A change represents a single commit under review. Each change is identified by a Change-Id.

Multiple git commits can share the same Change-Id, allowing you to update a change as you receive feedback through the code review process. In Gerrit, commits that share the same Change-Id are referred to as patch sets. When a change is approved, only the latest version of a commit is submitted to the repository.

You can view a specific change using Gerrit’s Review screen. This screen provides the following information for each change:

  • Current and previous patch sets
  • Change properties, such as owner, project, and target branch
  • Comments
  • Votes on Review Labels
  • The Change-Id

Change properties

When you open a change in Gerrit, the Review screen displays a number of properties about that change.

Change Properties

Property

Description

Updated

The date on which the change was last updated.

Owner

The contributor who created the change.

Assignee

The contributor responsible for the change. Often used when a change has mulitple reviewers to identify the individual responsible for final approval.

Reviewers

A list of one or more contributors responsible for reviewing the change.

CC

A list of one or more contributors who are kept informed about the change, but are not required to review it.

Project

The name of the Gerrit project.

Branch

The branch on which the change was made.

Topic

An optional topic.

Strategy

The submit strategy for the change.

Code Review

Displays the Code Review status for the change.

In addition, Gerrit displays the status of any additional labels, such as the Verified label, that have been configured for the server. See Review Labels for more information.

Change Message

Next to the list of change properties is the change message. This message contains user-supplied information regarding what the change does. To modify the change message, click the Edit link.

By default, the change message contains the Change-Id. This ID contains a permanent link to a search for that Change-Id in Gerrit.

In some cases, a change may be dependent on another change. These changes are listed next to the change message. These related changes are grouped together in several categories, including:

  • Relation Chain. These changes are related by parent-child relationships, regardless of topics.

  • Merge Conflicts. These are changes in which there is a merge conflict with the current change.

  • Submitted Together. These are changes that share the same topics.

An arrow indicates the change you are currently viewing.

Topics

Changes can be grouped by topics. Topics make it easier to find related changes by using the topic search operator. Changes with the same topic also appear in the Relation Chain section of the Review screen.

Grouping changes by topics can be helpful when you have several changes that, when combined, implement a feature.

Assigning a topic to a change can be done in the change screen or through a git push command.

Submit strategies

Each project in Gerrit can employ a specific submit strategy. This strategy is listed in the change properties section of the Review screen.

The following table lists the supported submit strategies.

Submit Strategies

Strategy

Description

Fast Forward Only

No merge commits are produced. All merges must be handled on the client, before submitting the change.

To submit a change, the change must be a strict superset of the destination branch.

Merge If Necessary

The default submit strategy. If the change being submitted is a strict superset of the destination branch, then the branch is fast-forwarded to the change. If not, a merge commit is automatically created at submit time. This is identical to the git merge --ff command.

Always Merge

Always produce a merge commit, even if the change is a strict superset of the destination branch. This is identical to the git merge --no-ff command. It is often used when users of the project want to be able to read the history of submits by running the git log --first-parent command.

Cherry Pick

Always cherry pick the patch set, ignoring the parent lineage and instead creating a new commit on top of the current branch.

When cherry picking a change, Gerrit automatically appends a short summary of the change’s approvals and a link back to the change. The committer header is also set to the submitter, while the author header retains the original patch set author.

NOTE: Gerrit ignores dependencies between changes when using this submit type unless change.submitWholeTopic is enabled and depending changes share the same topic. This means submitters must remember to submit changes in the right order when using this submit type.

Rebase if Necessary

If the change being submitted is a strict superset of the destination branch, the branch is fast-forwarded to the change. If not, the change is automatically rebased and the branch is fast-forwarded to the change.

Rebase Always

Similar to Rebase If Necessary, but creates a new patch set even if fast forward is possible. This strategy is also similar to Cherry Pick; however, Rebase Always does not ignore dependencies.

Any project owner can use the Project screen to modify the method Gerrit uses to submit a change.

Change-Id

Gerrit uses a Change-Id to identify which patch sets belong to the same review. For example, you make a change to a project. A reviewer supplies some feedback, which you address in a second commit. By assigning the same Change-Id to both commits, Gerrit can attach those commits to the same change.

Change-Ids are appended to the end of a commit message, and resemble the following:

commit 29a6bb1a059aef021ac39d342499191278518d1d
Author: A. U. Thor <author@example.com>
Date: Thu Aug 20 12:46:50 2009 -0700

    Improve foo widget by attaching a bar.

    We want a bar, because it improves the foo by providing more
    wizbangery to the dowhatimeanery.

    Bug: #42
    Change-Id: Ic8aaa0728a43936cd4c6e1ed590e01ba8f0fbf5b
    Signed-off-by: A. U. Thor <author@example.com>
    CC: R. E. Viewer <reviewer@example.com>

Gerrit requires that the Change-Id is in the footer (last paragraph) of a commit message. It can be combined with a Signed-off-by, CC, or other lines. For instance, the previous example has a Change-Id, along with a Signed-off-by and CC line.

Notice that the Change-Id is similar to the commit id. To avoid confusing the two, a Change-Id typically begins with an I.

While there are several ways you can add a Change-Id, the standard method uses git’s commit-msg hook to automatically add the Change-Id to each new commit.