Feature flags: How we control exposure in VS Team Services

One question that I often get from customers is how we manage exposing features in the service. Features may not be complete or need to be revealed at a particular time. We may want to get early feedback. With the team working in master and deploying every three-week sprint, let’s take a look at how we do this for Team Services.

Goals

Our first goal is decoupling deployment and exposure. We want to be able to control when a feature is available to users without having to time when the code is committed. This allows engineering the freedom to implement the feature based on our needs while also allowing control for the business on when a feature is announced. Next we want to be able to change the setting at any scope from globally to particular scale units to accounts to individual users. This granularity gives us a great deal of flexibility. We can deploy a feature and then expose it to select users and accounts. That allows us to get feedback early, which includes not only what users tell us but also how the feature is used based on aggregated telemetry. Additionally, we want to be able to react quickly if a feature causes issues and be able to turn it off quickly.

To make all of this work well, we need to be able to change a feature flag’s state without re-deploying any of our services. We need each service to react automatically to the change to minimize the propagation delay.

As a result, we have the following goals.

  • Decouple deployment and exposure
  • Control down to an individual user
  • Get feedback early
  • Turn off quickly
  • Change without redeployment

Feature flags

Feature flags, sometimes called feature switches, allow us to achieve our goals. At the core, a feature flag is nothing more than an input to an if statement in the code: if the flag is enabled, execute a new code path, and else if not, execute the existing code path.

Let’s look at an actual example. In this case I want to control whether a new feature to revert a pull request is available to the user. I’ve highlighted the Revert button in the screen shot.

Continue reading…

Author: Buck Hodges

Director of Engineering for Visual Studio Team Services and Team Foundation Server

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