Contributing to the NDN Codebase

Getting Started

The NDN team is very glad you are interested in contributing! The NDN (Named Data Networking) codebase is composed of many projects, with multiple universities across the world contributing. This documentation will help you understand how we work and what we expect from contributors.

Code of Conduct

NDN codebase development adheres to the Contributor Covenant, located in CODE_OF_CONDUCT.md. By contributing, you are expected to also adhere to this code. If you feel someone has breached this code of conduct, please email us.

What can I do?

The NDN codebase continually needs help with, among other things:

Implementing new features
Writing fixes for bugs
Writing documentation for features that have recently changed or that change quickly. Sometimes people forget to update documentation!
Code review

Finding Documentation

The NDN team maintains a community (Redmine site) for issue tracking and documentation hosting. Redmine is the hub for design activity. In particular, all design discussion and decisions will either occur or be copied there.

Why did they do it this way? When writing code, if you ever have questions about why some design decision was made, the best approach is to use git blame on a file to inspect the last commit for some line of code. In this view there will be a commit hash in hex (e.g. 0ab12e3a) in the first column. Using our example, git show 0ab12e3a would show the commit made for that line. In the commit view there should be a reference number (e.g. refs: #1234). Using this number, you can find a pertaining issue on the Redmine (e.g., https://redmine.named-data.net/issues/1234).

Additionally, in some cases there are published papers you can read in order to gain a better understanding of the project. Searching for these papers is not difficult; in many cases you can find a pertaining paper or technical report listed on the main NDN website. Usually the title of a project will be a good keyword.

If you cannot find an answer to your question, the best place to go is the mailing lists. There are multiple different lists for different interests, so be sure you are mailing the right list for a quick response.

Tracking work on Redmine

As mentioned above, Redmine is the organizational hub of the NDN projects. As such, extensive use of it is made, and learning how it works will help you. In particular, there is a workflow associated with Redmine. Generally it is:

An issue is reported by someone. At this point they should describe the issue, including relationships to other issues. Unless they know what they're doing, they should leave particulars blank. Such particulars are priority, target version, assignees, and categories.
Design discussion about the issue occurs, and the issue is either accepted or rejected.
Someone either is assigned to, or assigns themselves to work on the issue, setting the status to "In progress." Note: You must be a "Developer" of the project before you are able to toggle the Status field. A user should contact a project manager to be granted Developer privilege. Project managers can be found on "Overview" page of the project.
After every useful chunk of progress, notes are made on the issue and the progress percentage is updated to reflect this. Remember that notes are actually Markdown, so it is advised to use the 'preview' button to ensure your note looks right.
The changes are uploaded to Gerrit for review. Once all changes pertaining to an issue are on Gerrit, an issues status should change to "Code review."
Once all changes are cleared for merging, they either can be merged, or held for feedback, at which point the issue status should become "Feedback."
After all changes have been merged, the issue should be set to "Closed."

Redmine provides other facilities for managing your work on NDN projects, too, such as time logging and time estimation.

Contributing Guidelines

Repositories

All NDN projects are hosted on GitHub, at various organizations, using git for version control:

If you are unfamiliar with git, some kind of tutorial on git should be your first step.

There is also a Git game you can play. Finally, there is a wiki page on NFD's Redmine that has more useful links.

There are a lot of different projects, so take some time to look through them for ones that pertain to your interests. NDN projects use Gerrit for code review purposes.

Occasionally there will be other repositories used privately to test changes, but this is uncommon. In general, you will not need to refer to these repositories.

Style

Most NDN projects are written in C++, and there is a style guide. In general the NDN style is similar to the GNU style, but there are some significant changes. This style guide is not exhaustive, and in all cases not covered by the guide you should emulate the current style of the project.

There is a partial styleguide for Python. It applies many of the provisions from the C++ guide, in addition to some other Python-specific things.

Some NDN projects are writtin in JavaScript, and there is a style guide for that language, too.

Commit Messages

Commit messages are very important, as they are usually the first thing (besides the changelog file) a developer looks at when trying to understand the timeline of a project. Commit messages should:

Have a short, descriptive first line, starting with the module they change. A good rule of thumb is a maximum length of 65 characters.
If including a body, leave a blank line between the first line and the rest, and ensure that it's no longer than 72 characters.
Include Redmine issue numbers. The exact syntax is given below.
Be written in an imperative mood. E.g. "Make foo a bar" and not "Foo is now a bar"
Use the present tense. E.g. "Make foo a bar" and not "Made foo a bar"

To explain, the anatomy of a typical commit message is like this:

docs: write contributing guide and code of conduct

refs: #3898
Change-Id: Ife89360305027dba9020f0b298793c1121ae1fd6c

Explaining this:

docs is the module that the commit affects. We want this because it lets someone know at a glance what part of the project it changes. For some projects, there will be only one module or only very small other modules. This practice should be observed in those cases, too.
#3898 is the Redmine issue number. Gerrit transforms these into clickable links, and it is useful to reviewers to gain background understanding of the issue. You can have multiple by separating them with commas.
Change-Id should be filled automatically. It is used by Gerrit to track changes.

Unit Tests

With a few exceptions, every patch needs to have unit tests that accompany them. For C++, we use the Boost unit test framework to help us out. Note that this link points to the newest version of the Boost Test library documentation, and you may need to refer to older documentation if you are using an older version of Boost. More information on this is available in the NFD Dev Guide

When designing and writing tests, a few things need to be kept in mind:

Unit tests are design tools, not debug tools. Just because your code passes some unit tests does not mean it is bug-free. Unit tests are tools to convince you that your code does what you think it does.
It can be difficult to know when you should test something. If you find that you are having a hard time designing a test for something, ask yourself whether it is because it doesn't make sense to test what you've just written, or if it's because the way you designed it makes it difficult to test. Consider a second look at your design if you think it's the second one.
The method that gave the world unit tests says that unit tests should be written before the code they test. This is a contentious issue, so writing them either before or after is acceptable. If writing them later, keep the tests in mind when writing the code, and it will help design good interfaces.
At the very least, write the test for a completed module/unit of code before moving on. Doing this will ensure that the test gets written, and will help you think about the interface that you need to examine. If you don't, you may forget exactly what each piece is responsible for when you're looking at the whole system afterward.
Separate your I/O, as it is very hard to test, so I/O should be isolated if possible. Consider something like separating a function that invokes I/O into two functions: one that does the I/O and calls the other one, which takes that I/O result. This will make it easier to test that second function than if they were combined.

Writing unit tests using the Boost framework is quite simple, and you can refer to existing unit tests for examples. (This is a good example.)

Building and running unit tests

This is mentioned in greater depth in the developer's readme, but the basic procedure is:

In the base directory of the project, run ./waf distclean. This removes all prior build files.
Run ./waf configure --with-tests. This configures the next build to also build the tests.
Run ./waf. This will build the tests.
Then, the unit tests will be in the build directory, and will be named unit-tests-<module name, e.g. unit-tests-nlsr or unit-tests-rib. To run just one test suite, run ./unit-tests-exampleModule -t ExampleTestSuite. To run a specific test in a suite, use ./unit-tests-exampleModule -t ExampleTestSuite/ExampleTestCase.

Gerrit: Uploading Patches and Code Review

As mentioned above, NDN projects use Gerrit for code review. This is a web-hosted, open code review platform that allows for interactive code review, rebasing, and cross-linking with the Redmine, and a developer interacts with it using the familiar git. For issues that require a tracker reference, a particularly useful feature is that the refs: #... becomes a clickable link to the issue for design discussion.

First-time Setup

The first-time Gerrit setup goes like this:

Log in to Gerrit. You can authenticate using many different methods, including GitHub OAuth. You need to ensure that you have a Gerrit username:
1. Log in.
2. Click on your name in the top right corner and click "Settings" in the pop-up box.
3. In the Username box, check for a name. If there's already one there, great.
4. If not, type one in, and this will be used in later steps.
Set up your Gerrit credentials. This will depend on how you configured your Gerrit remote in step 1. Among other things, you need to set up identities so that the email on your Gerrit profile matches whatever email you will be committing with on your git repo. Note: We only support using SSH access to Gerrit.
This shows what the identities panel looks like. If you do not see the email here that you have configured git to use, you cannot upload to Gerrit.
In that case, add it under contact information panel. .
Gerrit itself has extensive documentation regarding error messages, and this identity-based one is by far the most common. This is the documentation for an identity error.
Clone the source for a project from Gerrit.
1. While logged in to Gerrit, there will be a link in the top-left area, Projects.
2. From this project view, click on the Clone with commit-msg hook tab.
3. Click on the ssh tab to select cloning over SSH. Note: This will require setting up SSH access to Gerrit first.
4. Copy-and-paste the git clone ... command into your terminal to fetch the project source.
After this, your remotes will be set up correctly, and pushing to Gerrit will only require git push origin HEAD:refs/for/master after committing work. This section has some git tips and tricks to type less.

Uploading patches

Most patches should have a corresponding Redmine issue that they can reference. If you search the Redmine and notice there is no relevant issue for a patch you are writing, please create an issue first. You will need a Redmine account, which can be created there.

After writing some changes, commit them locally as normal. After saving and exiting your editor, the commit hook will insert a unique Change-Id to the message.

Once you have a commit message you are happy with, simply run git push HEAD:refs/for/master.

Note: Gerrit separates commits into patch sets by the unique Change-Ids. As a result, it is important that you either:

Squash your various commits into one with git rebase -i <initial commit>, ensuring that the ultimate Change-Id in the commit is the one on the patch set on Gerrit. This workflow is generally preferred.
Amend your commit with any new changes using git commit --amend.

If you do not do this, what will happen is that each commit will be interpreted by Gerrit as a separate patch set. This is probably not what you want.

Code Review

Dealing with Code Review

It is important to remember that code review is about improving the quality of code contributed, and nothing else. Further, code review is highly important, as every line of code that's committed comes with a burden of maintenance. Code review helps minimize the burden of that maintenance. Consider that when you are receiving comments, those comments are influenced by two things:

How the reviewer personally feels about the change ("Would I be happy to see this code in a year?" or "Would I be happy if I wrote this?") and
How the reviewer feels the change abides by style and usage requirements ("I may not personally mind, but we have to do it this way.")

Remembering these things when reading comments helps to separate what can feel like needless negativity.

Doing Code Review and Writing Comments

Code review is extremely important! We need every bit of code review you can give. In many cases this is the bottleneck for new contributors who do not fully understand how certain language constructs should be used, what best practices are, etc. In these cases it is important to give constructive feedback.

Writing comments is somewhat counter-intuitive on Gerrit. If you are signed in through a modern browser, you can leave a comment in a file either by clicking on the line number, by selecting some text you want to comment and pressing 'c', or by clicking on someone else's comment. After typing, click the Save button there. After navigating through the patch set with the arrows at the top-right of the Gerrit UI, you then must click the up-arrow to get back to the Change screen. At this point your comments have not been made yet! You must then click the Reply... button, and assign a score. If you correctly saved your comments, they will be shown at the bottom of that box. Once you click post, the comments will be made public to others.

Minimally, a review must include:

A score (usually -1, 0, or +1)
An "itemized" commentary on each objection you have, or a justification for a whole-change objection.

Optimally, a review should include:

A useful explanation of why you object to some item. This is more important than it first appears. Consider that although you may have been writing code since before you could count, not everyone has. Some things may not be obvious to others, and sharing your knowledge is key to making an open-source project work.
Comments about good design decisions. These help motivate developers when otherwise a review is entirely negative comments.

Expediting the Code Review Process

If you observe that code review takes a long time, there are a few things that you can do to to expedite the process:

Think about language best practices.
Ask yourself how you would feel about this code, if you saw it "in the wild."
When making code decisions, ask yourself "why shouldn't I do it this way?"

Responding to Code Review

There are a few things to remember when responding to code review, including:

Don't click "Done" on a comment if you agree with a comment and are updating the code accordingly. These comments are essentially useless and only clutter the discussion. Gerrit has a convenient mechanism to check the differences between two patch sets, so reviewers are expected to check that their comments have been addressed.
Don't blindly follow what is suggested. Review is just that: a review. Sometimes what is suggested is functionally equivalent to what you have implemented. Other times, the reviewer has not considered something when writing their review, and their suggestion will not work. Lastly, if you disagree with a suggestion for some other reason, feel free to object, but you may be asked to provide an explanation.
How to reply. To reply to a comment, click on the comment in the Gerrit interface, type your reply, and then press save. After replying, you must remember to go to the main patch set change screen, and press Reply.... Your comments will be shown as drafts, and you must click Post to make them visible to others.

Jenkins

As a supplement to the code review process, every patch set is automatically compiled and tested on multiple platforms using our instance of Jenkins, the continuous integration system. Interacting with Jenkins is not usually necessary, as Jenkins automatically picks up new patch sets and posts the results. Typically the only interaction needed with Jenkins is when some kind of glitch occurs and a build needs to be retriggered.

It is expected that code is checked locally. Reviewers may wait until Jenkins checks the code before doing a more functional review of the code. Since Jenkins checks can take a while, you can save some time by checking yourself first.

Gerrit Change-Id Commit Hook

If you encounter an error like this when trying to push work:

    remote: ERROR: [4311462] missing Change-Id in commit message footer
    remote:
    remote: Hint: To automatically insert Change-Id, install the hook:
    remote:   gitdir=$(git rev-parse --git-dir); scp -p -P 29418 someuser@gerrit.named-data.net:hooks/commit-msg ${gitdir}/hooks/
    remote: And then amend the commit:
    remote:   git commit --amend

Then your commit hook has not been set up correctly, somehow. Follow the instructions in the terminal to resolve this.

Code Style Robot

As part of CI, there is a script that checks every patch uploaded to Gerrit for code style consistency. Checking these kinds of things is notoriously difficult, and while the robot is very good in general, it will occasionally report false positives and miss things. So all issues it raises should be inspected manually to ensure they really are style problems. Further, since this automated check is not perfect, you should take steps (e.g., configure your editor or run a linter before you commit) to ensure you adhere to code style rules.

Typing Less to Upload Patches to Gerrit

If typing HEAD:refs/for/master feels repetitive, you have a few options:

You can configure git to do the work for you, as described in this blog post
Git has an alias system that you can use to specify certain commands. For example, you can use git config --global alias.pg "push origin HEAD:refs/for/master" to make git pg push to Gerrit. You can learn about git aliases.
If using Linux or macOS, it is relatively simple to create a shell alias: alias gs="git status" for example, or alias gitpush="git push origin HEAD:refs/for/master. The alias itself must be one word, but it can represent multiple words. To persist your aliases, they need to go in your environment file, which is generally at ~/.bashrc or ~/.zshrc or ~/.profile. The exact syntax of aliases and the environment variables file will vary by shell. You should be careful not to overuse aliases, however.