Improving iOS App Build Times: Spotify's Transition to Bazel

In 2020, Spotify made a significant change to their build process for the iOS app by adopting Bazel as their official build system. The decision came after three years of experimentation with Bazel and has proven to be a game-changer for the company. Spotify engineer Patrick Balestra explains that this transition has resulted in a staggering reduction in build times, making the development process faster and more efficient.

The Need for Faster Build Times

Spotify, as one of the leading music streaming services, constantly strives to improve user experience on their iOS app. One critical factor in achieving this goal is reducing the time it takes to build the app from source code to a functional application. Prior to adopting Bazel, Spotify faced significant challenges with their existing build system, which caused delays in delivering updates and new features to their users.

With the increasing complexity of the iOS app and a growing number of engineers working on it, Spotify recognized the need for a more scalable and efficient build system. They sought a solution that could handle large codebases, enable parallelized builds, and provide faster incremental builds. After thorough evaluation and experimentation, Bazel emerged as the most promising option.

The Bazel Advantage

Bazel is an open-source build tool developed by Google to address the challenges of large-scale software development. Its key advantage lies in its ability to build projects incrementally and in parallel, optimizing build times even for massive codebases. For Spotify, this meant that their engineers could make changes to the codebase and see the results faster than ever before.

One of the key features of Bazel is its compatibility with different programming languages. Since Spotify's iOS app is primarily developed using Swift, Bazel seamlessly integrated with the existing toolchain and build processes. Spotify engineers found Bazel's declarative build configuration language easy to work with, allowing them to define build targets, dependencies, and build rules intuitively.

Reducing Build Times

The transition to Bazel had a significant impact on Spotify's iOS app's build times. According to Patrick Balestra, build times were reduced to a fourth of their original duration. This means that tasks that previously took hours to complete now finish in a fraction of that time. The faster builds allowed engineers to iterate more quickly, resulting in faster delivery of new features and fixes to users.

Bazel achieves this remarkable speedup by leveraging its caching mechanism. It intelligently tracks changes at a granular level and only rebuilds the necessary components affected by those changes. This optimization eliminates the need for rebuilding the entire codebase, resulting in faster incremental builds. Additionally, Bazel's parallelization capabilities enable multiple tasks to execute simultaneously, further boosting efficiency.

Streamlining Development Processes

Beyond the immediate benefit of faster build times, Spotify's transition to Bazel has also streamlined their development processes. Bazel's modular and scalable nature has made it easier for engineers to manage dependencies, allowing for stricter control over code quality and compatibility. The ability to define fine-grained build targets has improved the efficiency of testing and deployment pipelines, reducing the risk of issues in the production environment.

Bazel's integration with Spotify's continuous integration and deployment workflows has further facilitated collaboration among teams. Engineers can easily share build artifacts, ensuring consistent and reproducible builds across different environments.

Conclusion

Spotify's decision to transition to Bazel as their official build system for the iOS app has paid off tremendously. The significant reduction in build times has allowed Spotify to expedite the delivery of new features, bug fixes, and improvements to their millions of iOS app users. By leveraging Bazel's powerful caching and parallelization capabilities, Spotify has achieved remarkable efficiency gains in their development processes, ultimately enhancing the overall user experience.

As Spotify continues to innovate and improve their services, their transition to Bazel stands as a testament to the importance of choosing the right build system to support efficient software development. The impact on Spotify's iOS app serves as inspiration for other organizations seeking to optimize their own build processes and deliver high-quality software products faster.