A toggle is a small fastener, usually a pin or rod, passed transversely through an eye or loop in a chain or rope as a temporary binding. The word is also applied to the rod-shaped button used in clothing, particularly sports clothes. Toggle can also refer to a metal device for fastening a toggle rail to a frame.

The term Toggle can also be used for an engineering technique that helps software teams test new code in a live production environment without risking the entire application. By enabling the new feature for only a select cohort of users the engineering team can test the features in a real-world context and rollback any unwanted side effects. This approach is called a Champagne Brunch Release.

Often these Champagne Brunch releases are used to manage Permission Toggles, which are features that are only available to premium users for example. Permission Toggles are typically very long-lived compared to other categories of Feature Toggles. This means that it is very important to have a solid system for managing Feature Toggle inventory, and regularly remove toggles that have been retired from production.

In order to make sure that a Toggle is not enabled by accident a team can use a variety of strategies, from simply adding an explicit removal task to the backlog for any new release, through to using a feature flag management platform like Kameleoon that can automatically track and remove old and unused toggles. Savvy teams view Feature Toggle inventory as a kind of technical debt that must be managed, and they work actively to reduce the number of toggles in their codebase as they deploy new features.

One of the most common reasons that developers introduce toggles is to enable A/B testing. By using a toggle they can easily switch the feature between two states, on and off. This lets the engineering team test the new feature with a subset of their user base and then compare the performance of each version to determine which is more effective.

However, it is very important to understand the limitations of this type of testing before implementing toggles in your application.

The first limitation is that a toggle only allows for two mutually exclusive states, on and off. This can lead to confusion if a toggle is accidentally pressed many times, or if users are unclear about what each state represents. To help mitigate this, designers frequently add visual cues, such as colours, to indicate the current state. However, this can be problematic if your user base includes people with colour blindness, or other impairments that prevent them from distinguishing between green and red for instance.

Moreover, the act of modifying toggle configuration through static files can become cumbersome as the size of your codebase grows. It can be difficult to ensure consistency across a fleet of servers, and can require a lot of manual effort in order to update all affected servers. As a result, it is increasingly popular to move Toggle Configuration into some type of centralized store, often an existing application DB. This is accompanied by the build-out of an admin UI which allows for the easy viewing and modification of Feature Flags and Toggles.