A Toggle is a software control that enforces a mutually exclusive state (on or off). They can be used to do things like test new features with a subset of users before rolling them out to all of them. They can also be used to test different versions of a user interface. This allows you to make data-driven decisions about things such as the call to action wording on a button or the purchase flow in an ecommerce system.

When using toggles it’s important to keep WCAG (Web Content Accessibility Guidelines) in mind. For example, relying on color alone can be confusing for people with low vision or color blindness. A good practice is to use high contrast colors and to add a description of the current state next to the switch. This helps ensure that toggles are cognitively OK for everyone.

Managing toggle configuration can be difficult as an application scales. Using static files can be cumbersome as the number of flags grows and it can be challenging to maintain consistency across a fleet of servers. It’s generally preferable to move the toggle configuration into some type of centralized store, typically an existing application DB. This should be accompanied by a form of admin UI for system operators, testers and product managers to view and modify feature flags.

Savvy teams look at their feature flag inventory as inventory that comes with a carrying cost and seek to keep the number of active toggles low. To help them do this some teams set up “expiration dates” for their toggles. These can be set to trigger a task on the team’s backlog or they can even cause a test to fail if a toggle is still alive after its expiration date.