with Alberto Bacchelli

Distributed software development projects employ collaboration models and patterns to streamline the process of integrating incoming contributions. Classic forms of code contributions to collaborative projects include change sets sent to development mailing lists or issue tracking systems and direct access to the version control system. More recently however, a big portion of open source development happens on GitHub. One of the main reasons for this is the fact that contributing to a GitHub project is a relatively pain-free experience. Or is it?

In Apr 2014, we run a survey among contibutors (also: integrators) to Github projects trying to understand how they use pull requests and what issues they face while doing so. We got 760 responses, which after lots of preprocessing were reduced to 650.

What motivates contributors?

The main motivation for contributing to a project is its usage. This usage can be a dependency from another project the contributor is developing or fixing an end user bug. Altruistic motives (still) play a role: 33% of the respondents mentioned that they want to devote their time to a good cause. Developers also contribute for natural interest and personal development reasons, for example to sharpen their programming skills or for the intellectually stimulating fun of it. Finally, approximately 35% of the respondents related contributions to career development, raging from enriching their profile to attraching new customers.

R121: Making contributions to [project] makes it easier for me to get new clients.

What does the pull request process look like for contributors?

Initially, most (76%) contributors look up for open issues related to their changes or whether similar pull requests where submitted recently (59%). Half of them try to communicate their changes to the core team. Moreover, trying to be good citizens, they check for pull request guidelines in the project. Only few (32%) check for similar open PRs; this might explain the fact that we found in another study that many PRs are closed due to being duplicate.

After coding the change, contributors (81%) expect tests to run, while they try to honour the project guidelines (79%). Again, only very few check whether other pull requests where opened in the mean time (37%).

To communicate with the project team, contributors prefer tools that are tied to the GitHub process, namely issues and pull requests (>70%). Fewer than 50% will use email and even fewer means of synchronous communications such as IRC and Skype/Hangouts. It looks like that developers value asynchrony rather than immediacy at least when working with pull requests.

How do contributors assess the quality of their contributions?

While our question was specific on how contributors evaluate the quality, the analysis of the results also revealed what contributors examine in their PRs.

One of the top priorities for contributors when examining PR quality is compliance, which manifests as either compliance to the pull request guidelines or to de facto code formatting and system architecture patterns that they implicitly extract by studying the code. Code and commit quality are also other views of compliance: contributors try to deviate from the project norms as less as possible to increase their chances of acceptance.

It is interesting however that compliance is only checked manually; functionality on the other hand is tested by means of automated testing. Contributors expect to find a test suite (see also above) and run those tests. Very few contributors actually run automated static analysis tools, and the majority that do try to lint code (e.g. using PEP8). Contributors also mentioned that they rely on explicit, code reviews (self done or by asking peers for help).

What are the challenges of contributing with pull requests?

To find the pain-points emerging when contributing using the pull-based model, we asked a mandatory open question, in which we ask respondents to state the biggest challenge they experience when contributing PRs. We found three broad categories of challenges: code related, tool and model related and social ones (which are the most common!).

WRT code, the contributor's biggest challenge is to understand the code base. Contributors have trouble assessing the impact of a change and obtaining and maintaining awareness of what happens in the project while coding their change. Apparently, Github's pull request interface is not perfect in that respect, which is also what many contributors have been complaining about.

R564: (my biggest challenge is to) Read others code and get understanding of the project design.

WRT tools, contributors have trouble in two fronts: using git and especially doing (conflict resolution) in the face of multiple branches and, more importantly, creating the required setup to build a project and run its test suite (infrastructure setup).

Finally, many problems are social. The most prominent one is responsiveness: more than 15% of the survey participants find that getting a timely feedback, if any, for their pull requests is hard and they mostly mention people-related causes. Respondents specify that they would rather receive a clear reject than having no response for their PRs. If nothing else, prompt feedback reassures contributors that their effort is not in vain and helps them predict future communication patterns.

Communication is also hard over Github's pull request mechanism (see also above). It is therefore hard to explain rationale for a change. Typical issues prevalent in online forums are also prevalent in pull requests: politics, divergent opinions, bikeshedding and general rudeness have been reported by contributors, but admittedly not too often.

R526: (my biggest challenge is) Politics, or project owners not wanting a fix or change, or not actively maintaining it


After examining all aspects (projects, integrators and, now, contributors) of pull request work, I have the following to propose to make the pull request process more streamlined and pleasant for everyone involved.



  • Minimize friction: make contributions small and isolated. Restrict them to one subsystem. Adhere to code and programming style. Follow the contribution process.
  • Build your profile: Produce contributions that are accepted, engage in project community activities (e.g. discussions)

This blog post is a brief account of our findings. An in-depth analysis, including a description of our analysis methods and the original survey can be found in our technical report.

If you liked this post, you will also like our previous work on:


26 June 2015



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