Feature Flag Best Practices

Chapter 2. The Moving Parts of a Feature-Flagging System

At its core, feature flagging is about your software being able to choose between two or more different execution paths, based upon a flag configuration, often taking into account runtime context (i.e., which user is making the current web request). A toggle router decides the execution path based on runtime context and flag configuration.

Creating Separate Code Paths

Let’s break this down using a working example. Imagine that we work for an ecommerce site called acmeshopping.com. We want to use our feature-flagging system to perform some A/B testing of our checkout flow. Specifically, we want to see whether a user is more likely to click the “Place your order!” button if we enlarge it, as illustrated in Figure 2-1.

Figure 2-1. acmeshopping.com A/B testing

To achieve this, we modify our checkout page rendering code so that there are two different execution paths available at a specific toggle point:

renderCheckoutButton(){
  if( 
    features
      .for({user:currentUser})
      .isEnabled(“showReallyBigCheckoutButton”) 
  ){
    return renderReallyBigCheckoutButton();
  }else{
    return renderRegularCheckoutButton();
  }
}

Every time the checkout page is rendered our software will use that if statement (the toggle point) to select an execution path. It does this by asking the feature-flagging system’s toggle router whether the showReallyBigCheckoutButton ...