It's always been clear to developers that a project's source code and how to build that source code are inextricably linked. After all, we've been including
Makefiles (and, more recently, declarative build specifications like
pom.xml for Maven and
stack.yaml for Haskell Stack) with our source code since time immemorial (well, 1976).
What has been less clear is that the build process and environment are also important, especially when using Continuous Integration. First-generation CI systems such as Jenkins CI and Bamboo have you use a web-based UI in order to set up the build variables, jobs, stages, and triggers. You set up an agent/slave running on a machine that has tools, system libraries, and other prerequisites installed (or just run the builds on the same machine as the CI system). When a new version of your software needs a change to its build pipeline, you log into the web UI and make that change. If you need a newer version of a system library or tool for your build, you SSH into the build agent and make the changes on the command line (or maybe you use a configuration management tool such as Chef or Ansible to help).
New generation CI systems such as Travis CI, AppVeyor, and Gitlab's integrated CI instead have you put as much of this information as possible in a file inside the repository (e.g. named
.gitlab-ci.yml). With the advent of convenient cloud VMs and containerization using Docker, each build job can specify a full OS image with the required prerequisites, and the services it needs to have running. Jobs are fully isolated from each other. When a new version of your software needs a change to its build pipeline, that change is made right alongside the source code. If you need a newer version of a system library or tool for your build, you just push a new Docker image and instruct the build to use it.
Tracking the build pipeline and environment alongside the source code rather than through a web UI has a number advantages:
If a new version of your software needs a different build pipeline, or needs a newer system tool installed in order to build, that change is right alongside the new version of the source code.
You can make changes to the build pipeline and environment in a feature branch, test them there without risk of interfering with other branches, and then a simple merge applies those changes to the master branch. No need to carefully coordinate merge timing with making manual changes to the build pipeline and environment.
Building old versions of your project "just works" without needing to worry that the changes you made to support a newer version break building the old version, since the old version will build using the pipeline and environment that was in place at the time it was tagged.
Developers can manage the build pipeline and environment themselves, rather than having to ask a CI administrator to make changes for them.
There is no need to manage what is installed on build agent machines except for the bare minimum to support the agent itself, since the vast majority of build requirements are included in an image instead of directly on the machine.
Developers can more easily build and test locally in an environment that is identical that used by CI by reusing images and build scripts (although "inlined" scripts in a
.travis.yml are not usable directly on a developer workstation, so the CI metadata should reference scripts stored elsewhere in the repo). However, for thorough testing (rather than rapid iteration) in a real environment, developers will often prefer to just push to a branch and let the CI system take care of building and deploying instead (especially using review apps).
A history of changes to the CI configuration are retained, and it's easy to revert bad changes. Web based UIs may keep an audit log, but this is harder to deal with than a Git commit history of a text file.
You can include comments with the CI configuration, which web based UIs usually don't have room for aside from perhaps a "description" textarea which is nowhere near the actual bit of configuration that the comment applies to.
Making changes to machine-readable text files is less error prone than clicking around a web UI.
It's easier to move your project to a different CI server, since most of the CI configuration does not need to be re-created on the new server since it is in the code repo. With a web UI, you end up spending a lot of time clicking around trying to make the new server's job configuration look the same as the old server's, copying-and-pasting various bits of text, and it's easy to miss something.
There are also potential pitfalls to be aware of:
You need to consider whether any aspects of the build pipeline may change "externally" and should therefore not be inside the repo itself. For example, you might decide to migrate your Docker images to a different registry, in which case having the docker registry's hostname hard-coded in your repo would make it complicated to build older versions. It is best to using your CI system's facility to pass variables to a build for any such external values.
Similarly, credentials should never be hard coded in your repo, and should always be passed in as variables from your CI system.
Of course, nothing described here is entirely new. You could be judicious about having the a first generation CI system only make very simple call-outs to scripts in your repo, and those scripts could use VMs or
chrooted environments themselves. In fact, these have been long considered best practices. Jenkins has plug-ins to integrate with any VM or containerization environment you can think of, as well as plugins to support in-repo pipelines. The difference is that the newer generation of CI systems make this way of operating the default rather than something you have to do extra work to achieve (albeit with a loss of some of the flexibility of the first generation tools).
CI has always been a important part of the FP Complete development and DevOps arsenal, and these principles are at the core of our approach regardless of which CI system is being used. We have considerable experience converting existing CI pipelines to these principles in both first-generation and newer generation CI systems, and we offer consulting and training.