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Nothing in this document is meant to state that +one or the other might be better, but only aims to help new users understand +Gerrit more readily. We use Github as the point of comparison since it seems +to be the most popular service. +==== + +To illustrate the differences in a meaningful order, we will walk you through +the process of cloning a repo, making a change, asking for code review, +iterating on the code and finally having it submitted to the code base. This +document also does not aim to describe all features of Gerrit. Please refer to +the link:intro-gerrit-walkthrough.html[Basic Gerrit Walkthrough] or +link:index.html[the rest of the documentation] for a more complete overview and additional pointers. + +[[tldr]] +== tl;dr + +Here’s how getting code reviewed and submitted with Gerrit is different from +doing the same with GitHub: + +* You need the add a commit-msg hook script when you clone a repo for the first +time using a snippet you can find e.g. https://gerrit-review.googlesource.com/admin/repos/gerrit[here]; +* Your review will be on a single commit instead of a branch. You use +`git commit --amend` to modify a code change. +* Instead of using the Web UI to create a pull request, you use +`git push origin HEAD:refs/for/master` to upload new local commits that are +ready for review to Gerrit. You will find the URL to the review in the output of +the push command. +* As a reviewer, Gerrit offers a number of so-called labels to vote on, one of +which is Code-Review. You indicate a negative, neutral or positive review using +a -1, 0 or +1 vote. +* To be able to submit (== merge) a change, you usually need a +2 Code-Review +vote and possibly additional positive votes, depending on the configuration of +the project you are contributing to. + +[[clone]] +== 1. Cloning a Repository + +[NOTE] +==== +Both GitHub and Gerrit provide simple Git repository hosting (of course both can +do much more). In the simplest setup, you could just use both as such without +any code review to push code. We will assume that this is not what you want to +do and focus on the use case where your change requires a review. +==== + +The first step to working with the code is to clone the repo. For both, Gerrit +and GitHub, you can simply use the `git clone` command. + +For Gerrit, there is an additional step before you can start making changes. For +reasons we explain below, you’ll have to add a https://gerrit-review.googlesource.com/Documentation/user-changeid.html[commit-msg hook] script. This will +append the Gerrit Change-Id to every commit message such that Gerrit can track +commits through the review process. To make this process a little easier in +Gerrit, you can find a command snippet for cloning and adding the commit-msg +hook on the repository page (e.g. https://gerrit-review.googlesource.com/admin/repos/gerrit[here]). + +[[create-change]] +== 2. Making a Change + +*Branches* + +Now that you have the code in the git repo on your machine, you can start making +changes. With GitHub, you would usually create a new branch and then start +committing to it. This branch would then contain all the changes you share with +your code reviewers in the next step. Your local branch will usually also be +pushed to the remote server. This can be handy to back up your work or hand-off +work to another device or developer. + +With Gerrit, you can also create a new local branch to develop in. While not +required, it can be considered a best practice to sandbox this change from other +changes you might be making. In contrast to the GitHub model, your local branch +will not have to be pushed to the remote in Gerrit, at least not for the +purposes of code review. + +*Commits* +In Gerrit, a single commit is the unit of code that will be reviewed. With +GitHub, you can commit to your branch as much as you like and the sum of all +your commits on that branch will get reviewed. As a single commit gets reviewed +in Gerrit, you need to `git commit --amend` when you iterate on the same change as +opposed to only using `git commit` with GitHub (see Section 5 for more). You can, +however, also add another commit on top of your existing commit in Gerrit, which +will create a second change (and thus another review) that is based on your +first change. Gerrit will show the relationship between these two changes as a +so-called relation chain. This also means that your second change can only be +submitted after the first was successfully merged. In many basic use cases, this +situation is however not what you want. + +image::images/user-review-ui-change-relation-chain.png[Relation chain display on the change page.] + +With GitHub, you may be pushing your branch to the remote for non-code-review +purposes, as mentioned above. You usually do not do this with Gerrit, as +Gerrit-managed repos often only have one or a few branches on the server that +can only be merged into via code review. + +[[request-review]] +== 3. Asking for Code Review + +After you are satisfied with the changes you made, you’ll usually want/need to +get your code reviewed. In GitHub, you would push your branch to the remote, go +to the Web UI and create a pull request. In Gerrit, you need to push your commit +(or the series of changes/commits) to the remote first, since you usually +develop in a local branch only. While you can often just use git push with +GitHub, you need to do a slightly different thing for Gerrit. Gerrit uses a +“magic” branch that tells the server that this code is supposed to be reviewed. +To send the changes you made on your local branch to review and being eventually +merged into the remote’s master branch, you use +`git push origin HEAD:refs/for/master`. There are also link:user-upload.html#_git_push[a number of Gerrit change +options] you can trigger from the CLI this way. + +After successfully pushing your change to Gerrit, you will already find the URL +for viewing your change in Gerrit’s Web UI in the response you get from the +server. The description of the Gerrit code review that was just created is equal +to the commit message of that one commit the change is based on. In GitHub, you +might have described your change in the message you can create when creating the +pull request in the GitHub Web UI. + +Next, you would go and visit your Gerrit change in the Web UI to get your change +ready for review (choose reviewers, cc people, check for failing CI builds or +tests, etc.), very similar to what you do on Github. Reviewers will be notified +via email once you add them. By default, anyone can add reviewers to a Gerrit +change. In GitHub, this ability is reserved for certain users, so you may have +relied on others adding reviewers for you before. This can be the case in a +Gerrit project, but it is also often expected that the change owner (usually the +creator of the change) adds reviewers to get the review process started. + +[[reviewing]] +== 4. Reviewing a Change + +Switching perspectives briefly, reviewing a change is fairly similar between +GitHub and Gerrit. You, as a reviewer, will be notified of a change you have +been added to via email or see an “incoming” change on your Gerrit dashboard. +The dashboard is the central overview of changes going on within a Gerrit +instance. By default, the dashboard shows changes that you are involved in, in +any way. You can also see all changes on a Gerrit server by using the top menu +(“Changes” -> “Open”). This view is more similar to what you see on Github, when +you navigate to the Pull Requests tab of the project/repository you are working +on. Note, however, that a single Gerrit instance can host multiple projects +(also referred to as repositories; a list can be found, for example, https://gerrit-review.googlesource.com/admin/repos[here]). Your +dashboard and other lists of changes will show all changes across the +projects/repositories by default. + +Back to your dashboard, you can click on the change you want to review. You can +also access this from the email you received. You will see the same view that +you saw as an author. In the middle of the change page, you can find the list of +files that have been modified, just like what you find in the “Files changed” +tab of GitHub. Also similarly, you can leave comments by highlighting a piece of +the code and pressing ‘c’. All comments you make are in a draft state and thus +only visible to you, like on GitHub. When you are done with your review, you +need to click the “Reply” button at the top of the change page to send your +assessment to the change owner alongside a “change message” summarizing your +findings and/or adding higher level comments. Replying to a change makes your +draft comments and the change message visible on the change page for everyone +that has view access to this change. This again is fairly similar to GitHub, +except for Gerrit’s voting labels. + +image::images/user-review-ui-change-reply-dialogue.png[Reply dialogue for a Gerrit change.] + +As you can see in the screenshot of the reply dialogue, the voting labels are in +the bottom part of the dialogue. They can be fairly simple as in this case, but +there can also be a larger number of labels you might be able to vote on. Labels +can be used to distinguish different aspects of a review (e.g. whether or not +the licensing of included libraries is okay), outcome of CI systems (e.g. +whether or not a format checker passed, a build completed successfully, etc.) or +as a flag that is read by bots to do something with a change. An example of a +more complex label setup can be seen in this screenshot from the Android Gerrit +instance. + +image::images/user-review-ui-change-complex-reply-dialogue.png[Reply dialogue for a change on the Android project.] + +In the simplest case shown above, voting -1 on the Code-Review label equals +requesting changes on a GitHub pull request, 0 equals just having comments and ++1 means that you think this change looks good. Usually, Gerrit changes require +a +2 vote on the Code-Review label to be submitted (merged in GitHub terms, see +Section 6 below). Being able to vote +2 on Code-Review is often restricted to +maintainers of a given project, so they can have a final say on a change. These +practices can however vary between projects, as labels and voting permissions +are configurable. + +[[iterate]] +== 5. Iterating on the Change + +After your reviewers got back to you as a change owner, you realize that you +need to make a few updates to the code in your change. As mentioned in Section 2 +(Making a Change), you’ll have to amend the commit that this review was based +on. To do that, you might have to checkout the respective commit first if it is +not at the tip of your local branch, for example if you stacked multiple changes +on top of each other. Another common use case is to not have a local branch but +to work in the so-called https://www.git-tower.com/learn/git/faq/detached-head-when-checkout-commit["detached HEAD"] mode. In that case you can use the +“Download” button on the files tab to copy a command that fetches and checks out +the commit underlying your change. Make sure to select the latest patchset, +though! + +image::images/user-review-ui-change-page-download.png[Using the “Download” button to copy a command that checks out a given patchset for a change.] + +After checking out the commit, you then make the changes as usual. When you +think you are done, you can commit with the `--amend` flag to change the commit +you currently have checked out. + +When you `git commit --amend` to iterate on your change, you might be worried that +you are changing your previous commit and may thus lose that state of your work. +However, here the Change-Id appended to your commit message comes into play. +While the SHA1 hash of your change (the commit ID used by Git) might change, the +Change-Id stays the same (in fact it is the SHA1 hash of the very first version +of that commit). When this amended commit is uploaded to the Gerrit server, +Gerrit knows that this commit is really an iteration of that previous commit +(and the associated review) and will preserve both, the old and the new state. +All previous states of your commit will be visible in the Gerrit UI as so-called +patchsets (and link:intro-user.html#change-ref[from the Git repo]). + +image::images/user-review-ui-change-page-patchset-dropdown.png[Screenshot of the patchset dropdown above the file list, showing all iterations a commit went through.] + +After iterating as much as needed, your reviewers will finally be satisfied. +With GitHub, you would have a string of additional commits in the branch you +used for opening the pull request. In Gerrit, you still only have that one +commit in your local branch. All the iterations are available as patchsets in +the Web UI as well as from the special branch mentioned above. + +[[submit]] +== 6. Submitting a Change + +Finally, it is time to submit your change. As mentioned above, the precondition +for this in Gerrit is usually at least a +2 vote on the Code-Review label. With +GitHub, an authorized person must have given an “Approve” vote. Once this +precondition has been met, anyone with submit permission can submit the change +in Gerrit. To do that, you click the “Submit” button in the Gerrit Web UI just +as you would click the “Merge Pull Request” button in GitHub. Both, Gerrit and +GitHub, allow different merge strategies, that can be enabled by project +administrators. In Gerrit, a merge strategy is configured for each project and +cannot be changed at submit time while this may be possible with GitHub, +depending on project configuration. + +A merge can fail due to conflicts with competing edits on the target branch. +With GitHub, you may be able to resolve some simple conflicts directly from the +Web UI. In Gerrit, you can attempt to rebase a change from the Web UI. If there +are no conflicts, a new patchset will automatically appear. Otherwise, similar +to GitHub, you need to resolve conflicts on the command line with your local +clone of the repository. While you resolve conflicts that arise from a +`git merge` for GitHub, you will need to link:intro-user.html#rebase[use `git rebase` with your change] on +Gerrit. + +After resolving locally, with GitHub, you end up with another commit on your +pull request branch and push it to the server, which should then allow you to +finish merging the change. With Gerrit, resolving the conflict through rebasing +your commit/change results in another amended version of that same commit and +you upload it to Gerrit, resulting in a new patchset just like your previous +iterations addressing reviewer comments. This new patchset will usually require +another round of reviewer votes, as Gerrit will not copy votes from a previous +patchset by default. + + +GERRIT +------ + +Part of link:index.html[Gerrit Code Review] + +SEARCHBOX +---------