Code Review Guidelines

This guide contains advice and best practices for performing code review, and having your code reviewed.

All merge requests for GitLab CE and EE, whether written by a GitLab team member or a wider community member, must go through a code review process to ensure the code is effective, understandable, maintainable, and secure.

Getting your merge request reviewed, approved, and merged

Before you begin:

• Familiarize yourself with the contribution acceptance criteria.
• If you need some guidance (for example, if it's your first merge request), feel free to ask one of the Merge request coaches.

As soon as you have code to review, have the code reviewed by a reviewer. This reviewer can be from your group or team, or a domain expert. The reviewer can:

• Give you a second opinion on the chosen solution and implementation.
• Help look for bugs, logic problems, or uncovered edge cases.

For assistance with security scans or comments, include the Application Security Team (@gitlab-com/gl-security/appsec).

The reviewers use the reviewer functionality in the sidebar. Reviewers can add their approval by approving additionally.

Depending on the areas your merge request touches, it must be approved by one or more maintainers. The Approved button is in the merge request widget.

Getting your merge request merged also requires a maintainer. If it requires more than one approval, the last maintainer to review and approve merges it.

Read more about author responsibilities below.

Domain experts

Domain experts are team members who have substantial experience with a specific technology, product feature, or area of the codebase. Team members are encouraged to self-identify as domain experts and add it to their team profiles.

When self-identifying as a domain expert, it is recommended to assign the MR changing the .yml file to be merged by an already established Domain Expert or a corresponding Engineering Manager.

We make the following assumption with regards to automatically being considered a domain expert:

• Team members working in a specific stage/group (for example, create: source code) are considered domain experts for that area of the app they work on
• Team members working on a specific feature (for example, search) are considered domain experts for that feature

We default to assigning reviews to team members with domain expertise. When a suitable domain expert isn't available, you can choose any team member to review the MR, or simply follow the Reviewer roulette recommendation.

Team members' domain expertise can be viewed on the engineering projects page or on the GitLab team page.

Reviewer roulette

The Danger bot randomly picks a reviewer and a maintainer for each area of the codebase that your merge request seems to touch. It only makes recommendations and you should override it if you think someone else is a better fit!

It picks reviewers and maintainers from the list at the engineering projects page, with these behaviors:

• It doesn't pick people whose Slack or GitLab status:
  • Contains the string OOO, PTO, Parental Leave, or Friends and Family.
  • GitLab user Busy indicator is set to True.
  • Emoji is from one of these categories:
    • On leave - 🌴 :palm_tree:, 🏖️ :beach:, ⛱ :beach_umbrella:, 🏖 :beach_with_umbrella:, 🌞 :sun_with_face:, 🎡 :ferris_wheel:
    • Out sick - 🌡️ :thermometer:, 🤒 :face_with_thermometer:
    • At capacity - 🔴 :red_circle:
    • Focus mode - 💡 :bulb: (focusing on their team's work)
• It doesn't pick people who are already assigned a number of reviews that is equal to or greater than their chosen "review limit". The review limit is the maximum number of reviews people are ready to handle at a time. Set a review limit by using one of the following as a Slack or GitLab status:
  • 0️⃣ - :zero: (similar to :red_circle:)
  • 1️⃣ - :one:
  • 2️⃣ - :two:
  • 3️⃣ - :three:
  • 4️⃣ - :four:
  • 5️⃣ - :five:
• Team members whose Slack or GitLab status emoji is 🔵 :large_blue_circle: are more likely to be picked. This applies to both reviewers and trainee maintainers.
  • Reviewers with 🔵 :large_blue_circle: are two times as likely to be picked as other reviewers.
  • Trainee maintainers with 🔵 :large_blue_circle: are three times as likely to be picked as other reviewers.
• People whose GitLab status emoji is 🔶 :large_orange_diamond: or 🔸 :small_orange_diamond: are half as likely to be picked.
• It always picks the same reviewers and maintainers for the same branch name (unless their out-of-office (OOO) status changes, as in point 1). It removes leading ce- and ee-, and trailing -ce and -ee, so that it can be stable for backport branches.

The Roulette dashboard contains:

• Assignment events in the last 7 and 30 days.
• Currently assigned merge requests per person.
• Sorting by different criteria.
• A manual reviewer roulette.
• Local time information.

For more information, review the roulette README.

Approval guidelines

As described in the section on the responsibility of the maintainer below, you are recommended to get your merge request approved and merged by maintainers with domain expertise.

1. If your merge request includes ~backend changes (1), it must be approved by a backend maintainer.
2. If your merge request includes database migrations or changes to expensive queries (2), it must be approved by a database maintainer. Read the database review guidelines for more details.
3. If your merge request includes ~frontend changes (1), it must be approved by a frontend maintainer.
4. If your merge request includes (~UX) user-facing changes (3), it must be approved by a Product Designer. See the design and user interface guidelines for details.
5. If your merge request includes adding a new JavaScript library (1)...
   • If the library significantly increases the bundle size, it must be approved by a frontend foundations member.
   • If the license used by the new library hasn't been approved for use in GitLab, the license must be approved by a legal department member. More information about license compatibility can be found in our GitLab Licensing and Compatibility documentation.
6. If your merge request includes a new dependency or a file system change, it must be approved by a Distribution team member. See how to work with the Distribution team for more details.
7. If your merge request includes documentation changes, it must be approved by a Technical writer, based on assignments in the appropriate DevOps stage group.
8. If your merge request includes changes to development guidelines, follow the review process and get the approvals accordingly.
9. If your merge request includes end-to-end and non-end-to-end changes (4), it must be approved by a Software Engineer in Test.
10. If your merge request only includes end-to-end changes (4) or if the MR author is a Software Engineer in Test, it must be approved by a Quality maintainer
11. If your merge request includes a new or updated application limit, it must be approved by a product manager.
12. If your merge request includes Product Intelligence (telemetry or analytics) changes, it should be reviewed and approved by a Product Intelligence engineer.
13. If your merge request includes an addition of, or changes to a Feature spec, it must be approved by a Quality maintainer or Quality reviewer.
14. If your merge request introduces a new service to GitLab (Puma, Sidekiq, Gitaly are examples), it must be approved by a product manager. See the process for adding a service component to GitLab for details.
15. If your merge request includes changes related to authentication or authorization, it must be approved by a Manage:Authentication and Authorization team member. Check the code review section on the group page for more details. Patterns for files known to require review from the team are listed in the in the Authentication and Authorization section of the CODEOWNERS file, and the team will be listed in the approvers section of all merge requests that modify these files.

• (1): Specs other than JavaScript specs are considered ~backend code. Haml markup is considered ~frontend code. However, Ruby code within Haml templates is considered ~backend code.
• (2): We encourage you to seek guidance from a database maintainer if your merge request is potentially introducing expensive queries. It is most efficient to comment on the line of code in question with the SQL queries so they can give their advice.
• (3): User-facing changes include both visual changes (regardless of how minor), and changes to the rendered DOM which impact how a screen reader may announce the content.
• (4): End-to-end changes include all files within the qa directory.

Acceptance checklist

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for high-impact risks to quality, performance, reliability, security, observability, and maintainability.

Using checklists improves quality in software engineering. This checklist is a straightforward tool to support and bolster the skills of contributors to the GitLab codebase.

Quality

See the test engineering process for further quality guidelines.

1. I have self-reviewed this MR per code review guidelines.
2. For the code that this change impacts, I believe that the automated tests (Testing Guide) validate functionality that is highly important to users (including consideration of all test levels).
3. If the existing automated tests do not cover the above functionality, I have added the necessary additional tests or added an issue to describe the automation testing gap and linked it to this MR.
4. I have considered the technical aspects of this change's impact on GitLab.com hosted customers and self-managed customers.
5. I have considered the impact of this change on the frontend, backend, and database portions of the system where appropriate and applied the ~ux, ~frontend, ~backend, and ~database labels accordingly.
6. I have tested this MR in all supported browsers, or determined that this testing is not needed.
7. I have confirmed that this change is backwards compatible across updates, or I have decided that this does not apply.
8. I have properly separated EE content from FOSS, or this MR is FOSS only.
   • Where should EE code go?
9. I have considered that existing data may be surprisingly varied. For example, a new model validation can break existing records. Consider making validation on existing data optional rather than required if you haven't confirmed that existing data will pass validation.

Performance, reliability, and availability

1. I am confident that this MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
2. I have added information for database reviewers in the MR description, or I have decided that it is unnecessary.
   • Does this MR have database-related changes?
3. I have considered the availability and reliability risks of this change.
4. I have considered the scalability risk based on future predicted growth.
5. I have considered the performance, reliability, and availability impacts of this change on large customers who may have significantly more data than the average customer.

Observability instrumentation

1. I have included enough instrumentation to facilitate debugging and proactive performance improvements through observability. See example of adding feature flags, logging, and instrumentation.

Documentation

1. I have included changelog trailers, or I have decided that they are not needed.
   • Does this MR need a changelog?
2. I have added/updated documentation or decided that documentation changes are unnecessary for this MR.
   • Is documentation required?

Security

1. I have confirmed that if this MR contains changes to processing or storing of credentials or tokens, authorization, and authentication methods, or other items described in the security review guidelines, I have added the ~security label and I have @-mentioned @gitlab-com/gl-security/appsec.
2. I have reviewed the documentation regarding internal application security reviews for when and how to request a security review and requested a security review if this is warranted for this change.

Deployment

1. I have considered using a feature flag for this change because the change may be high risk.
2. If I am using a feature flag, I plan to test the change in staging before I test it in production, and I have considered rolling it out to a subset of production customers before rolling it out to all customers.
   • When to use a feature flag
3. I have informed the Infrastructure department of a default setting or new setting change per definition of done, or decided that this is unnecessary.

The responsibility of the merge request author

The responsibility to find the best solution and implement it lies with the merge request author. The author or directly responsible individual (DRI) stays assigned to the merge request as the assignee throughout the code review lifecycle. If you are unable to set yourself as an assignee, ask a reviewer to do this for you.

Before requesting a review from a maintainer to approve and merge, they should be confident that:

• It actually solves the problem it was meant to solve.
• It does so in the most appropriate way.
• It satisfies all requirements.
• There are no remaining bugs, logical problems, uncovered edge cases, or known vulnerabilities.

The best way to do this, and to avoid unnecessary back-and-forth with reviewers, is to perform a self-review of your own merge request, following the Code Review guidelines. During this self-review, try to include comments in the MR on lines where decisions or trade-offs were made, or where a contextual explanation might aid the reviewer in more easily understanding the code.

To reach the required level of confidence in their solution, an author is expected to involve other people in the investigation and implementation processes as appropriate.

They are encouraged to reach out to domain experts to discuss different solutions or get an implementation reviewed, to product managers and UX designers to clear up confusion or verify that the end result matches what they had in mind, to database specialists to get input on the data model or specific queries, or to any other developer to get an in-depth review of the solution.

If your merge request touches more than one domain (for example, Dynamic Analysis and GraphQL), ask for reviews from an expert from each domain.

If an author is unsure if a merge request needs a domain expert's opinion, then that indicates it does. Without it, it's unlikely they have the required level of confidence in their solution.

Before the review, the author is requested to submit comments on the merge request diff alerting the reviewer to anything important as well as for anything that demands further explanation or attention. Examples of content that may warrant a comment could be:

• The addition of a linting rule (RuboCop, JS etc).
• The addition of a library (Ruby gem, JS lib etc).
• Where not obvious, a link to the parent class or method.
• Any benchmarking performed to complement the change.
• Potentially insecure code.

If there are any projects, snippets, or other assets that are required for a reviewer to validate the solution, ensure they have access to those assets before requesting review.

Avoid:

• Adding TODO comments (referenced above) directly to the source code unless the reviewer requires you to do so. If TODO comments are added due to an actionable task, include a link to the relevant issue.
• Adding comments which only explain what the code is doing. If non-TODO comments are added, they should explain why, not what.
• Requesting maintainer reviews of merge requests with failed tests. If the tests are failing and you have to request a review, ensure you leave a comment with an explanation.
• Excessively mentioning maintainers through email or Slack (if the maintainer is reachable through Slack). If you can't add a reviewer for a merge request, @ mentioning a maintainer in a comment is acceptable and in all other cases adding a reviewer is sufficient.

This saves reviewers time and helps authors catch mistakes earlier.

The responsibility of the reviewer

Review the merge request thoroughly.

Verify that the merge request meets all contribution acceptance criteria.

If a merge request is too large, fixes more than one issue, or implements more than one feature, you should guide the author towards splitting the merge request into smaller merge requests.

When you are confident that it meets all requirements, you should:

• Select Approve.
• @ mention the author to generate a to-do notification, and advise them that their merge request has been reviewed and approved.
• Request a review from a maintainer. Default to requests for a maintainer with domain expertise, however, if one isn't available or you think the merge request doesn't need a review by a domain expert, feel free to follow the Reviewer roulette suggestion.
• Remove yourself as a reviewer.

The responsibility of the maintainer

Maintainers are responsible for the overall health, quality, and consistency of the GitLab codebase, across domains and product areas.

Consequently, their reviews focus primarily on things like overall architecture, code organization, separation of concerns, tests, DRYness, consistency, and readability.

Because a maintainer's job only depends on their knowledge of the overall GitLab codebase, and not that of any specific domain, they can review, approve, and merge merge requests from any team and in any product area.

A maintainer should ask the author to make a merge request smaller if it is:

• Too large.
• Fixes more than one issue.
• Implements more than one feature.
• Has a high complexity resulting in additional risk.

The maintainer, any of the reviewers, or a merge request coach can step up to help the author to divide work into smaller iterations, and guide the author on how to split the merge request. The author may choose to request that the current maintainers and reviewers review the split MRs or request a new group of maintainers and reviewers.

Maintainers do their best to also review the specifics of the chosen solution before merging, but as they are not necessarily domain experts, they may be poorly placed to do so without an unreasonable investment of time. In those cases, they defer to the judgment of the author and earlier reviewers, in favor of focusing on their primary responsibilities.

If a maintainer feels that an MR is substantial enough that it warrants a review from a domain expert, and it is unclear whether a domain expert have been involved in the reviews to date, they may request a domain expert's review before merging the MR.

If a developer who happens to also be a maintainer was involved in a merge request as a reviewer, it is recommended that they are not also picked as the maintainer to ultimately approve and merge it.

Maintainers should check before merging if the merge request is approved by the required approvers. If still awaiting further approvals from others, remove yourself as a reviewer then @ mention the author and explain why in a comment. Stay as reviewer if you're merging the code.

Maintainers must check before merging if the merge request is introducing new vulnerabilities, by inspecting the list in the merge request Security Widget. When in doubt, a Security Engineer can be involved. The list of detected vulnerabilities must be either empty or containing:

• dismissed vulnerabilities in case of false positives
• vulnerabilities converted to issues

Maintainers should never dismiss vulnerabilities to "empty" the list, without duly verifying them.

Note that certain merge requests may target a stable branch. These are rare events. These types of merge requests cannot be merged by the Maintainer. Instead, these should be sent to the Release Manager.

After merging, a maintainer should stay as the reviewer listed on the merge request.

Dogfooding the Reviewers feature

On March 18th 2021, an updated process was put in place aimed at efficiently and consistently dogfooding the Reviewers feature.

Here is a summary of the changes, also reflected in this section above.

• Merge request authors and DRIs stay as Assignees
• Authors request a review from Reviewers when they are expected to review
• Reviewers remove themselves after they're done reviewing/approving
• The last approver stays as Reviewer upon merging

Best practices

Everyone

• Be kind.
• Accept that many programming decisions are opinions. Discuss tradeoffs, which you prefer, and reach a resolution quickly.
• Ask questions; don't make demands. ("What do you think about naming this :user_id?")
• Ask for clarification. ("I didn't understand. Can you clarify?")
• Avoid selective ownership of code. ("mine", "not mine", "yours")
• Avoid using terms that could be seen as referring to personal traits. ("dumb", "stupid"). Assume everyone is intelligent and well-meaning.
• Be explicit. Remember people don't always understand your intentions online.
• Be humble. ("I'm not sure - let's look it up.")
• Don't use hyperbole. ("always", "never", "endlessly", "nothing")
• Be careful about the use of sarcasm. Everything we do is public; what seems like good-natured ribbing to you and a long-time colleague might come off as mean and unwelcoming to a person new to the project.
• Consider one-on-one chats or video calls if there are too many "I didn't understand" or "Alternative solution:" comments. Post a follow-up comment summarizing one-on-one discussion.
• If you ask a question to a specific person, always start the comment by mentioning them; this ensures they see it if their notification level is set to "mentioned" and other people understand they don't have to respond.

Having your merge request reviewed

Please keep in mind that code review is a process that can take multiple iterations, and reviewers may spot things later that they may not have seen the first time.

• The first reviewer of your code is you. Before you perform that first push of your shiny new branch, read through the entire diff. Does it make sense? Did you include something unrelated to the overall purpose of the changes? Did you forget to remove any debugging code?
• Write a detailed description as outlined in the merge request guidelines. Some reviewers may not be familiar with the product feature or area of the codebase. Thorough descriptions help all reviewers understand your request and test effectively.
• If you know your change depends on another being merged first, note it in the description and set a merge request dependency.
• Be grateful for the reviewer's suggestions. ("Good call. I'll make that change.")
• Don't take it personally. The review is of the code, not of you.
• Explain why the code exists. ("It's like that because of these reasons. Would it be more clear if I rename this class/file/method/variable?")
• Extract unrelated changes and refactorings into future merge requests/issues.
• Seek to understand the reviewer's perspective.
• Try to respond to every comment.
• The merge request author resolves only the threads they have fully addressed. If there's an open reply, an open thread, a suggestion, a question, or anything else, the thread should be left to be resolved by the reviewer.
• It should not be assumed that all feedback requires their recommended changes to be incorporated into the MR before it is merged. It is a judgment call by the MR author and the reviewer as to if this is required, or if a follow-up issue should be created to address the feedback in the future after the MR in question is merged.
• Push commits based on earlier rounds of feedback as isolated commits to the branch. Do not squash until the branch is ready to merge. Reviewers should be able to read individual updates based on their earlier feedback.
• Request a new review from the reviewer once you are ready for another round of review. If you do not have the ability to request a review, @ mention the reviewer instead.

Requesting a review

When you are ready to have your merge request reviewed, you should request an initial review by selecting a reviewer from your group or team. However, you can also assign it to any reviewer. The list of reviewers can be found on Engineering projects page.

When a merge request has multiple areas for review, it is recommended you specify which area a reviewer should be reviewing, and at which stage (first or second). This will help team members who qualify as a reviewer for multiple areas to know which area they're being requested to review. For example, when a merge request has both backend and frontend concerns, you can mention the reviewer in this manner: @john_doe can you please review ~backend? or @jane_doe - could you please give this MR a ~frontend maintainer review?

You can also use workflow::ready for review label. That means that your merge request is ready to be reviewed and any reviewer can pick it. It is recommended to use that label only if there isn't time pressure and make sure the merge request is assigned to a reviewer.

When your merge request receives an approval from the first reviewer it can be passed to a maintainer. You should default to choosing a maintainer with domain expertise, and otherwise follow the Reviewer Roulette recommendation or use the label ready for merge.

Sometimes, a maintainer may not be available for review. They could be out of the office or at capacity. You can and should check the maintainer's availability in their profile. If the maintainer recommended by the roulette is not available, choose someone else from that list.

It is the responsibility of the author for the merge request to be reviewed. If it stays in the ready for review state too long it is recommended to request a review from a specific reviewer.

Volunteering to review

GitLab engineers who have capacity can regularly check the list of merge requests to review and add themselves as a reviewer for any merge request they want to review.

Reviewing a merge request

Understand why the change is necessary (fixes a bug, improves the user experience, refactors the existing code). Then:

• Try to be thorough in your reviews to reduce the number of iterations.
• Communicate which ideas you feel strongly about and those you don't.
• Identify ways to simplify the code while still solving the problem.
• Offer alternative implementations, but assume the author already considered them. ("What do you think about using a custom validator here?")
• Seek to understand the author's perspective.
• If you don't understand a piece of code, say so. There's a good chance someone else would be confused by it as well.
• Ensure the author is clear on what is required from them to address/resolve the suggestion.
  • Consider using the Conventional Comment format to convey your intent.
  • For non-mandatory suggestions, decorate with (non-blocking) so the author knows they can optionally resolve within the merge request or follow-up at a later stage.
  • There's a Chrome/Firefox add-on which you can use to apply Conventional Comment prefixes.
• Ensure there are no open dependencies. Check linked issues for blockers. Clarify with the authors if necessary. If blocked by one or more open MRs, set an MR dependency.
• After a round of line notes, it can be helpful to post a summary note such as "Looks good to me", or "Just a couple things to address."
• Let the author know if changes are required following your review.

WARNING: If the merge request is from a fork, also check the additional guidelines for community contributions.

Merging a merge request

Before taking the decision to merge:

• Set the milestone.
• Consider warnings and errors from danger bot, code quality, and other reports. Unless a strong case can be made for the violation, these should be resolved before merging. A comment must be posted if the MR is merged with any failed job.
• If the MR contains both Quality and non-Quality-related changes, the MR should be merged by the relevant maintainer for user-facing changes (backend, frontend, or database) after the Quality related changes are approved by a Software Engineer in Test.

If a merge request is fundamentally ready, but needs only trivial fixes (such as typos), consider demonstrating a bias for action by making those changes directly without going back to the author. You can do this by using the suggest changes feature to apply your own suggestions to the merge request. Note that:

• If the changes are not straightforward, please prefer allowing the author to make the change.
• Before applying suggestions, edit the merge request to make sure squash and merge is enabled, otherwise, the pipeline's Danger job fails.
  • If a merge request does not have squash and merge enabled, and it has more than one commit, then see the note below about rewriting commit history.

As a maintainer, if a merge request that you authored has received all required approvals, it is acceptable to show a bias for action and merge your own MR, if:

• The last maintainer to review intended to start the merge and did not, OR
• The last maintainer to review started the merge, but some trivial chore caused the pipeline to break. For example, the MR might need a rebase first because of unrelated pipeline issues, or some files might need to be regenerated (like gitlab.pot).
  • "Trivial" is a subjective measure but we expect project maintainers to exercise their judgement carefully and cautiously.

When ready to merge:

WARNING: If the merge request is from a fork, also check the additional guidelines for community contributions.

• Consider using the Squash and merge feature when the merge request has a lot of commits. When merging code, a maintainer should only use the squash feature if the author has already set this option, or if the merge request clearly contains a messy commit history, it will be more efficient to squash commits instead of circling back with the author about that. Otherwise, if the MR only has a few commits, we'll be respecting the author's setting by not squashing them.
• Start a new merge request pipeline with the Run pipeline button in the merge request's "Pipelines" tab, and enable "Merge When Pipeline Succeeds" (MWPS). Note that:
  • If the default branch is broken, do not merge the merge request except for very specific cases. For other cases, follow these handbook instructions.
  • If the latest pipeline was created before the merge request was approved, start a new pipeline to ensure that full RSpec suite has been run. You may skip this step only if the merge request does not contain any backend change.
  • If the latest merged results pipeline finished less than 2 hours ago, you may merge without starting a new pipeline as the merge request is close enough to main.
• When you set the MR to "Merge When Pipeline Succeeds", you should take over subsequent revisions for anything that would be spotted after that.
• For merge requests that have had Squash and merge set, the squashed commit's default commit message is taken from the merge request title. You're encouraged to select a commit with a more informative commit message before merging.

Thanks to merged results pipelines, authors no longer have to rebase their branch as frequently anymore (only when there are conflicts) because the Merge Results Pipeline already incorporate the latest changes from main. This results in faster review/merge cycles because maintainers don't have to ask for a final rebase: instead, they only have to start a MR pipeline and set MWPS. This step brings us very close to the actual Merge Trains feature by testing the Merge Results against the latest main at the time of the pipeline creation.

Community contributions

WARNING: Review all changes thoroughly for malicious code before starting a merged results pipeline.

When reviewing merge requests added by wider community contributors:

• Pay particular attention to new dependencies and dependency updates, such as Ruby gems and Node packages. While changes to files like Gemfile.lock or yarn.lock might appear trivial, they could lead to the fetching of malicious packages.
• Review links and images, especially in documentation MRs.
• When in doubt, ask someone from @gitlab-com/gl-security/appsec to review the merge request before manually starting any merge request pipeline.
• Only set the milestone when the merge request is likely to be included in the current milestone. This is to avoid confusion around when it'll be merged and avoid moving milestone too often when it's not yet ready.

If the MR source branch is more than 1,000 commits behind the target branch:

• Ask the author to rebase it, or consider taking a bias-for-action and rebasing it yourself if the MR has "Allows commits from members who can merge to the target branch" enabled.
• Reviewing MRs in the context of recent changes can help prevent hidden runtime conflicts and promote consistency. Depending on the nature of the change, you might also want to rebase if the MR is less than 1,000 commits behind.
• A forced push could throw off the contributor, so it's a good idea to communicate that you've performed a rebase, or check with the contributor first when they're actively working on the MR.
• The rebase can usually be done inside GitLab with the /rebase quick action.

Taking over a community merge request

When an MR needs further changes but the author is not responding for a long period of time, or is unable to finish the MR, GitLab can take it over in accordance with our Closing policy for issues and merge requests. A GitLab engineer (generally the merge request coach) will:

1. Add a comment to their MR saying you'll take it over to be able to get it merged.
2. Add the label ~"coach will finish" to their MR.
3. Create a new feature branch from the main branch.
4. Merge their branch into your new feature branch.
5. Open a new merge request to merge your feature branch into the main branch.
6. Link the community MR from your MR and label it as ~"Community contribution".
7. Make any necessary final adjustments and ping the contributor to give them the chance to review your changes, and to make them aware that their content is being merged into the main branch.
8. Make sure the content complies with all the merge request guidelines.
9. Follow the regular review process as we do for any merge request.

The right balance

One of the most difficult things during code review is finding the right balance in how deep the reviewer can interfere with the code created by a author.

• Learning how to find the right balance takes time; that is why we have reviewers that become maintainers after some time spent on reviewing merge requests.
• Finding bugs is important, but thinking about good design is important as well. Building abstractions and good design is what makes it possible to hide complexity and makes future changes easier.
• Enforcing and improving code style should be primarily done through automation instead of review comments.
• Asking the author to change the design sometimes means the complete rewrite of the contributed code. It's usually a good idea to ask another maintainer or reviewer before doing it, but have the courage to do it when you believe it is important.
• In the interest of Iteration, if your review suggestions are non-blocking changes, or personal preference (not a documented or agreed requirement), consider approving the merge request before passing it back to the author. This allows them to implement your suggestions if they agree, or allows them to pass it onto the maintainer for review straight away. This can help reduce our overall time-to-merge.
• There is a difference in doing things right and doing things right now. Ideally, we should do the former, but in the real world we need the latter as well. A good example is a security fix which should be released as soon as possible. Asking the author to do the major refactoring in the merge request that is an urgent fix should be avoided.
• Doing things well today is usually better than doing something perfectly tomorrow. Shipping a kludge today is usually worse than doing something well tomorrow. When you are not able to find the right balance, ask other people about their opinion.

GitLab-specific concerns

GitLab is used in a lot of places. Many users use our Omnibus packages, but some use the Docker images, some are installed from source, and there are other installation methods available. GitLab.com itself is a large Enterprise Edition instance. This has some implications:

1. Query changes should be tested to ensure that they don't result in worse performance at the scale of GitLab.com:
   1. Generating large quantities of data locally can help.
   2. Asking for query plans from GitLab.com is the most reliable way to validate these.
2. Database migrations must be:
   1. Reversible.
   2. Performant at the scale of GitLab.com - ask a maintainer to test the migration on the staging environment if you aren't sure.
   3. Categorized correctly:
      • Regular migrations run before the new code is running on the instance.
      • Post-deployment migrations run after the new code is deployed, when the instance is configured to do that.
      • Background migrations run in Sidekiq, and should only be done for migrations that would take an extreme amount of time at GitLab.com scale.
3. Sidekiq workers cannot change in a backwards-incompatible way:
   1. Sidekiq queues are not drained before a deploy happens, so there are workers in the queue from the previous version of GitLab.
   2. If you need to change a method signature, try to do so across two releases, and accept both the old and new arguments in the first of those.
   3. Similarly, if you need to remove a worker, stop it from being scheduled in one release, then remove it in the next. This allows existing jobs to execute.
   4. Don't forget, not every instance is upgraded to every intermediate version (some people may go from X.1.0 to X.10.0, or even try bigger upgrades!), so try to be liberal in accepting the old format if it is cheap to do so.
4. Cached values may persist across releases. If you are changing the type a cached value returns (say, from a string or nil to an array), change the cache key at the same time.
5. Settings should be added as a last resort. If you're adding a new setting in gitlab.yml:
   1. Try to avoid that, and add to ApplicationSetting instead.
   2. Ensure that it is also added to Omnibus.
6. File system access is not possible in a cloud-native architecture. Ensure that we support object storage for any file storage we need to perform. For more information, see the uploads documentation.

Review turnaround time

Because unblocking others is always a top priority, reviewers are expected to review merge requests in a timely manner, even when this may negatively impact their other tasks and priorities.

Doing so allows everyone involved in the merge request to iterate faster as the context is fresh in memory, and improves contributors' experience significantly.

Review-response SLO

To ensure swift feedback to ready-to-review code, we maintain a Review-response Service-level Objective (SLO). The SLO is defined as:

Review-response SLO = (time when first review is provided) - (time MR is assigned to reviewer) < 2 business days

If you don't think you can review a merge request in the Review-response SLO time frame, let the author know as soon as possible in the comments (no later than 36 hours after first receiving the review request) and try to help them find another reviewer or maintainer who is able to, so that they can be unblocked and get on with their work quickly. Remove yourself as a reviewer.

If you think you are at capacity and are unable to accept any more reviews until some have been completed, communicate this through your GitLab status by setting the 🔴 :red_circle: emoji and mentioning that you are at capacity in the status text. This guides contributors to pick a different reviewer, helping us to meet the SLO.

Of course, if you are out of office and have communicated this through your GitLab.com Status, authors are expected to realize this and find a different reviewer themselves.

When a merge request author has been blocked for longer than the Review-response SLO, they are free to remind the reviewer through Slack or add another reviewer.

Customer critical merge requests

A merge request may benefit from being considered a customer critical priority because there is a significant benefit to the business in doing so.

Properties of customer critical merge requests:

• The VP of Development (@clefelhocz1) is the DRI for deciding if a merge request qualifies as customer critical.
• The DRI applies the customer-critical-merge-request label to the merge request.
• It is required that the reviewers and maintainers involved with a customer critical merge request are engaged as soon as this decision is made.
• It is required to prioritize work for those involved on a customer critical merge request so that they have the time available necessary to focus on it.
• It is required to adhere to GitLab values and processes when working on customer critical merge requests, taking particular note of family and friends first/work second, definition of done, iteration, and release when it's ready.
• Customer critical merge requests are required to not reduce security, introduce data-loss risk, reduce availability, nor break existing functionality per the process for prioritizing technical decisions.
• On customer critical requests, it is recommended that those involved consider coordinating synchronously (Zoom, Slack) in addition to asynchronously (merge requests comments) if they believe this may reduce the elapsed time to merge even though this may sacrifice efficiency.
• After a customer critical merge request is merged, a retrospective must be completed with the intention of reducing the frequency of future customer critical merge requests.

Examples

How code reviews are conducted can surprise new contributors. Here are some examples of code reviews that should help to orient you as to what to expect.

"Modify DiffNote to reuse it for Designs": It contained everything from nitpicks around newlines to reasoning about what versions for designs are, how we should compare them if there was no previous version of a certain file (parent vs. blank sha vs empty tree).

"Support multi-line suggestions": The MR itself consists of a collaboration between FE and BE, and documenting comments from the author for the reviewer. There's some nitpicks, some questions for information, and towards the end, a security vulnerability.

"Allow multiple repositories per project": ZJ referred to the other projects (workhorse) this might impact, suggested some improvements for consistency. And James' comments helped us with overall code quality (using delegation, &. those types of things), and making the code more robust.

"Support multiple assignees for merge requests": A good example of collaboration on an MR touching multiple parts of the codebase. Nick pointed out interesting edge cases, James Lopez also joined in raising concerns on import/export feature.

Credits

Largely based on the thoughtbot code review guide.