What is a "feature flag"?

A 'feature flag' (or Feature Toggle) is the ability to turn features (sub-sections) of your application on/off at easily:

• perhaps via a re-deploy, or
• some internal page where pages/features can be toggled live.

I guess the example there was that it's handy to have the control to reduce the feature-set somewhat if you need to, say, reduce db queries if the load is too high.

There are heaps of other reasons you would want to use this though - one of the main being enabling Continuous Delivery: pushing things into production/live yet having the feature disabled/toggled until it's completed. We often use what we call a 'dev cookie' to show uncompleted features to just the dev team. This way we can test partially completed work in production (oh yeh! is there better integration?) over multiple releases/deployments before we 'untoggle' (completed) it and it becomes visible to the public.

Here's a simple package that helps you do this in ASP.NET MVC land: https://github.com/cottsak/DevCookie (full disclosure: I'm the author)

Fowler also has a much longer article than the one linked above with a lot more details.

This post (on Fowler's site also) explains the various types of toggle strategies. DevCookie supports the mainline/trunk-based strategy and is called a "