Friday, December 21, 2012

Webhook Call On Git Branch Change

Most modern build systems support webhook calls; simply put, your source control system does a command or makes a web service/url call to initiate a build, rather than the traditional SCM polling or timed/cron based build cycle from your build system.
The advantages of using hooks are that sometimes polling your SCM system is expensive in either time, bytes, or logging; or sometimes your build software doesn't support your SCM system directly and can't poll it. Or if you're trying to refine your continuous integration and you want to try to ensure each commit is processed separately rather than in batches, per your polling timer.
Usually the build system exposes this via a job-specific url; different SCM systems expose this in different ways. Git uses a "hooks" subfolder in your repo, here's fairly complete documentation on it:

On our main git server, we use post-receive text files to initiate builds for the projects that ask for webhooks vs cron or scm polling. Here's a typical post-receive text file, calling a Jenkins job:

/usr/bin/curl -u username:password

Fairly straightforward. The other day I was asked the other day to setup a webhook for branch commits in git. I did some googling around, stole some ideas and this is the result:

while read oldrev newrev refname
    branch=$(git rev-parse --symbolic --abbrev-ref $refname)
    if [ "master" == "$branch" ]; then
        /usr/bin/curl -u username:password
    if [ "feature" == "$branch" ]; then
        /usr/bin/curl -u username:password


This will watch for changes on "master" and a feature branch called "feature", and kick off different builds respectively.  You could also only look for master, or just for feature, or extend this considerably.
Prior to this I had only thought of the hook scripts as web calls, not as full-blown executable scripts with the ability to do logic. I had a harder time trying to figure out how to google the question than putting the pieces together once I started getting relevant results back.

-Kelly Schoenhofen

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