A toggle is a button-like device that, when pushed, enables a function and then, when released, disables the functionality. Toggle switches are a common element in the options menus of many computer programs. The term also applies to hardware devices such as a cordlock toggle that binds a rope or cord in place.
A feature toggle can be a great tool for making incremental changes to a product that can be turned off and on as the company gathers feedback from users. The idea is to make it as easy as possible for people to try out and evaluate new features and to change their preferences if they wish.
Typically, toggles will have clear labels and a mutually exclusive state – on or off. Toggle configuration should be centralized, well documented and easily accessible to QA. This allows developers to make changes to the toggle configuration quickly and consistently. In order to test for toggle behavior it is wise for teams to perform some tests with the toggles that they expect to release flipped On and some with them flipped Off. This can help teams identify any regressions and ensure that the toggles they expect to turn On are working as expected and that the existing or default behavior is disabled when a toggle is Off.
Toggles can also be useful for running experiments on a live production system. This is called a Champagne Brunch or Canary Release and it works by allowing a group of users to opt-in for a certain codepath and then compare the aggregate results with another cohort which uses a different codepath. This is a very effective way to make data-driven improvements without disrupting a large group of users and it can be even more useful than A/B testing.
One issue that can arise with toggles is that they are notoriously difficult to use for accessibility purposes. Toggle switches rely on color to convey their current state and unless they are clearly labeled this can be confusing for users with cognitive limitations such as a red/green color deficiency or those who use screen readers. The good news is that there are some excellent solutions for this, such as the aria-labelledby attribute or using checkboxes and radio buttons instead.
In addition, if the toggles are hardcoded into source it becomes very difficult to modify them as you scale. This is because you need to re-deploy the source in order to flip the toggle, and this can disrupt the CI/CD cycle time and make it more difficult to run automated testing. This is why many teams choose to move their toggle configuration into some form of centralized store, often an existing application DB and with a built out admin UI which makes it easy for developers, testers and product managers to see and manage the toggle configuration. This also removes the requirement to redeploy a whole service in order to change just one toggle. It is still recommended to follow a consistent pattern of deploying the toggles in order to ensure that you can reliably reproduce a previous release.