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How to Become a Patch Guru?

When you are working on implementing a new feature or adding documentation to an already existing one it is easy to get carried away by the work itself and forget about the unwritten rules of constructing your changes.

This section will guide you through how to create patches that people will want to review.

Allows you to:

  • Know how to structure a patch that makes it easier to maintain throughout the review process

  • Know how to structure a patch that is easier for community members to review

The Right Size

Reviewing large patches is very inconvenient and time consuming therefore we always suggest to break down your changes into smaller blocks.

While there is no magic number try to keep the size of your changes as small as possible, but under a few hundreds of lines changed total. Patches that are test heavy with little code change require as much effort as code heavy changes.

In rare occasions when there is no good logical breakdown for a change and your patch can grow to a thousand lines or more. In some cases it is acceptable as you cannot extract the related test changes to another patch for instance, but it’s not highly recommended.

Always try to extract as much as you can into other patches, like documentation or logical parts of the functionality that do not depend on common functions in a lower layer.

Longer patches require more time to review; wherever you can, keep the length reasonable. And where you can’t, you can help the reviewers by adding code comments and writing a detailed commit message to describe the changes you introduced in your patch.

Patch Chains, Depends-On Tag and Gerrit Topics

In the case of complex feature implementation work when you need to introduce changes to multiple modules of the same project or multiple projects you need to be very careful with managing the dependencies.

There are multiple options to choose from depending on the structure of your design. You can either organize the changes in patch chains or you can use the “Depends-On” tag.

Depends-On Tag

When you have changes in multiple project repositories you can mark dependent patches with the “Depends-On” tag. The tag will appear as a link in the commit message which helps you and also the reviewers to track and navigate between the dependencies of your changes.

The “Depends-On” tag is a marker on your changes and when used a patch cannot be merged until all its dependencies are landed.

The tag can be applied to patches proposed for the same repository as well. In that case the changes are separate enough to be kept independent which means that if you need to fix changes from review comments you can do it on a per patch basis. It is also true for rebasing each patch.


In case you use the “Depends-On” tag you need to download all the changes for a feature implementation or documentation change to test the feature or build the documentation with all the changes applied. Git will not take care of handling the dependencies automatically in this case.

Patch Chains

In some cases when you break down the required changes to smaller blocks you cannot avoid having direct dependencies between them that prevents you from having independent changes. You need to organize your changes in a chain to maintain the dependencies which requires some additional care when you work with these changes.

Even if you have a chain of patches you still need to keep code changes and related tests in one patch as you cannot guarantee that both land in time for a release.

When you have a chain Gerrit helps you by listing all the commit titles in the “Related Changes” column on the top right corner of the Gerrit UI. The titles are also links which help you navigate between the changes to review them when you upload a new version.

How to Handle Chains?

A patch chain is easy to handle if you keep in mind a few recommendations:

  • Always have a local branch for these changes to ensure that you don’t mix it together with changes related to another feature or bug fix.

  • Always handle a chain as one block of changes by rebasing the whole chain and keep it up to date when you modify a patch to fix review comments or add changes to it.

  • To modify a patch within a chain you will need to use interactive rebase:

git rebase -i HEAD^

You need as many “^” as the number of the patch you want to edit first from the top of the chain. Alternatively you may wish to use git-restack, which figures out the appropriate git rebase command for you.

Gerrit also provides you options to edit the patch itself or only the commit message and a few more for more advanced changes, like modifying the author.

  • To download the full chain you need to download the top patch and Git will automatically download all the dependent patches in the chain.

  • If you only need to modify the top patch in the chain that can be done the same way as you update individual patches.

  • When you upload changes in a chain only the patches that got updated will be uploaded. This also means that the review scores on lower patches in the chain will be untouched.

  • Always check the changes you made to each patch and be careful that you applied the changes in the right one as patches still get merged individually and there is no guarantee that the whole chain gets landed at the same time.

For a more in-depth look at managing patch chains, see Tutorial: Developing Changes In A Series.

Gerrit Topics

You have the option to specify a topic for your changes when you upload them for review. While Gerrit topics will not add dependency to your patches you can apply a search based on the topic that will show you all the relevant changes in all the projects where there are patches with the same topic. Gerrit will also show them to you in the “Same Topic” column on the top right corner of the page of a review.

This is a good way to help tracking related changes, let that be a feature implementation, bugfix or documentation work.

How to Structure Your Changes?

When your work item grows above a particular size and you need to upload multiple patches it is crucial to structure it well in case of both patch chains and independent changes.

It is a good practice to group changes by modules in a project, for instance virt driver changes, compute manager and api changes in case of OpenStack Compute.

By grouping the changes per module you can also construct the chain or dependencies by the hierarchy of the components and always keep the API changes last as that will enable the new functionality and that change will depend on everything else you needed to touch for your design.

Beyond this you can also look into the functionality to find smaller building blocks and make your changes smaller. For instance changes to an object can be implemented first that you will use later when you implement new API functionality.

Structural split of Changes

The cardinal rule for creating good commits is to ensure there is only one «logical change» per commit. There are many reasons why this is an important rule:

  • The smaller the amount of code being changed, the quicker and easier it is to review and identify potential flaws.

  • If a change is found to be flawed later, it may be necessary to revert the broken commit. This is much easier to do if there are not other unrelated code changes entangled with the original commit.

  • When troubleshooting problems using Git’s bisect capability, small well defined changes will aid in isolating exactly where the code problem was introduced.

  • When browsing history using git annotate or git blame, small well defined changes also aid in isolating exactly where and why a piece of code came from.

The Right Content

Changes that are not related to any feature implementation or bug report can be uploaded but are less welcomed by reviewers.

Improvement to either the code or documentation should be part of a larger effort, like if you would like to fix typos in documentation then you should do it for a larger block, like a whole guide. It is also preferred to report a story with tasks for a work item like this that can be tracked later.

It is similar for code improvements. As the community is large and world-wide we have coding guidelines, but the style of each individual can still be very different. We don’t enforce a particular coding style, therefore changes related to fix that are less welcomed and are sources of very opinionated arguments that should be avoided.

In case of code refactoring work which makes the code more readable and easier to maintain by restructuring methods and deleting unused code snippets it is highly encouraged to consult with the project team and report a story in StoryBoard first and then upload the relevant changes to Gerrit for review.