In the world of software development, pull requests (PRs) are essential for ensuring code quality and enabling collaboration across teams. However, they can also become major bottlenecks, slowing down release cycles, causing merge conflicts, and adding friction to the development process.
To tackle this challenge, Microsoft, in collaboration with Delft University of Technology, conducted a comprehensive study, analyzing 22,875 pull requests to uncover the key factors that contribute to delays in PR completion. The findings, which are detailed in the paper “Nudge: Accelerating Overdue Pull Requests toward Completion”, led to the development of an automated system designed to send timely reminders—or "nudges"—to authors and reviewers. The results were groundbreaking: the Nudge system reduced PR completion times by 60%.
This article dives into the key insights from the research and explores how nudges can transform your code review process.
1. The Anatomy of Pull Request Bottlenecks
Microsoft’s study identified the top factors that contribute to prolonged PR completion times. After analyzing 28 features of PRs, four key issues emerged as major bottlenecks:
- Day of the week: PRs created later in the week, especially on Thursdays or Fridays, tend to linger over weekends and take longer to resolve.
- New contributors: Authors unfamiliar with the repository or its processes often have slower PRs due to the learning curve.
- Number of reviewers: While adding more reviewers can improve code quality, it frequently leads to more comments, discussions, and questions, which increases the time taken to approve a PR.
- Critical file changes: Modifications to significant project files (like package.json in JavaScript projects or .csproj in C#) usually indicate substantial updates, which require more attention and take longer to review.
By recognizing these factors, teams can better manage their PR workflows, either by limiting the number of reviewers or assigning experienced contributors to critical PRs.
2. The Impact of Overdue Pull Requests
When PRs stay open for too long, they create a range of problems that affect the entire development cycle. Microsoft and Delft University’s research highlights the following risks of overdue PRs:
- Communication breakdowns: The longer a PR remains unresolved, the less visibility the rest of the team has into what changes are in progress. Quick merges allow code to be reused and discussed more actively.
- Outdated assumptions: In large teams, PRs that stay open for too long can leave developers working with outdated code, increasing the risk of bugs and regressions.
- Merge conflicts: Long-lived branches are significantly more likely to encounter difficult merge conflicts. In fact, Klissiomara Dias’s research into over 70,000 merge conflicts revealed that longer PRs are a major factor in causing these issues.
- Delayed releases: Slow PRs delay new features, bug fixes, and other important updates, frustrating both internal teams and external users.
The lesson here is clear: faster pull request resolution leads to smoother workflows, fewer problems, and quicker value delivery to your users.
3. Microsoft’s Nudge System: A Game-Changer for Pull Requests
To address these issues, Microsoft developed the Nudge system, an automated bot designed to send reminders to the relevant actors—whether they are the PR author or the reviewer—whenever action is overdue. This system was deployed to over 8,000 repositories and sent 210,000 nudges in just one year, achieving a 60% reduction in PR completion times.
Here’s how the Nudge system works:
Prediction Model
The Nudge system uses a machine learning model that predicts the time required to complete a PR based on two years of historical data. It considers 28 features, including the top four factors that slow down PRs and the four that help speed them up, to ensure accurate predictions.
Activity Detection
This module continuously monitors whether the PR is seeing any activity from the author or reviewer. It detects changes in the PR’s state (e.g., whether it’s approved or rejected), comments from reviewers or the author, and any new commits that are pushed to the branch. If no activity is detected within 24 hours, the system sends a nudge to the relevant party.
Actor Identification
The key to the Nudge system is identifying the "change blocker"—whether it’s the author or the reviewer—and sending targeted notifications. For example, the system will notify the author when:
- The PR has been approved and is ready to merge.
- There are unresolved review comments that need attention.
- The author has pending action items requested by the reviewer.
Conversely, the system notifies the reviewer when:
- The review hasn’t started.
- The author has addressed all comments, and the changes need to be verified.
This targeted approach ensures that PRs don’t stagnate due to miscommunication or inactivity.
4. The Results: Faster Code Reviews, Happier Teams
The results of implementing the Nudge system were remarkable. Microsoft reduced PR completion times by 60%, cutting the average time from 197.2 hours to just 77.65 hours. The system’s predictive capabilities and activity detection modules ensured that the right person was reminded at the right time, preventing PRs from falling into the backlog.
Additionally, Microsoft achieved a 71% satisfaction rate from developers regarding the usefulness of the notifications. By helping teams prioritize PRs and reduce delays, the Nudge system has improved productivity, sped up development cycles, and reduced the frustration associated with overdue pull requests.
5. How You Can Apply These Insights to Your Team
Whether you’re a small team or managing a large-scale codebase, the insights from Microsoft’s study can help you optimize your code review process:
- Prioritize critical PRs: Assign experienced developers to review critical project file changes and bug fix PRs, which are typically resolved faster.
- Limit reviewer count: Avoid adding too many reviewers, as this can slow down the process. Keep the number of reviewers to a manageable level to reduce back-and-forth communication.
- Automate reminders: Implement a simple nudge system to send reminders to authors and reviewers when PRs are overdue. Automation can drastically reduce review times and keep the team focused on what’s important.
- Encourage quick merges: Remind your team to merge frequently to avoid long-lived branches and reduce the likelihood of merge conflicts.
Conclusion: The Future of Code Reviews
Microsoft’s Nudge system provides a blueprint for how automation, combined with data-driven insights, can transform the way teams handle code reviews. By identifying bottlenecks, predicting PR completion times, and sending timely reminders to the right people, the Nudge system has proven that a little nudge can go a long way in accelerating pull requests.
Incorporating these principles into your team’s workflow could be the key to faster merges, fewer merge conflicts, and ultimately, happier developers and users. The future of efficient code reviews is here—and it starts with a nudge.
References:
- Nudge: Accelerating Overdue Pull Requests toward Completion (Link)
