Pull Request Process

Explains the process and best practices for submitting a pull request to the United Manufacturing Hub project and its associated sub-repositories. It should serve as a reference for all contributors, and be useful especially to new or infrequent submitters.

This doc explains the process and best practices for submitting a pull request to the United Manufacturing Hub project and its associated sub-repositories. It should serve as a reference for all contributors, and be useful especially to new and infrequent submitters.

Before You Submit a Pull Request

This guide is for contributors who already have a pull request to submit. If you’re looking for information on setting up your developer environment and creating code to contribute to United Manufacturing Hub, or you are a first-time contributor, see the Contributor Guide to get started.

Make sure your pull request adheres to our best practices. These include following project conventions, making small pull requests, and commenting thoroughly. Please read the more detailed section on Best Practices for Faster Reviews at the end of this doc.

The Pull Request Submit Process

Merging a pull request requires the following steps to be completed before the pull request will be merged.

Marking Unfinished Pull Requests

If you want to solicit reviews before the implementation of your pull request is complete, you should hold your pull request to ensure that a maintainer does not merge it prematurely.

There are three methods to achieve this:

  1. You may add the status: in-progress or status: on-hold labels
  2. You may add or remove a WIP or [WIP] prefix to your pull request title
  3. You may open your pull request in a draft state

While either method is acceptable, we recommend using the status: in-progress label.

How the e2e Tests Work

United Manufacturing Hub runs a set of end-to-end tests (e2e tests) on pull requests. You can find an overview of the tests in the CI documentation.

Why was my pull request closed?

Closed pull requests are easy to recreate, and little work is lost by closing a pull request that subsequently needs to be reopened. We want to limit the total number of pull requests in flight to:

  • Maintain a clean project
  • Remove old pull requests that would be difficult to rebase as the underlying code has changed over time
  • Encourage code velocity

Best Practices for Faster Reviews

Most of this section is not specific to United Manufacturing Hub, but it’s good to keep these best practices in mind when you’re making a pull request.

You’ve just had a brilliant idea on how to make United Manufacturing Hub better. Let’s call that idea Feature-X. Feature-X is not even that complicated. You have a pretty good idea of how to implement it. You jump in and implement it, fixing a bunch of stuff along the way. You send your pull request - this is awesome! And it sits. And sits. A week goes by and nobody reviews it. Finally, someone offers a few comments, which you fix up and wait for more review. And you wait. Another week or two go by. This is horrible.

Let’s talk about best practices so your pull request gets reviewed quickly.

Familiarize yourself with project conventions

Is the feature wanted? File a United Manufacturing Hub Enhancement Proposal

Are you sure Feature-X is something the UMH team wants or will accept? Is it implemented to fit with other changes in flight? Are you willing to bet a few days or weeks of work on it?

It’s better to get confirmation beforehand.

Even for small changes, it is often a good idea to gather feedback on an issue you filed, or even simply ask in UMH Discord channel to invite discussion and feedback from code owners.


Sometimes we need to remind each other of core tenets of software design - Keep It Simple, You Aren’t Gonna Need It, Minimum Viable Product, and so on. Adding a feature “because we might need it later” is antithetical to software that ships. Add the things you need NOW and (ideally) leave room for things you might need later - but don’t implement them now.

Smaller Is Better: Small Commits, Small Pull Requests

Small commits and small pull requests get reviewed faster and are more likely to be correct than big ones.

Attention is a scarce resource. If your pull request takes 60 minutes to review, the reviewer’s eye for detail is not as keen in the last 30 minutes as it was in the first. It might not get reviewed at all if it requires a large continuous block of time from the reviewer.

Breaking up commits

Break up your pull request into multiple commits, at logical break points.

Making a series of discrete commits is a powerful way to express the evolution of an idea or the different ideas that make up a single feature. Strive to group logically distinct ideas into separate commits.

For example, if you found that Feature-X needed some prefactoring to fit in, make a commit that JUST does that prefactoring. Then make a new commit for Feature-X.

Strike a balance with the number of commits. A pull request with 25 commits is still very cumbersome to review, so use your best judgment.

Breaking up Pull Requests

Or, going back to our prerefactoring example, you could also fork a new branch, do the prefactoring there and send a pull request for that. If you can extract whole ideas from your pull request and send those as pull requests of their own, you can avoid the painful problem of continually rebasing.

Multiple small pull requests are often better than multiple commits. Don’t worry about flooding us with pull requests. We’d rather have 100 small, obvious pull requests than 10 unreviewable monoliths.

We want every pull request to be useful on its own, so use your best judgment on what should be a pull request vs. a commit.

As a rule of thumb, if your pull request is directly related to Feature-X and nothing else, it should probably be part of the Feature-X pull request. If you can explain why you are doing seemingly no-op work (“it makes the Feature-X change easier, I promise”) we’ll probably be OK with it. If you can imagine someone finding value independently of Feature-X, try it as a pull request. (Do not link pull requests by # in a commit description, because GitHub creates lots of spam. Instead, reference other pull requests via the pull request your commit is in.)

Open a Different Pull Request for Fixes and Generic Features

Put changes that are unrelated to your feature into a different pull request

Often, as you are implementing Feature-X, you will find bad comments, poorly named functions, bad structure, weak type-safety, etc.

You absolutely should fix those things (or at least file issues, please) - but not in the same pull request as your feature. Otherwise, your diff will have way too many changes, and your reviewer won’t see the forest for the trees.

Look for opportunities to pull out generic features

For example, if you find yourself touching a lot of modules, think about the dependencies you are introducing between packages. Can some of what you’re doing be made more generic and moved up and out of the Feature-X package? Do you need to use a function or type from an otherwise unrelated package? If so, promote! We have places for hosting more generic code.

Likewise, if Feature-X is similar in form to Feature-W which was checked in last month, and you’re duplicating some tricky stuff from Feature-W, consider prerefactoring the core logic out and using it in both Feature-W and Feature-X. (Do that in its own commit or pull request, please.)

Comments Matter

In your code, if someone might not understand why you did something (or you won’t remember why later), comment it. Many code-review comments are about this exact issue.

If you think there’s something pretty obvious that we could follow up on, add a TODO.

Read up on GoDoc - follow those general rules for comments.


Nothing is more frustrating than starting a review, only to find that the tests are inadequate or absent. Very few pull requests can touch the code and NOT touch tests.

If you don’t know how to test Feature-X, please ask! We’ll be happy to help you design things for easy testing or to suggest appropriate test cases.


Your reviewer has finally sent you feedback on Feature-X.

Make the fixups, and don’t squash yet. Put them in a new commit, and re-push. That way your reviewer can look at the new commit on its own, which is much faster than starting over.

We might still ask you to clean up your commits at the very end for the sake of a more readable history, but don’t do this until asked: typically at the point where the pull request would otherwise be tagged LGTM.

Each commit should have a good title line (<70 characters) and include an additional description paragraph describing in more detail the change intended.

For more information, see squash commits.

General squashing guidelines

  • Sausage => squash

    Do squash when there are several commits to fix bugs in the original commit(s), address reviewer feedback, etc. Really we only want to see the end state, and commit message for the whole pull request.

  • Layers => don’t squash

    Don’t squash when there are independent changes layered to achieve a single goal. For instance, writing a code munger could be one commit, applying it could be another, and adding a precommit check could be a third. One could argue they should be separate pull requests, but there’s really no way to test/review the munger without seeing it applied, and there needs to be a precommit check to ensure the munged output doesn’t immediately get out of date.

Commit Message Guidelines

PR comments are not represented in the commit history. Commits and their commit messages are the “permanent record” of the changes being done in your PR and their commit messages should accurately describe both what and why it is being done.

Commit messages are comprised of two parts; the subject and the body.

The subject is the first line of the commit message and is often the only part that is needed for small or trivial changes. Those may be done as “one liners” with the git commit -m or the --message flag, but only if the what and especially why can be fully described in that few words.

The commit message body is the portion of text below the subject when you run git commit without the -m flag which will open the commit message for editing in your preferred editor. Typing a few further sentences of clarification is a useful investment in time both for your reviews and overall later project maintenance.

This is the commit message subject

Any text here is the commit message body
Some text
Some more text

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch example
# Changes to be committed:
#   ...

Use these guidelines below to help craft a well formatted commit message. These can be largely attributed to the previous work of Chris Beams, Tim Pope, Scott Chacon and Ben Straub.

Follow the conventional commit format

The conventional commit format is a lightweight convention on top of commit messages. It provides an easy set of rules for creating an explicit commit history; which makes it easier to write automated tools on top of.

The commit message should be structured as follows:

<type>[optional scope]: <description>

[optional body]

[optional footer(s)]

The type and description fields are mandatory, the scope field is optional. The body and footer are optional and can be used to provide additional context.

Find more information on the conventional commits website.

Try to keep the subject line to 50 characters or less; do not exceed 72 characters

The 50 character limit for the commit message subject line acts as a focus to keep the message summary as concise as possible. It should be just enough to describe what is being done.

The hard limit of 72 characters is to align with the max body size. When viewing the history of a repository with git log, git will pad the body text with additional blank spaces. Wrapping the width at 72 characters ensures the body text will be centered and easily viewable on an 80-column terminal.

Do not end the commit message subject with a period

This is primary intended to serve as a space saving measure, but also aids in driving the subject line to be as short and concise as possible.

Use imperative mood in your commit message subject

Imperative mood can be be thought of as a “giving a command”; it is a present-tense statement that explicitly describes what is being done.

Good Examples:

  • fix: x error in y
  • feat: add foo to bar
  • Revert commit “baz”
  • docs: update pull request guidelines

Bad Examples:

  • fix: Fixed x error in y
  • feat: Added foo to bar
  • Reverting bad commit “baz”
  • docs: Updating the pull request guidelines
  • Fixing more things

Add a single blank line before the commit message body

Git uses the blank line to determine which portion of the commit message is the subject and body. Text preceding the blank line is the subject, and text following is considered the body.

Wrap the commit message body at 72 characters

The default column width for git is 80 characters. Git will pad the text of the message body with an additional 4 spaces when viewing the git log. This would leave you with 76 available spaces for text, however the text would be “lop-sided”. To center the text for better viewing, the other side is artificially padded with the same amount of spaces, resulting in 72 usable characters per line. Think of them as the margins in a word doc.

Do not use GitHub keywords or (@)mentions within your commit message

GitHub Keywords

Using GitHub keywords followed by a #<issue number> reference within your commit message will automatically apply the do-not-merge/invalid-commit-message label to your PR preventing it from being merged.

GitHub keywords in a PR to close issues is considered a convenience item, but can have unexpected side-effects when used in a commit message; often closing something they shouldn’t.

Blocked Keywords:

  • close
  • closes
  • closed
  • fix
  • fixes
  • fixed
  • resolve
  • resolves
  • resolved

(@)mentions within the commit message will send a notification to that user, and will continually do so each time the PR is updated.

Use the commit message body to explain the what and why of the commit

Commits and their commit messages are the “permanent record” of the changes being done in your PR. Describing why something has changed and what effects it may have. You are providing context to both your reviewer and the next person that has to touch your code.

If something is resolving a bug, or is in response to a specific issue, you can link to it as a reference with the message body itself. These sorts of breadcrumbs become essential when tracking down future bugs or regressions and further help explain the “why” the commit was made.

Additional Resources:

It’s OK to Push Back

Sometimes reviewers make mistakes. It’s OK to push back on changes your reviewer requested. If you have a good reason for doing something a certain way, you are absolutely allowed to debate the merits of a requested change. Both the reviewer and reviewee should strive to discuss these issues in a polite and respectful manner.

You might be overruled, but you might also prevail. We’re pretty reasonable people.

Another phenomenon of open-source projects (where anyone can comment on any issue) is the dog-pile - your pull request gets so many comments from so many people it becomes hard to follow. In this situation, you can ask the primary reviewer (assignee) whether they want you to fork a new pull request to clear out all the comments. You don’t HAVE to fix every issue raised by every person who feels like commenting, but you should answer reasonable comments with an explanation.

Common Sense and Courtesy

No document can take the place of common sense and good taste. Use your best judgment, while you put a bit of thought into how your work can be made easier to review. If you do these things your pull requests will get merged with less friction.

Trivial Edits

Each incoming Pull Request needs to be reviewed, checked, and then merged.

While automation helps with this, each contribution also has an engineering cost. Therefore it is appreciated if you do NOT make trivial edits and fixes, but instead focus on giving the entire file a review.

If you find one grammatical or spelling error, it is likely there are more in that file, you can really make your Pull Request count by checking the formatting, checking for broken links, and fixing errors and then submitting all the fixes at once to that file.

Some questions to consider:

  • Can the file be improved further?
  • Does the trivial edit greatly improve the quality of the content?
Last modified July 2, 2024: Fix ENG-919 (16e32ef)