jj/docs/glossary.md

Glossary

Anonymous branch

An anonymous branch is a chain of commits that doesn't have any named branches pointing to it or to any of its descendants. Unlike Git, Jujutsu keeps commits on anonymous branches around until they are explicitly abandoned. Visible anonymous branches are tracked by the view, which stores a list of heads of such branches.

Backend

A backend is an implementation of the storage layer. There are currently two builtin commit backends: the Git backend and the native backend. The Git backend stores commits in a Git repository. The native backend is used for testing purposes only. Alternative backends could be used, for example, if somebody wanted to use jj with a humongous monorepo (as Google does).

There are also pluggable backends for storing other information than commits, such as the "operation store backend" for storing the operation log.

Branch

A branch is a named pointer to a commit. They automatically follow the commit if it gets rewritten. Branches are sometimes called "named branches" to distinguish them from anonymous branches, but note that they are more similar to Git's branches than to Mercurial's named branches. See here for details.

Change

A change is a commit as it evolves over time.

Change ID

A change ID is a unique identifier for a change. They are typically 16 bytes long and are often randomly generated. By default, jj log presents them as a sequence of 12 letters in the k-z range, at the beginning of a line. These are actually hexadecimal numbers that use "digits" z-k instead of 0-9a-f.

For the git backend, Change IDs are currently maintained only locally and not exchanged via push/fetch operations.

Commit

A snapshot of the files in the repository at a given point in time (technically a tree object), together with some metadata. The metadata includes the author, the date, and pointers to the commit's parents. Through the pointers to the parents, the commits form a Directed Acyclic Graph (DAG) .

Note that even though commits are stored as snapshots, they are often treated as differences between snapshots, namely compared to their parent's snapshot. If they have more than one parent, then the difference is computed against the result of merging the parents. For example, jj diff will show the differences introduced by a commit compared to its parent(s), and jj rebase will apply those changes onto another base commit.

The word "revision" is used as a synonym for "commit".

Commit ID

A commit ID is a unique identifier for a commit. They are 20 bytes long when using the Git backend. They are presented in regular hexadecimal format at the end of the line in jj log, using 12 hexadecimal digits by default. When using the Git backend, the commit ID is the Git commit ID.

Co-located repos

When using the Git backend and the backing Git repository's .git/ directory is a sibling of .jj/, we call the repository "co-located". Most tools designed for Git can be easily used on such repositories. jj and git commands can be used interchangeably.

See here for details.

Conflict

Conflicts can occur in many places. The most common type is conflicts in files. Those are the conflicts that users coming from other VCSs are usually familiar with. You can see them in jj status and in jj log (the red "conflict" label at the end of the line). See here for details.

Conflicts can also occur in branches. For example, if you moved a branch locally, and it was also moved on the remote, then the branch will be in a conflicted state after you pull from the remote. See here for details.

Similar to a branch conflict, when a change is rewritten locally and remotely, for example, then the change will be in a conflicted state. We call that a divergent change.

Divergent change

A divergent change is a change that has more than one visible commit.

Head

A head is a commit with no descendants. The context in which it has no descendants varies. For example, the heads(X) revset function returns commits that have no descendants within the set X itself. The view records which anonymous heads (heads without a branch pointing to them) are visible at a given operation. Note that this is quite different from Git's HEAD.

Hidden commits, abandoned commits

See visible commits.

Operation

A snapshot of the visible commits and branches at a given point in time (technically a view object), together with some metadata. The metadata includes the username, hostname, timestamps, and pointers to the operation's parents.

Operation log

The operation log is the DAG formed by operation objects, much in the same way that commits form a DAG, which is sometimes called the "commit history". When operations happen in sequence, they form a single line in the graph. Operations that happen concurrently from jj's perspective result in forks and merges in the DAG.

Repository

Basically everything under .jj/, i.e. the full set of operations and commits.

Remote

TODO

Revision

A synonym for Commit.

Revset

Jujutsu supports a functional language for selecting a set of revisions. Expressions in this language are called "revsets". See here for details. We also often use the term "revset" for the set of revisions selected by a revset.

Rewrite

To "rewrite" a commit means to create a new version of that commit with different contents, metadata (including parent pointers), or both. Rewriting a commit results in a new commit, and thus a new commit ID, but the change ID generally remains the same. Some examples of rewriting a commit would be changing its description or rebasing it. Modifying the working copy rewrites the working copy commit.

Root commit

The root commit is a virtual commit at the root of every repository. It has a commit ID consisting of all '0's (00000000...) and a change ID consisting of all 'z's (zzzzzzzz...). It can be referred to in revsets by the function root(). Note that our definition of "root commit" is different from Git's; Git's "root commits" are the first commit(s) in the repository, i.e. the commits jj log -r root()+ will show.

Tree

A tree object represents a snapshot of a directory in the repository. Tree objects are defined recursively; each tree object only has the files and directories contained directly in the directory it represents.

Visible commits

Visible commits are the commits you see in jj log -r 'all()'. They are the commits that are reachable from an anonymous head in the view. Ancestors of a visible commit are implicitly visible.

Intuitively, visible commits are the "latest versions" of a revision with a given change id. A commit that's abandoned or rewritten stops being visible and is labeled as "hidden". Such commits are no longer accessible using a change id, but they are still accessible by their commit id.

View

A view is a snapshot of branches and their targets, anonymous heads, and working-copy commits. The anonymous heads define which commits are visible.

A view object is similar to a tree object in that it represents a snapshot without history, and an operation object is similar to a commit object in that it adds metadata and history.

Workspace

A workspace is a working copy and an associated repository. There can be multiple workspaces for a single repository. Each workspace has a .jj/ directory, but the commits and operations will be stored in the initial workspace; the other workspaces will have pointers to the initial workspace. See here for details.

This is what Git calls a "worktree".

Working copy

The working copy contains the files you're currently working on. It is automatically snapshot at the beginning of almost every jj command, thus creating a new working-copy commit if any changes had been made in the working copy. Conversely, the working copy is automatically updated to the state of the working-copy commit at the end of almost every jj command. See here for details.

This is what Git calls a "working tree".

Working-copy commit

A commit that corresponds to the current state of the working copy. There is one working-copy commit per workspace. The current working-copy commits are tracked in the operation log.