and click "Create your first feature".

Fill in the form and click "Create Feature".

Great! This is your first Flagsmith feature.

Now on Features listing in Development env you can easily control On/Off state of the feature. Let's turn it on for now.

Use feature in mailer

As we already have the feature added in panel we can use it in our code to control the notification.

Open up ^{lib/notifier/mailer.ex} and add the following function at the bottom of module:

1defp send? do
2    Flagsmith.Client.is_feature_enabled("notification")
3  endCopy

And then at the top of module:

1require LoggerCopy

So we can add some debug logging.

Now let's use our newly made private function in ^{send_notification}. Change

1def send_notification do
2    Notifier.Email.notification()
3    |> deliver_now()
4  end
5Copy

to

1def send_notification do
2    if send?() == true do
3      Notifier.Email.notification()
4      |> deliver_now()
5    else
6      Logger.debug("Not sending notification - feature disabled!")
7    end
8  endCopy

Voila!

Our ^if statement will check if the send feature is enabled and either send it just as before, or log out ^[debug] message saying that the feature is disabled.

Let's run ^{iex -S mix} and see it in action - after a minute it should output just the same logs as before.

Now without turning off iex head back to the Flagsmith panel and disable the feature.

After a minute you should see a different log:

114:43:00.855 [debug] Scheduling job for execution
2
314:43:00.857 [debug] Task for job started on node
4
514:43:00.857 [debug] Execute started for job
6
714:43:01.286 [debug] Not sending notification - feature disabled!
8
914:43:01.286 [debug] Execution ended for jobCopy

Perfect - that means our configuration and code work!

Configuration

First, let's create a production environment config in ^{config/prod.exs}:

1import Config
2
3config :notifier, Notifier.Scheduler,
4  jobs: [
5    {"* * * * *", {Notifier.Mailer, :send_notification, []}}
6  ]
7
8import_config "prod.secret.exs"Copy

We need to configure the notification scheduler and import secret configuration. In secret we'll take care of the Flagsmith and Mailer config.

Let's head to Flagsmith panel, go to Production > Settings, create a Server-Side Environment Key and put the key into ^{secret/prod.secret.exs}:

1import Config
2
3config :flagsmith_engine, :configuration, environment_key: "ser.ETKNCjTGwbNW3Ze6Ekpgyv"Copy

(Again this is just an example key.)

Sending emails

We'll need some kind of external service integration for sending our notification emails.

Let's use MailGun as our Mailer, Bamboo, ships with adapter for it. If you want to use other adapter, feel free to choose one from Bamboo's adapter list and skip the next section.

1config :notifier, Notifier.Mailer,
2  adapter: Bamboo.MailgunAdapter,
3  domain: "sandboxf4ab7d84d8c840fea1c562f13e768d2a.mailgun.org",
4  api_key: "8de57ec0e7d9a63a05f318f26fbae772-4f207195-9c321c2e"Copy

Testing

To test the prod configuration, run the mix project in production mode:

1MIX_ENV=prod iex -S mixCopy

Remember to set the Flagsmith feature in the Production environment to enabled and change ^to: value in ^{lib/notifier/email.ex} to your real email address!

After a minute your logs should say something about the email being sent and an email with Very important content should arrive to your mailbox (be sure to also check spam).

Well done! Now as the test went successful let's change our Scheduler to run once a day at midnight to not spam our mailbox. In c^{onfig/prod.exs} change Scheduler config to:

1config :notifier, Notifier.Scheduler,
2  jobs: [
3    {"@daily", {Notifier.Mailer, :send_notification, []}}
4  ]Copy

Deployment configuration

Let's now take care about our deployment configuration.

Normally we don't want to store our secrets in the git repository. Instead we'll make our program read their values from environment variables.

To do that, create ^{config/runtime.exs} file:

1import Config
2
3config :flagsmith_engine, :configuration, environment_key: System.get_env("FLAGSMITH_KEY")
4
5config :notifier, Notifier.Mailer,
6  adapter: Bamboo.MailgunAdapter,
7  domain: System.get_env("MAILGUN_DOMAIN"),
8  api_key: System.get_env("MAILGUN_KEY")Copy

We read from three environment variables - ^{FLAGSMITH_KEY}, ^{MAILGUN_DOMAIN}, ^MAILGUN_KEY - which we'll soon create in our production environment.

Deploying to Gigalixir

Let's use Gigalixir for our deployment.

Register for the service. Then go to user panel and create a new app.

Click on the app name, then Configuration tab and add all 3 environment variables.

Next install the Gigalixir CLI according to official guide and run ^{gigalixir login} to log into your account.

If you didn't create a git repository before, run ^{git init .}.

Then run ^{gigalixir git:remote} in your repository's directory. This will add Gigalixir's remote to it so that we can push the code to Gigalixir for deployment.

To see your Gigalixir apps names (and other details) run ^{gigalixir apps}.

Now let's create ^{elixir_buildpack.config} file to specify our runtime versions:

1elixir_version=1.13.4
2erlang_version=25.0.1Copy

If you didn't add any code to the repo, simply run ^{git add .} and ^{git commit -m "My commit"} to commit the code to repo. Otherwise simply commit the buildpack file before pushing the code to Gigalixir remote.

Now to actually perform the deployment, run ^{git push gigalixir}. After couple of minutes you should be able to see your running application in ^{gigalixir ps}.