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Jujutsu compared

Jujutsu (jj) is a version control system that can share a repository with Git: in a colocated repository, .jj/ and .git/ live side by side, and Git commands still work. jj has no command of its own for a library in a folder: it doesn't support submodules, and the proposals for more aren't released (below).

This page runs jj on the same library and monorepo as the other comparisons, first with the library as a submodule, then as a splice. It shows what works today, and where git-splice stops. What git-splice needs to work there is a page of its own.

The setup: https://git.example.com/lib.git, a library with 30 commits, and a monorepo with an app. How these pages run.

$ jj --version
jj 0.46.0-7d382314f9420b4b8d932245b49c99492c812b9f

A submodule: jj doesn't look inside​

$ git submodule add -q https://git.example.com/lib.git vendor/lib &&
>   git commit -q -m "add lib as a submodule"
$ jj git init --colocate
Done importing changes from the underlying Git repo.
ignoring git submodule at "vendor/lib"
Initialized repo in "."

You fix the library and use the fix in the app:

$ echo "fix" >>vendor/lib/src/parse.txt && echo "call parse()" >>app/main.txt

jj sees the app's change only. The submodule stays at its commit, and the change in it doesn't show, not even as a dirty submodule the way git status shows it:

$ jj diff --stat
app/main.txt | 1 +
1 file changed, 1 insertion(+), 0 deletions(-)

A splice: jj sees the files​

A splice is files in a folder, plus .splice, so jj tracks them like any other:

$ cd ../splice-monorepo && git splice clone https://git.example.com/lib.git vendor/lib
===  vendor/lib: fetching https://git.example.com/lib.git
ok   vendor/lib: cloned bafd496 from main
$ jj git init --colocate
Done importing changes from the underlying Git repo.
Setting the revset alias `trunk()` to `main@origin`.
Hint: The following remote bookmarks aren't associated with the existing local bookmarks:
  main@origin
Hint: Run the following command to keep local bookmarks updated on future pulls:
  jj bookmark track main@origin
Initialized repo in "."
$ echo "fix" >>vendor/lib/src/parse.txt &&
>   echo "call parse()" >>app/main.txt &&
>   jj diff --stat
app/main.txt             | 1 +
vendor/lib/src/parse.txt | 1 +
2 files changed, 2 insertions(+), 0 deletions(-)
$ jj commit -m "fix parse() and call it"
Working copy  (@) now at: qpvuntsm 9df2497b (empty) (no description set)
Parent commit (@-)      : sqpuoqvx eb9ac31c fix parse() and call it

git-splice stops at the detached HEAD​

jj keeps Git's HEAD detached, at the parent of the working-copy commit (@-):

$ git status | head -n 1
Not currently on any branch.

git-splice follows the upstream branch named like the current branch, and here there is none:

$ git splice push vendor/lib
!!   not on a branch (detached HEAD) -- git splice requires a named branch
[1]

Back on a branch, by hand​

jj's main bookmark is still at the clone, since a commit doesn't move a bookmark. Moved to @-, and checked out with Git, it's a branch again:

$ jj bookmark set main -r @-
Moved 1 bookmarks to sqpuoqvx eb9ac31c main* | fix parse() and call it
$ git checkout -q main && git splice push vendor/lib
ok   vendor/lib: pushed 5069701 to main

That's the same commit a push from plain Git sends (Sapling).

The library gets a release:

$ git clone -q https://git.example.com/lib.git ../lib-dev &&
>   echo "1.3" >../lib-dev/VERSION &&
>   git -C ../lib-dev add VERSION &&
>   git -C ../lib-dev commit -q -m "release 1.3" &&
>   git -C ../lib-dev push -q
$ git splice pull vendor/lib
ok   vendor/lib fetched (main moved 5069701..9a471d2)
ok   vendor/lib: pulled 9a471d2

jj imports the pull's commit at its next command, as it does with any commit made with Git:

$ jj log --no-graph -r '::@- ~ ::main@origin' -T 'description.first_line() ++ "\n"'
Reset the working copy parent to the new Git HEAD.
Done importing changes from the underlying Git repo.
splice: pull vendor/lib from main at 9a471d2
fix parse() and call it
splice: clone vendor/lib from main at bafd496

Git stays on main until jj moves the working copy, with jj new or jj commit, so the bookmark and the checkout come before every sync:

$ git status | head -n 1
On branch main
$ jj new --quiet && git status | head -n 1
HEAD detached from eb9ac31

What jj plans​

None of these is released:

  • Submodules. jj's design docs plan submodule support, tracked in #1405 and its sibling issues.
  • Composable histories. An RFC for histories of several repositories in one, closed as a draft (#10179).
  • Views. A filter that derives a folder's repository from the monorepo's history, keeping the upstream's commit hashes, like Josh; closed unmerged (#9889).
  • jj graft. A prototype discussed on jj's Discord: merges between a folder and another repository, with the commit they came from recorded in an extra commit header. That's Sapling's design (Sapling), and the weak parent Git lacks.