dev-docs/working-between-major-versions.adoc

= Working between Multiple Major versions
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Working between multiple major versions of `lucene` is often a necessary part of committing code, due to backports and
testing. For some versions, this is an even bigger issue because `8.x` and `9.x` use different build systems, ant and
gradle. Switching between these branches will result in many files left around that are not tracked by the other branch.
Even when the build system between branches is the same, major refactoring can produce the same issues.
These orphaned files can impact the use of `precommit`, IntelliJ, and other tools.

== Git Worktree

https://git-scm.com/docs/git-worktree[Git worktree] is a feature of git that allows you to have different directories
store separate checkouts of the same repository, at the same time. The git metadata is shared between the different
directories, so any remotes added or local commits made from one worktree are available to all other worktrees as well.

For Lucene, this allows us to have separate directories (worktrees) that manage the checkouts of `main` and `branch_8x`
(or any other major branch). One can make a commit on `main`, then easily switch directories and cherry-pick the commit
onto `branch_8x` without having to worry about gradle or ant files. This setup also allows the commit to be tested on
`main` and `branch_8x` simultaneously.

=== Setup

Wherever you store your source code, create a root folder for lucene.

[source]
----
mkdir lucene
----

This folder is not a git folder. Instead, it will hold all of our lucene git checkouts.

[source,bash]
----
cd lucene
# main will be the main lucene checkout, that all worktrees stem from.
git clone git@github.com:apache/lucene.git main
cd main
# For each branch that you want a separate directory created for, add a worktree
git worktree add ../9x branch_9x
----

=== Using the Worktrees

It's not necessary to create a worktree for every branch you are working on.
Creating repositories for each relevant major version is likely sufficient, because the differences between minor
versions is likely not great enough to require a whole new folder.

Therefore, most developers will only need two: main and the latest major version. Whenever working on a minor release
branch, you can easily use the worktree that corresponds to the same major version.

If you are using IntelliJ, you will likely want to load each of the worktrees as a separate project.
That way when you switch between them, IntelliJ will not have to re-build the project fully.