The power of Jenkins

I first was exposed to Jenkins while working for an open source software project out of my university that wanted to use continuous integration as a part of their workflow. While services like Travis and Semaphore can be excellent if you can accomplish your build within the constraints of the systems they provide, they could not meet the needs of that project. Enter Jenkins, “the leading open source automation server”.

Jenkins is an powerful tool, and much of this power comes from the fact that it is very customizable. Configuring jobs via the web UI that Jenkins provides leaves many things to be desired, however. First, the task is fraught with human error. It is very easy to forget to expand that one nested menu and set a critical value. Small mistakes like this can lead to faulty jobs that lead to misleading results or a broken CI/CD pipeline causing a headache for all involved. Secondly, once you have constructed your house of cards, you have only to experience a hard drive failure to lose all your work. Even more likely, if something changes in your set up you may have to hand update dozens of jobs so carefully configured with hours of mouse clicks.

Jenkins Job Builder (JJB)

To scale the process of Jenkins job definition it becomes obvious that one must be able to generate Jenkins jobs based on templates. This allows the use of variables so that you might test multiple versions against multiple OSes, or vary other conditions while allowing yourself to share common steps and code among jobs. Moreover, these assets should be under version control and stored in some location other than the machine Jenkins is running on. The good news is, this tool already exists!

Jenkins Job Builder, installable via pip allows the user to generate the job definitions from YAML files and upload the XML job definitions to your Jenkins instance.

PulpQE stores the YAML files and scripts used to generate the pulp jenkins jobs with jenkins-job-builder in the pulp_packaging repo.

While working on changes to these files, it is desirable to be able to try them out locally, so I decided to give it a shot!

I have Docker set up on my Fedora 25 machine, so I spun up a Jenkins instance to test it out. There is an official Jenkins image on DockerHub so I pulled it down and served it up on my localhost. (More detailed guides for setting up jenkins from this docker image are available on the Jenkins DockerHub page)

docker pull jenkins
docker run -p 8080:8080 -p 50000:50000 jenkins

I then visited the Jenkins web UI at http://localhost:8080 and was directed to login with an admin account created during the install. After changing the admin password, I chose to “download recommended plugins”.

JJB documentation guided me to create a virtual environment to download and run jenkins job builder in.

cd $HOME
python3 -m venv jjb-env
source ~/jjb-env/bin/activate
pip install jenkins-job-builder

Then, I needed to set up a configuration file for jenkins-job-builder.

cat >> $HOME/jjb.ini << 'EOF'
> [jenkins]
> user=myadminaccount
> password=myterribleplaintextpassword
> url=http://localhost:8080
> query_plugins_info=False
>
> [job_builder]
> include_path=ci/jobs/scripts
> EOF

With those pieces in place, I cloned the pulp_packaging repo and tried to upload a job to my local Jenkins instance.

cd $HOME
git clone https://github.com/pulp/pulp_packaging
cd pulp_packaging
jenkins-jobs --conf $HOME/jjb.ini --ignore-cache update ci/jobs pulp-installer

Happily, I was greeted with the messages:

INFO:root:Updating jobs in ['ci/jobs'] (['pulp-installer'])
... output omitted ...
INFO:jenkins_jobs.builder:Number of jobs generated:  1
INFO:jenkins_jobs.builder:Creating jenkins job pulp-installer
INFO:root:Number of jobs updated: 1

And lo and behold, there was a new job that popped up in the Jenkins dashboard! Now I am ready to work on creating my own Jenkins jobs and getting them fine tuned before shipping them off to our real Jenkins server.

Limitations

While Jenkins Job Builder does a lot to remedy concerns around being able to migrate jobs from one instance of Jenkins to another without data loss or excessive manual configuration, it does not manage all aspects of a complete Jenkins set up. One should not, for example, manage credential migration from one Jenkins to another with JJB. When credentials are stored in Jenkins, they can then be referenced in JJB YAML files via their ID. If you set up a new Jenkins instance you will have to put your credentials in place on the Jenkins manually and then update the ID references in your JJB job definitions. If you fail to update SSH key information or other data stored on Jenkins and mapped to JJB via its ID, and your jobs are broken because of this incorrect data, JJB has no way of knowing about this or informing you of the error.

Furthermore, this may be a case where in a local install of Jenkins you would have to modify this data for testing purposes, and then switch it back to the correct data for your official Jenkins instance.