Abstract
Collaborative software development platforms (such as GitHub and GitLab) have become increasingly popular as they have attracted thousands of external contributors to contribute to open source projects. The external contributors may submit their contributions via pull requests, which must be reviewed before being integrated into the central repository. During the review process, reviewers provide feedback to contributors, conduct tests and request further modifications before finally accepting or rejecting the contributions. The role of reviewers is key to maintain the effective review process of the project. However, the number of decisions that reviewers can make is far superseded by the increasing number of pull requests submissions. To help reviewers to perform more decisions on pull requests within their limited working time, we propose a learning-to-rank (LtR) approach to recommend pull requests that can be quickly reviewed by reviewers. Different from a binary model for predicting the decisions of pull requests, our ranking approach complements the existing list of pull requests based on their likelihood of being quickly merged or rejected. We use 18 metrics to build LtR models and we use six different LtR algorithms, such as ListNet, RankNet, MART and random forest. We conduct empirical studies on 74 Java projects to compare the performances of the six LtR algorithms. We compare the best performing algorithm against two baselines obtained from previous research regarding pull requests prioritization: the first-in-and-first-out (FIFO) baseline and the small-size-first baseline. We then conduct a survey with GitHub reviewers to understand the perception of code reviewers regarding the usefulness of our approach. We observe that: (1) The random forest LtR algorithm outperforms other five well adapted LtR algorithms to rank quickly merged pull requests. (2) The random forest LtR algorithm performs better than both the FIFO and the small-size-first baselines, which means our LtR approach can help reviewers make more decisions and improve their productivity. (3) The contributor's social connections and contributor's experience are the most influential metrics to rank pull requests that can be quickly merged. (4) The GitHub reviewers that participated in our survey acknowledge that our approach complements existing prioritization baselines to help them to prioritize and to review more pull requests.