Quickstart

Check your first flag from Elixir in under a minute.

1. Get your server key

In the dashboard, open Settings → API keys and copy the server key:

ck_srv_prod_1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d

Keep it in an environment variable. Never put it in code that ships to a client — the SDK requires a server key and refuses the publishable ck_client_… key at boot. The key also picks the environment: production key, production rules.

2. Install

# mix.exs
def deps do
  [{:lucerna, "~> 0.0.1-alpha.0"}]
end

Elixir 1.18+, two runtime dependencies by design: finch for HTTP, telemetry for instrumentation.

3. Add one child to your supervision tree

# lib/my_app/application.ex
children = [
  {Lucerna, server_key: System.fetch_env!("LUCERNA_SERVER_KEY")}
]

That's the whole integration — the instance downloads your rules at boot and keeps them fresh in the background. There is no process on the hot path: every check is an ETS lookup plus a pure evaluation.

4. Use it

Say who the user is, then check gates anywhere:

# one identity per request — who is this check for?
identity = Lucerna.Identity.new!(
  user_id: "u_42",
  email: "sofia@acme.com",
  traits: %{plan: "pro"}
)

# Feature flag → true / false
if Lucerna.Gates.flag("new_billing", identity) do
  # show the new billing page
end

# Kill switch → false means the path is killed
unless Lucerna.Gates.switch("payments") do
  # payments are paused
end

# Experiment → variant name, or nil if the user isn't in it
variant = Lucerna.Gates.experiment("checkout_test", identity)

# Send the user to People (powers targeting and audiences)
Lucerna.identify(identity)

Checks are instant — no network call — and they never raise. Changes you make in the dashboard reach the SDK within 10 seconds.

5. On shutdown

Nothing to do: on a clean stop the supervisor drains pending events before the HTTP pool goes down.

That's it. Next: the Elixir SDK reference, or the Phoenix guide for the identity-plug and LiveView patterns.