= Gerrit Code Review - A Quick Introduction

Gerrit is a web-based code review tool built on top of the
https://git-scm.com/[git version control system]. This introduction provides
an overview of Gerrit and describes how Gerrit integrates into a typical
development workflow. It also provides a brief tutorial that shows how to manage
a change using Gerrit.

== What is Gerrit?

Gerrit makes code review easy by providing a lightweight framework for reviewing
commits before they are accepted by the codebase. Gerrit works equally well for
projects where approving changes is restricted to selected users, as is typical
for Open Source software development, as well as projects where all contributors
are trusted.

== Gerrit and the developer workflow

To understand how Gerrit fits into and enhances the developer workflow, consider
a typical project. This project has a central source repository, which serves as
the authoritative copy of the project's contents.

.Central Source Repository
image::images/intro-quick-central-repo.png[Authoritative Source Repository]

Gerrit takes the place of this central repository and adds an additional
concept: a _store of pending changes_.

.Gerrit in place of Central Repository
image::images/intro-quick-central-gerrit.png[Gerrit in place of Central Repository]

With Gerrit, when a developer makes a change, it is sent to this store of
pending changes, where other developers can review, discuss and approve the
change. After enough reviewers grant their approval, the change becomes an
official part of the codebase.

In addition to this store of pending changes, Gerrit captures notes
and comments about each change. These features allow developers to review
changes at their convenience, or when conversations about a change can't
happen face to face. They also help to create a record of the conversation
around a given change, which can provide a history of when a change was made and
why.

Like any repository hosting solution, Gerrit has a powerful
link:access-control.html[access control model.] This model allows you to
fine-tune access to your repository.

== Working with Gerrit: An example

To understand how Gerrit works, let's follow a change through its entire
life cycle. This example uses a Gerrit server configured as follows:

* *Hostname*: gerrithost
* *HTTP interface port*: 8080
* *SSH interface port*: 29418

In this walkthrough, we'll follow two developers, Max and Hannah, as they make
and review a change to a +RecipeBook+ project. We'll follow the change through
these stages:

. Making the change.
. Creating the review.
. Reviewing the change.
. Reworking the change.
. Verifying the change.
. Submitting the change.

NOTE: The project and commands used in this section are for demonstration
purposes only.

=== Making the Change

Our first developer, Max, has decided to make a change to the +RecipeBook+
project he works on. His first step is to get the source code that he wants to
modify. To get this code, he runs the following `git clone` command:

----
$ git clone ssh://gerrithost:29418/RecipeBook.git RecipeBook
Cloning into RecipeBook...
----

After he clones the repository, he makes a change to the file and commits it
locally.

NOTE: At this point, the workflow is exactly the same as it would be if Max was
not using Gerrit.

Max is ready to create a commit message for his change. When he does, he
includes a link:user-changeid.html[Change-Id]. This ID allows Gerrit to link
together different versions of the same change being reviewed.

=== Creating the Review

Max's next step is to push his change to Gerrit so other contributors can review
it. He does this using the `git push origin HEAD:refs/for/master` command, as
follows:

----
$ <work>
$ git commit
[master 9651f22] Change to a proper, yeast based pizza dough.
 1 files changed, 3 insertions(+), 2 deletions(-)
$ git push origin HEAD:refs/for/master
Counting objects: 5, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 542 bytes, done.
Total 3 (delta 0), reused 0 (delta 0)
remote:
remote: New Changes:
remote:   http://gerrithost:8080/68
remote:
To ssh://gerrithost:29418/RecipeBook.git
 * [new branch]      HEAD -> refs/for/master
----

The `refs/for/master` branch is a symbolic branch that Gerrit uses to create
reviews for the master branch. If Max opted to push to a different branch, he
would have modified his command to
`git push origin HEAD:refs/for/<branch_name>`. Gerrit automatically creates a
`refs/for/<branch_name>` for every branch that it tracks.

The output of this command also contains a link to a web page Max can use to
review this commit. Clicking on that link takes him to a screen similar to
the following.

.Gerrit Code Review Screen
image::images/intro-quick-new-review.jpg[Gerrit Review Screen]

This is the Gerrit code review screen, where other contributors can review
his change. Max can also perform tasks such as:

* Looking at the link:user-review-ui.html#diff-preferences[diff] of his change
* Writing link:user-review-ui.html#inline-comments[inline] or
  link:user-review-ui.html#reply[summary] comments to explain what he did and
  why
* link:intro-user.html#adding-reviewers[Adding a list of people] that should
  review the change

In this case, Max opts to manually add the senior developer on his team, Hannah,
to review his change.

=== Reviewing the Change

Let's now switch to Hannah, the senior developer who will review Max's change.

As mentioned previously, Max chose to manually add Hannah as a reviewer. Gerrit
offers other ways for reviewers to find changes, including:

* Using the link:user-search.html[search] feature that to find changes
* Setting up link:user-notify.html[email notifications] to stay informed of
  changes even if you are not added as a reviewer

Because Max added Hannah as a reviewer, she receives an email telling her about
his change. She opens up the Gerrit code review screen and selects Max's change.

.Gerrit Code Review Screen
image::images/intro-quick-new-review.jpg[Gerrit Review Screen]

Notice the two "Need" lines:

----
* Need Verified
* Need Code-Review
----

These two lines indicate what checks must be completed before the change is
accepted. The default Gerrit workflow requires two checks:

* *Code-Review*. This check requires that someone look at the code and ensures
  that it meets project guidelines, styles, and other criteria.
* *Verified*. This check means that the code actually compiles, passes any unit
  tests, and performs as expected.

In general, the *Code-Review* check requires an individual to look at the code,
while the *Verified* check is done by an automated build server, through a
mechanism such as the
link:https://wiki.jenkins-ci.org/display/JENKINS/Gerrit+Trigger[Gerrit Trigger
Jenkins Plugin].

IMPORTANT: The Code-Review and Verified checks require different permissions
in Gerrit. This requirement allows teams to separate these tasks. For example,
an automated process can have the rights to verify a change, but not perform a
code review.

With the code review screen open, Hannah can begin to review Max's change. She
can choose one of two ways to review the change: unified or side-by-side.
Both views allow her to perform tasks such as add
link:user-review-ui.html#inline-comments[inline] or
link:user-review-ui.html#reply[summary] comments.

Hannah opts to view the change using Gerrit's side-by-side view:

.Side By Side Patch View
image::images/intro-quick-review-line-comment.jpg[Adding a Comment]

Hannah reviews the change and is ready to provide her feedback. She clicks the
*Review* button on the change screen. This allows her to vote on the change.

.Reviewing the Change
image::images/intro-quick-reviewing-the-change.jpg[Reviewing the Change]

A code review vote is essentially a numerical score between -2 and 2. The
possible options are:

* `+2 Looks good to me, approved`
* `+1 Looks good to me, but someone else must approve`
* `0 No score`
* `-1 I would prefer that you didn't submit this`
* `-2 Do not submit`

By default, a change must have at least one `+2` vote and no `-2` votes before
it can be submitted.

IMPORTANT: Although votes use numerical values, they do not accumulate. Two
`+1` votes do not equate to a `+2`.

Hannah notices a possible issue with Max's change, so she selects a `-1` vote.
She uses the *Cover Message* text box to provide Max with some additional
feedback. When she is satisfied with her review, Hannah clicks the
*Published Comments* button. At this point, her vote and cover message become
visible to to all users.

=== Reworking the Change

Later in the day, Max decides to check on his change and notices Hannah's
feedback. He opens up the source file and incorporates her feedback. Because
Max set up the link:user-changeid.html[Change-Id commit-msg hook]
before he uploaded the change, all he has to do to upload the re-worked change
is push another commit with the same Change-Id in the message. To accomplish
this, Max needs only perform the following tasks:

* Check out the commit
* Amend the commit
* Push the commit to Gerrit

----
$ <checkout first commit>
$ <rework>
$ git commit --amend
$ git push origin HEAD:refs/for/master
Counting objects: 5, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 546 bytes, done.
Total 3 (delta 0), reused 0 (delta 0)
remote: Processing changes: updated: 1, done
remote:
remote: Updated Changes:
remote:   http://gerrithost:8080/68
remote:
To ssh://gerrithost:29418/RecipeBook.git
 * [new branch]      HEAD -> refs/for/master
----

Notice that the output of this command is slightly different from Max's first
commit. This time, the output verifies that the change was updated.

Having uploaded the reworked commit, Max can go back to the Gerrit web
interface and look at his change.

.Reviewing the Rework
image::images/intro-quick-review-2-patches.jpg[Reviewing the Rework]

Notice that there are now two patch sets associated with this change: the
initial submission and the rework.

When Hannah next looks at Max's change, she sees that he incorporated her
feedback. The change looks good to her, so she changes her vote to a `+2`.

=== Verifying the Change

Hannah's `+2` vote means that Max's change satisfies the *Needs Review*
check. It has to pass one more check before it can be accepted: the *Needs
Verified* check.

The Verified check means that the change was confirmed to work. This type of
check typically involves tasks such as checking that the code compiles, unit
tests pass, and other actions. You can configure a Verified check to consist
of as many or as few tasks as needed.

NOTE: Remember that this walkthrough uses Gerrit's default workflow. Projects
can add custom checks or even remove the Verified check entirely.

Verification is typically an automated process using the
link:https://wiki.jenkins-ci.org/display/JENKINS/Gerrit+Trigger[Gerrit Trigger Jenkins Plugin]
or a similar mechanism. However, there are still times when a change requires
manual verification, or a reviewer needs to check how or if a change works.
To accommodate these and other similar circumstances, Gerrit exposes each change
as a git branch. The Gerrit UI includes a
link:user-review-us.html#download[*download*] link in the Gerrit Code Review
Screen to make it easy for reviewers to fetch a branch for a specific change.
To manually verify a change, a reviewer must have the
link:config-labels.html#label_Verified[Verified] permission. Then, the reviewer
can fetch and checkout that branch from Gerrit. Hannah has this permission, so
she is authorized to manually verify Max's change.

NOTE: The Verifier can be the same person as the code reviewer or a
different person entirely.

.Verifying the Change
image::images/intro-quick-verifying.jpg[Verifying the Change]

Unlike the code review check, the verify check is pass/fail. Hannah can provide
a score of either `+1` or `-1`. A change must have at least one `+1` and no
`-1`.

Hannah selects a `+1` for her verified check. Max's change is now ready to be
submitted.

=== Submitting the Change

Max is now ready to submit his change. He opens up the change in the Code Review
screen and clicks the *Publish and Submit* button.

At this point, Max's change is merged into the main part of the repository and
becomes an accepted part of the project.

== Next Steps

This walkthrough provided a quick overview of how a change moves
through the default Gerrit workflow. At this point, you can:

* Read the link:intro-user.html[Users guide] to get a better sense of how to
  make changes using Gerrit
* Review the link:intro-project-owner.html[Project Owners guide] to learn more
  about configuring projects in Gerrit, including setting user permissions and
  configuring verification checks

GERRIT
------
Part of link:index.html[Gerrit Code Review]

SEARCHBOX
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