Starlark: The Language Behind Bazel
Starlark is a small, simple, and thread-safe programming language used primarily in the context of the Bazel build system. Designed with the intent to define build configurations and automate software builds, Starlark plays a pivotal role in the development lifecycle of large-scale software projects. While it shares similarities with Python, Starlark is not a general-purpose language. Instead, it focuses on specific domains such as configuration, scripting, and automation. This article explores Starlark’s characteristics, its role in Bazel, its design philosophy, and its adoption within the software development ecosystem.
The Origins of Starlark
Starlark was introduced in 2018 as a key component of Bazel, an open-source build and test tool developed by Google. Bazel is known for its scalability and ability to handle complex builds, and Starlark was created to help automate and manage these builds in a simple, concise manner. The language was developed by Laurent Le Brun and designed with a specific set of goals in mind: simplicity, thread-safety, and efficiency in managing build systems.
Although Starlark is heavily inspired by Python, it is a much more focused language, targeting only a subset of Python’s features. This makes Starlark both easy to learn for Python developers and efficient for use in configuration and automation contexts. The decision to limit the language’s features was intentional; it allows Starlark to perform its tasks within the confines of the build system without the overhead or complexity of a general-purpose language.
Key Features of Starlark
1. Syntactic Similarity to Python:
Starlark’s syntax is derived from Python 3, making it accessible for developers already familiar with Python. It uses familiar constructs such as variables, functions, and control flow statements like loops and conditionals. This Python-inspired syntax, however, omits certain features, making Starlark a simplified and specialized language.
2. Small and Simple:
One of the primary goals in designing Starlark was to ensure that it remains small and simple. This design philosophy allows developers to quickly understand the language and use it to automate complex build configurations without the need for an extensive learning curve. Starlark has a minimalistic set of data types and constructs, making it ideal for build scripting and configuration management.
3. Thread-Safety:
Thread-safety is an important aspect of Starlark, especially when used in multi-threaded build systems like Bazel. The language is designed to allow multiple operations to run in parallel without causing race conditions or corrupting data. This is crucial in large-scale builds, where many tasks are often performed simultaneously.
4. Limited Feature Set:
While Starlark is inspired by Python, it does not include many of Python’s features. For instance, it lacks object-oriented programming (OOP) features such as classes and inheritance. It also does not have built-in support for dynamic typing or other Python features that are unnecessary for its intended use case. Instead, Starlark focuses on a small set of features that are optimized for configuration, build automation, and scripting tasks.
5. Support for Comments and Line Comments:
Starlark supports the use of comments in scripts, allowing developers to add explanatory notes and improve code readability. Line comments are denoted using the hash symbol (#), and they can be placed on their own line or at the end of a code line. This feature is essential for documenting build scripts, making it easier for teams to understand the logic behind various build configurations.
6. No Semantic Indentation:
Unlike Python, which uses indentation as a critical part of its syntax, Starlark does not require semantic indentation. This means that the structure of code blocks is not dictated by indentation, allowing developers to format code in a way that is most convenient to them without worrying about accidental syntax errors related to indentation.
Starlark in the Context of Bazel
Bazel, developed by Google, is an open-source tool that automates the process of building and testing software. It is particularly well-suited for large codebases, offering scalability and speed. The language Starlark is used extensively within Bazel to define build configurations and automate processes like compiling code, managing dependencies, and testing software.
In Bazel, build configurations are written in Starlark, which allows developers to specify how different software components should be built, tested, and deployed. Starlark enables users to define complex build rules, set up dependencies between various components, and create custom scripts to handle specific build tasks. By using Starlark, Bazel users can automate repetitive tasks, optimize build times, and ensure that their software is built in a consistent and reproducible manner.
Bazel’s use of Starlark also contributes to its extensibility. Developers can extend Bazel’s functionality by writing custom build rules and macros in Starlark, tailoring the build system to meet their specific needs. This flexibility is one of the reasons why Bazel is favored for large-scale projects with complex build requirements.
Starlark’s Role in the Development Ecosystem
While Starlark is closely tied to Bazel, its use extends beyond just the build system. Many companies and open-source projects that rely on Bazel for their build processes also leverage Starlark to enhance the efficiency of their development workflows. By using Starlark for build automation, developers can create repeatable and reliable builds, ensuring that software is compiled and tested in a controlled environment.
Moreover, Starlark’s thread-safety and simplicity make it an attractive choice for developers working in continuous integration and continuous deployment (CI/CD) environments. As software development has increasingly moved toward automated pipelines, languages like Starlark provide the necessary tools for ensuring that code is built and tested quickly and reliably.
Starlark’s role in the broader development ecosystem is also reflected in its adoption. The language has garnered attention from both open-source projects and large enterprises that require scalable and efficient build systems. Its ease of use and integration with Bazel have made it an essential tool for software teams that prioritize automation, speed, and scalability in their workflows.
GitHub Repository and Community Engagement
Starlark’s development and maintenance are open-source, and its codebase is hosted on GitHub. The repository is actively managed, with contributions from the community and ongoing improvements to the language. As of the latest data, the Starlark GitHub repository has over 60 issues, which indicate ongoing discussion and development around the language’s features and improvements.
Though the repository is not widely discussed on Wikipedia, the GitHub repository itself serves as the central hub for developers seeking to contribute to or learn more about Starlark. The use of GitHub provides transparency in the development process and fosters a community-driven approach to the language’s evolution.
Conclusion
Starlark is a specialized, lightweight language designed to meet the needs of software developers working with Bazel. Its simplicity, thread-safety, and focus on build automation make it an ideal choice for managing complex build processes. While it shares similarities with Python, Starlark is a purpose-built language that omits many of Python’s features in favor of a more focused, efficient design.
As the software development industry continues to emphasize automation, scalability, and speed, languages like Starlark will remain critical in driving efficient build systems and improving developer productivity. Its role within the Bazel ecosystem demonstrates its utility in modern software development, making it a valuable tool for both individual developers and large organizations alike.
By leveraging Starlark for build configuration and automation tasks, software teams can streamline their workflows and focus more on writing high-quality code, knowing that their build system is capable of handling complex dependencies and tasks in an efficient, repeatable manner.