[JLBP-7] Make breaking transitions easy

When a library makes an in-place breaking change (meaning that the Java package is not renamed - see JLBP-6 for more discussion on renaming rules), this can create a lot of work for consumers to adapt to the change. If a change is “atomic” (meaning that the old version of something like a method is removed at the same time the new version is added), all usages of the old method must be converted to the new method in a single change to each consumer’s codebase. If the usage is widespread enough, and especially if usage spans packages by multiple owners and spans multiple levels of the dependency tree, the cost of transitioning may be too high to do at all.

Consequently, libraries should instead make two-phase breaking changes to stable features:

  1. Mark old methods/classes as @Deprecated at the same time as adding new methods/classes. This is a “stepping stone” release.
  2. Delete the deprecated methods/classes. This should wait until major consumers have stopped using the deprecated functionality, where major consumers means consumers with high usage or consumers that are deep in the dependency tree.

When a breaking change is introduced as two phases, consumers can adapt to the change in #1 incrementally. Then when they have completed the transition, adapting to #2 is trivial - upgrading to the version with the removed methods and classes shouldn’t cause any build failures. The ideal breaking-change release would have only removals of deprecated features.

The larger the number of places that use the old version’s code, and the deeper in the tree that the library is used, the longer the time needs to be between #1 and #2. Such a phase can even last years, depending on the release cadence of all the libraries and apps using the old code.

As an example, OkHttp 1.6 was released to help users upgrade to OkHttp 2. OkHttp 1.6 added some new 2.0 APIs which enabled users to transition large code bases to the OkHttp 2 API (in 1.6) before flipping from OkHttp 1 to OkHttp 2. (Source: https://medium.com/square-corner-blog/okhttp-2-0-6da3fe12c879)