Continuous Code Review

The high bar today

Continuous Code Review is where the team commits to processing proposed commits (to trunk) from teammates' trunk speedily.

The idea is that a system (the code portal probably) allows developers to package up commits for code review and get that in front of peers quickly. And that peer developers make a commitment to do code reviews objectively and quickly.

There is a cost to multi-tasking, so maybe someone in the dev team who is between work items at that moment should focus on the review before they start new work. With a continuous review ethos, it is critical that code reviews are not allowed to back up.

Companies doing Extreme Programming, often allow that pair of eyes to count as a review. Some companies require multiple reviews of code. For “the pair as reviewers too” scenario, one might have been enough and that commit will land in the trunk, without others looking at it. Five minutes and 20 seconds into Guido van Rossum’s famous 2006 Mondrian presentation, he states “code review is a best alternative to pair programming”, and that it is “basically asynchronous pair-programming”.

Pull Requests (PRs)

The pull-request (PR) model introduced by GitHub is the dominant code review model today. The concept was available from GitHub’s launch in 2008 and has revolutionized both open source and enterprise software development. Google were secretly doing the same thing with custom tooling around their Perforce install from about 2005, and Guido’s presentation on Mondrian in 2006 (as mentioned) leaked that to the world (see below).

A PR is one or more commits towards a goal described in an accompanying piece of text. The act of creating the PR from the branch signals the end (or a pause) in work, and the wish for the reviewers to get busy (and the CI daemon to wake up and build/verify the branch). There are caveats though.

Open Source contributions via PRs

These can come from anyone who has an account on GitHub (or equivalent). They will have forked your repository and the PR will be about commits that would come back to your repository. They may delete their repository after you consume their commits. If these are unsolicited you may well take your time reviewing them. Indeed you may never consume them, if you don’t like them. Hardly continuous, but open source is mostly a volunteer activity.

PRs from colleagues

Regardless of branching model, the wish is for the PR to be reviewed fairly quickly. On GitHub (and possibly others) the PR can come from a fork or a branch in the main repo. There is little difference to the processing of these. In Trunk-Based Development teams, the PR should be on a short-lived feature branch and processed very quickly by reviews towards merging back to trunk/main. A few minutes for the review is best, and tens of minutes acceptable. More than a hour or two, and you are negatively affecting cycle times.

The short-lived feature branch may have received many commits before the developer initiated the pull request. Some developers will squash (rebase) the changes into a single commit before starting code review. Some teams have a policy in favor of or against squash/rebase.

Common Code Owners

Commits being reviewed are never rejected for “Only I am allowed to change source in this package” reasons. Rejections must be for objective and published reasons.

Enterprise code review - as it was

In enterprises, if code review was done at all prior to 2008, it was done in a batch, and probably a group activity. It was often abhorred as it gave a lead developer/architect a moment to set an agenda, round on a large portion of the attendees and make sure that their own code flubs were not discussed at all.

Historically, open source teams never had the luxury of procrastinating about code review. They either did code reviews as they went (perhaps days were the review cadence, not hours or minutes), or they did not bother at all.

See also

See Game Changers - Google’s Mondrian and Game Changers - GitHub’s Pull Requests for the industry impact of continuous code review.

References elsewhere

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