mention-bot

Do you have a GitHub project that is too big for people to subscribe to all the notifications? The mention bot will automatically mention potential reviewers on pull requests. It helps getting faster turnaround on pull requests by involving the right people early on.

How To Use?

Go to your project on GitHub > Settings > Webhooks & services > Add Webhook
Payload URL: https://mention-bot.herokuapp.com/
Let me select individual events > Check Pull Request
Add Webhook

And you are done. Next time a pull request is opened, you should see the bot add a comment ;)

How Does It Work?

Every time there's a new pull request, GitHub wakes up the mention bot using Webhooks.

Once awakened, the bot will download the diff of the pull request and figure out which files and lines have been touched.

For these, it will download the associated blame to figure out who last touched that line last as they may be a good reviewer.

After running the algorithm described in the next section, it will comment on the pull request notifying those people and go back to sleep.

Algorithm

The problem of finding who the best reviewers are is really hard and I don't think that any algorithm will achieve perfection. Instead, what we want here is to be best effort. We want to notify people that are likely going to be interested and be good reviewers. If we ping a few too many people that's not the end of the world neither if we don't ping the exact right person.

We use two heuristics:

If a line was deleted or modified, the person that last touched that line is likely going to care about this pull request.
If a person last touched many lines in the file where the change was made, s/he will want to be notified.

Initialization

Create two empty hash map:

DeletedLines for authors of deleted lines.
AllLines for authors of lines in the changed files.

Filling the data structures

for each deleted line, find the author in the blame and increase its count by one in the DeletedLines map.
for each line in every file that was changed, find the author in the blame and increase its count by one in the AllLines map.

Since getting the blame information is sending an http request to GitHub it is pretty expensive. We first sort the files by number of deleted lines and only pick the top 5. Since we're only looking for 3 names and the algorithm is best effort, this greatly speeds up the algorithm in case of large pull requests for little loss in precision.

Putting it all together

delete from AllLines all the names that appear in both maps. We don't want to mention the same person twice.
sort each map by count
concat DeletedLines with AllLines
take the first three names

How To Contribute or Run Your Own Bot?

If you want to use a different account for the bot, change the message or extend it with more functionalities, we've tried to make it super easy:

git clone https://github.com/facebook/mention-bot.git
cd mention-bot
npm install
npm start
# Follow the instructions there

License