Versioning and Backward Compatibility

You might have noticed that sometimes when you open an app, it immediately asks you to update before you can continue. That’s because a new version has been released, and certain features or backend changes now require it. Unlike web apps, where updates take effect instantly for every user, mobile apps reside on individual devices and require active distribution for updates. Users can hold on to older versions for weeks, months, or even longer, often running side by side with the latest release. This makes versioning an essential concern in mobile System Design, which involves supporting and coordinating multiple active versions of the app at the same time. We, as System Designers, have to plan ahead to ensure our backend continues to work smoothly with all versions still in use, even as we keep evolving and shipping new ones. In other words, our systems need to handle older and newer app versions simultaneously without breaking either, a challenge known as backward compatibility.

The illustration below demonstrates a single backend supporting many app versions:

In this lesson, we’ll explore how to design systems that remain dependable in multi-version environments. We’ll cover strategies for building backward-compatible APIs, phasing out features safely, prompting users to update, and managing local data migrations during app upgrades. By the end, we’ll be equipped to design mobile systems that remain stable and user-friendly across versions.

Let’s now examine more closely why versioning and backward compatibility are crucial in mobile System Design.

Importance of versioning and backward compatibility

When we release a new version of a mobile app, it doesn’t instantly replace what’s already out there. Older versions often remain active as users do not actively update apps until explicitly required by the app, due to compatibility issues, such as in banking apps. This means our backend has to serve multiple app versions at the same time, each expecting slightly different behaviors.

Without careful version management, even a small back-end change can disrupt older clients still active in production. A removed API field, a tighter validation rule, or a new response structure might seem harmless during development, but can easily cause crashes or break functionality for users who haven’t updated. These risks directly impact user experience, trust, and overall system stability.

The following are common situations that highlight why versioning and backward compatibility are critical in mobile System Design:

• Offline users and delayed updates: Not everyone is always online. In regions with limited connectivity or high data costs, users may go weeks without updating. Meanwhile, they continue using an older version of the app that still needs to function reliably. If we design assuming everyone is on the latest build, we risk breaking the app for a significant part of our user base.

• Staged rollouts and A/B testing: We often release new features gradually, using staged rollouts or server-driven flags to test different behaviors in production. This means multiple app versions are active in production at the same time as part of our rollout strategy. Without backward compatibility, that kind of safe experimentation would be impossible. ...