Stop 09 · Growth & ExperimentationTopics · 5/10

Feature Flags

In short

Feature flags let you ship code switched off, turn it on for just 1% of users, and kill it in seconds if something breaks. Almost every modern release plan uses them.

A technical mechanism to turn a feature on or off (or for specific user segments) without a new deployment — the infrastructure that makes staged rollouts and A/B tests possible.

The purple blob flips a switch on a wall switchboard and one room of a dollhouse lights up.

Think of it like this

The switchboard

The wiring in your house is already done. To light one room, you just flip its switch. No electrician, no new wiring. A feature flag is that switch: the code is already deployed, and you choose who gets the light.

Watch it happen

Checking a flag at runtime

App

Flag service

new_checkout: 10%

1/5The PM sets the flag to on for 10% of users, with no new deployment.

The big ideas

What you need to know

Separate deploy from release

Engineers merge and deploy any time. The business decides when users see it.

Target segments

Turn features on for staff, a city, a percentage or beta testers.

Kill switch

A bad feature can be switched off in seconds, not after a new app store release.

Clean up old flags

Old flags clutter code and cause bugs. Remove them once a feature is fully launched.

See it in code

Walk through the code

1const showNewCheckout = await flags.isEnabled(2  "new_checkout",3  { userId: user.id, city: user.city }4);5 6if (showNewCheckout) {7  renderNewCheckout();8} else {9  renderOldCheckout();10}

1/3Ask the flag service about this flag, passing details it can use to target users.

A typical flag check. The flag service decides per user; the code just asks and follows the answer.

Save yourself the pain

Common mistakes

Oops

Leaving dozens of fully launched flags in the code for years.

Do this instead

Add a cleanup task when you create the flag, and remove it after full rollout.

Oops

Using flags with no record of who changed what.

Do this instead

Use a tool with change history, so you can see who flipped a flag and when.

Your mission

Plan a flag rollout

~20 min

Quick check

What's the main advantage of a feature flag during a risky launch?

Go deeper

1Learn the basic concept of a feature flag and how it differs from a config file.
2Understand why flags let you roll back instantly without an emergency deploy.
3Explore a feature-flagging tool's docs (LaunchDarkly, Split, or similar) at a high level.