Handling multiple branches in continuous integration

Question

I've been dealing with the problem of scaling CI at my company and at the same time trying to figure out which approach to take when it comes to CI and multiple branches. There is a similar question at stackoverflow, Multiple feature branches and continuous integration. I've started a new one because I'd like to get more of discussion and provide some analysis in the question.

So far I've found that there are 2 main approaches that I can take (or maybe some others???).

• Multiple set of jobs (talking about Jenkins/Hudson here) per branch
  • Write tooling to manage the extra jobs
    • Create/modify/delete Jobs in bulk
    • Custom settings for each job per branch (SCM url, dep management repos duplications)
    • Some examples of people tackling this problem with shell tools, ant scripts and Jenkins CLI. See:
      1. http://jenkins.361315.n4.nabble.com/Multiple-branches-best-practice-td2306578.html
      2. http://jenkins.361315.n4.nabble.com/Is-it-possible-to-handle-multiple-branches-where-some-jobs-should-run-on-each-one-without-duplicatin-td954729.html
      3. http://jenkins.361315.n4.nabble.com/Parallel-development-with-branches-td1013013.html
      4. Configure or Create hudson job automatically
  • Will cause more load on your CI cluster
  • Feedback cycle for devs slows down (if the infrastructure cannot handle the new load)
• Multiple set of jobs per 2 branches (dev & stable)
  • Manage the two sets manually (if you change the conf of a job then be sure to change in the other branch)
    • PITA but at least so few to manage
  • Other extra branches won't get a full test suite before they get pushed to dev
  • Unsatisfied devs. Why should a dev care about CI scaling problems. He has a simple request, when I branch I would like to test my code. Simple.

So it seems if I want to provide devs with CI for their own custom branches I need special tooling for Jenkins (API or shellscripts or something?) and handle scaling. Or I can tell them to merge more often to DEV and live without CI on custom branches. Which one would you take or are there other options?

Answer 1

When you talk about scaling CI you're really talking about scaling the use of your CI server to handle all your feature branches along with your mainline. Initially this looks like a good approach as the developers in a branch get all the advantages of the automated testing that the CI jobs include. However, you run into problems managing the CI server jobs (like you have discovered) and more importantly, you aren't really doing CI. Yes, you are using a CI server, but you aren't continuously integrating the code from all of your developers.

Performing real CI means that all of your developers are committing regularly to the mainline. Easy to say, but the hard part is doing it without breaking your application. I highly recommend you look at Continuous Delivery, especially the Keeping Your Application Releasable section in Chapter 13: Managing Components and Dependencies. The main points are:

• Hide new functionality until it's finished (A.K.A Feature Toggles).
• Make all changes incrementally as a series of small changes, each of which is releasable.
• Use branch by abstraction to make large-scale changes to the codebase.
• Use components to decouple parts of your application that change at different rates.

They are pretty self explanatory except branch by abstraction. This is just a fancy term for:

1. Create an abstraction over the part of the system that you need to change.
2. Refactor the rest of the system to use the abstraction layer.
3. Create a new implementation, which is not part of the production code path until complete.
4. Update your abstraction layer to delegate to your new implementation.
5. Remove the old implementation.
6. Remove the abstraction layer if it is no longer appropriate.

The following paragraph from the Branches, Streams, and Continuous Integration section in Chapter 14: Advanced Version Control summarises the impacts.

The incremental approach certainly requires more discipline and care - and indeed more creativity - than creating a branch and diving gung-ho into re-architecting and developing new functionality. But it significantly reduces the risk of your changes breaking the application, and will save your and your team a great deal of time merging, fixing breakages, and getting your application into a deployable state.

It takes quite a mind shift to give up feature branches and you will always get resistance. In my experience this resistance is based on developers not feeling safe committing code the the mainline and this is a reasonable concern. This in turn usually stems from a lack of knowledge, confidence or experience with the techniques listed above and possibly with the lack of confidence with your automated tests. The former can be solved with training and developer support. The latter is a far more difficult problem to deal with, however branching doesn't provide any extra real safety, it just defers the problem until the developers feel confident enough with their code.