Continuous Integration for Python

After beavering away at some ideas for Knowsis over the last 3 weeks and admittedly not really doing it test first , I spent this weekend finally getting round to setting up a CI server and some builds to run the pitiful number of tests that I have actually written to try and make me write more. It’s been bugging me all along but as the only developer at the moment it’s not been at the top of the priority list. However, my previous experience of setting this kind of thing up for legacy projects tells me that if I don’t get round to it soon, it will be infinitely more painful in the long run.

At 7digital we used Teamcity as  our CI build server, but knowing how much their build agent licensing can cost I thought I would look at the open source alternatives seeing as we’re bootstrapping. After some research a nailed it down to the either BuildBot or Jenkins (formerly Hudson) and digging a bit deeper it seems that people with experience of both would suggest using Jenkins first until you realise it can’t do something that you really need BB for as it can be quite painful to get set up; Jenkins on the other hand is very simple to get set up.

Jenkins CI Dashboard

Installing Jenkins

One of my favourite things about using Ubuntu, coming from a Windows background, is the ease of installing things using apt-get. These instructions are taken from the Jenkins site:

wget -q -O - http://pkg.jenkins-ci.org/debian/jenkins-ci.org.key | sudo apt-key add -
sudo sh -c 'echo deb http://pkg.jenkins-ci.org/debian binary/ > /etc/apt/sources.list.d/jenkins.list'
sudo aptitude update
sudo aptitude install jenkins

Jenkins gets installed and set up to run as a daemon at startup under a newly created user Jenkins. It’s now usable at http://127.0.0.1:8080, however I wanted to be able to skip the need for a port number so set up nginx to proxy requests for me. This isn’t a necessary step so I won’t go into it here but there are some simple guides available on line if you haven’t used nginx before (just remember to restart nginx after you change the config, it’ll save you hours of head scratching!).

Setting up Jenkins to work with git

This step isn’t necessary if you don’t use git, but i’ll go into it as I do and it took me a bit of figuring out, plus there wasn’t a huge amount of info out there on how to do it.

From the home screen of Jenkins go to the plugins section:

Manage Jenkins -> Manage Plugins

In the Available tab find the “Jenkins GIT plugin” and check the install checkbox. If you use github you can also install the “GitHub plugin” which creates a link from your project page to your github repository and also allows you to use GitHub’s post receive hooks to notuify Jenkins when code has been committed (not necessary as you can use polling to check for changes). Your Jenkins instance will need to be exposed publicly for this to work, so make sure you set up user authentication properly; there’s also a plugin to allow you to use your GitHub logins as authentication if you want to use that.

Once you have selected the required plugins click ‘Download now and install after restart’ which will install the plugins and restart Jenkins, should take no more than a minute to complete.

Create SSH keys for Jenkins

You now need to set up your ssh keys for the Jenkins user. Open up a terminal window and switch to the Jenkins user

sudo -su jenkins

You can run through the creation of your public private key pairs as normal which will be created in the Jenkins user home directory (/var/lib/jenkins). If you want a guide for this, I have always found the one on the GitHub site help pages to be the easiest to follow.

Now set up a user with your git repository for your build slaves to run as and copy the contents of the public key to it. You can use your own account if you wish but I would recommend using a separate one.

Creating Jobs

Creating a Job in Jenkins is really simple. From the Jenkins dasboard click “New Job”. Enter the name of your job and select “Build a freestyle software project“. Click Ok.

For now you can ignore the options at the top of the next screen, head  down to the source control section.

Source Code Management

If you installed the git plugin you should see git as an option here.  Select the option that’s relevant and point it to the location of your repository.

Build Triggers

Further down is the “Build Triggers” section, you should select the option ‘Poll SCM‘ option, this will then present you with a schedule box that will allow you to enter the frequency to poll your SCM, it uses the cron format. A few examples

Every minute:

* * * * *

Every 10 minutes:

*/10 * * * *

Every hour :

@hourly

At 15 mins past every hour:

15 * * * *

Build Steps

So far the job will just checkout when there are any changes to your code, so now you need to make the job actually do something interesting. You can set up one of more build steps to run for your unit tests, tp deploy your code to a test environment, run your system tests, deploy your code to live etc.

At this point i’ll just run the unit tests. If you have written some tests using unitest syntax you can use the nose test runner (nosetests) to automatically discover and run these tests. You can also get it to output the test results in Junit report format so that Jenkins can display your test results. You will need to make sure that nose is installed on your build server and slaves for this to work.

Select “Excute Shell” in the “Add Build Step” dropdown and add the following line:

nosetests --with-xunit

The shell script will run from the top level of your project (known as the workspace root) so if nose cannot auto discover your tests because they are buried in a folder tree structure you can always add a cd command to switch to that directory first. The  –with-xunit switch will output an xml report in the Junit format called nosetests.xml into the folder under which nosetests ran.

In the Jenkins set up there is a “Post-build Actions” section under which you should select “Publish Junit test report results” and enter nosetests.xml

If you use some other test format or want to use another test runner enter the shell command that would execute those tests remembering to install anything required onto your build server as well.

Now Go And Write Some Tests

That’s it, you now have a build set up to run your tests every time you check in changes, so there is no excuse not to write any. This is only a start and there a plenty of other things you might want to set up like failure notifications, test reports, dashboards etc so the best thing to do is explore the Jenkins site.