Maintaining binary compatibility in Scala libraries and applications is crucial to ensure that separate parts of a software environment can evolve at their own pace without breaking the whole system. When a library is updated, it should be possible to use the new version without recompiling the dependent code. This is especially important for large software ecosystems with shared libraries and multiple teams relying on the same set of dependencies.
1. Challenges
Several challenges make maintaining binary compatibility tricky in Scala:
1.1. Trait and class hierarchies
When evolving APIs in Scala, adding, removing or changing the hierarchy of traits and classes can lead to binary incompatibilities. For example, adding a method to a trait with a default implementation can break binary compatibility if a class that extends the trait is compiled against the older version.
1.2. Case classes and case objects
When a case class is defined, a series of additional methods and companion functions are generated by the compiler. Changing the structure of a case class or promoting a case class to a regular class can break binary compatibility.
1.3. Implicit parameters and conversions
Scala’s implicit resolution algorithm depends on the name and signature of implicit parameters and values. Changing the name or type signature of an implicit parameter or value can lead to binary incompatibilities if the dependent code depends on implicit resolution.
1.4. Constant folding and inlining
Scala’s optimizer can perform constant folding and inlining based on the values known at compile time. Changing these values or inlined methods in a library can lead to situations where dependent code still executes old, inlined code instead of updated implementations from the library causing a binary incompatibility issue.
1.5. Advanced language features
Scala’s advanced language features, such as macros, type-level programming, and path-dependent types, can pose additional challenges when maintaining binary compatibility as changes to these constructs are often harder to detect and may have subtle effects on the generated bytecode and dependent code.
2. Best practices
Several best practices can help manage and maintain binary compatibility in Scala libraries and applications:
2.1. Be conservative in API design
When defining public APIs, try to minimize the surface that could potentially cause binary compatibility issues. Limit the exposure of traits/classes and their hierarchies, prefer sealed traits or abstract classes over standard traits, and avoid exposing case classes unless necessary.
2.2. Avoid changing case classes and case objects
A case class should be treated as if its structure is set in stone. If you need to change a case class, consider creating a new case class, or deprecating the old one and creating a new non-case class.
2.3. Finalize non-extendable classes and methods
If a class or method is not meant to be extended or overridden, mark it as ‘final‘. This limits the potential surface of binary incompatibilities caused by changes in the class hierarchy or method implementations.
2.4. Use deprecation annotations
Utilize the ‘@deprecated‘ annotation when appropriate to inform users that a particular API is going to be removed or changed in future versions.
2.5. Use separate libraries for experimental features
If you’re experimenting with new features or language constructs, consider creating a separate library or module, so that potential binary compatibility problems can be confined to a smaller portion of your codebase.
2.6. Use a tool like MiMa
Take advantage of tools like ‘MiMa‘ (Migration Manager) to automatically check for binary compatibility issues between different versions of a library. With MiMa, you can ensure that you’re aware of potential binary compatibility problems before they happen.
3. Conclusion
Maintaining binary compatibility in Scala libraries and applications is a critical, ongoing challenge. By following best practices such as minimizing public APIs, avoiding changes to case classes, and leveraging tools like MiMa, you can minimize the risk of breaking binary compatibility and encourage a more maintainable and robust software ecosystem.