Jens Rantil's blog

Posted Ons 04 September 2013

GIT mirroring using Jenkins

For the past couple of weeks I've been annoyed by the high latency when connecting to Github over SSH. While the other GIT transports could be an option, I prefer to use keys to authenticate against Github.

This got me thinking about setting up a local GIT mirror at our office. To keep my project future proof, I planned to make this a cron-job-ish project; every so often it would checkout all repositories for a certain organization.

Then it struck me - I'm already running a Jenkins installation that pulls commits from Github regularly. Why not reuse those repositories? So far my GIT mirror setup has been working great - and has also made us more resilient to Github downtime. Here's how I've done it:

Serving the Jenkins checkouts

First you need to figure out where Jenkins has its working folder. In general this is stored in $HOME/.jenkins. Since I am running Jenkins as the user "jenkins" on my system, my Jenkins working directory is /home/jenkins/.jenkins.

Next, you need to start the GIT daemon that serves your GIT fetches. There are many ways to run this command, I'm using supervisord. Here's the command I've been using:

$ git daemon --reuseaddr --verbose --base-path=/home/jenkins/.jenkins/jobs --export-all

Note that the daemon will not do any authentication! Reads will be allowed by anyone. Writes, however, will be blocked. This means someone can pull changes from the Jenkins host, but not push any.

Configuring a GIT client

Configuring your GIT command line client is a two step process:

First, add a GIT remote pointing to your Jenkins server:

$ cd <your-local-git-clone>
$ git remote add --no-tags jenkins git://YOUR-JENKINS-SERVER/JOB-NAME/workspace/.git

JOB-NAME is the name you've given your job in Jenkins.

Secondly, you need to make a modification to <your-local-git-clone>/.git/config. Replace the following line:

fetch = +refs/heads/*:refs/remotes/jenkins/*

with:

fetch = +refs/remotes/origin/*:refs/remotes/jenkins/*

. What this change will do, is to ignore all the local branches on your Jenkins's host and instead instead only use your Jenkins's remote branches.

When you've done the above you can:

$ git fetch jenkins

If everything is succesful, you should now have a bunch of jenkins/* branches in your local repository.

Additional comments

  • If you choose to not include --no-tags (in git remote add) git fetch will fetch a tag for every unique build that Jenkins has build historically. If you'd like to checkout a specific build, you might want to drop --no-tags. I just felt I was bloating my local tags...
  • Only the branches specified to be fetched by Jenkins will be mirrored. If you would like to add more branches you do that under "Branches to build" at http://YOUR-JENKINS-SERVER/job/JOB-NAME/configure
Category: misc
Tags: git jenkins hudson
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