While exact definitions of Continuous Integration vary, the idea
is that your software project is automatically built and tested
very regularly, often many times per day. In practice, this usually
means every time a developer pushes their code to a central
repository, the CI process is performed. Ideally, this is performed
on every branch, even those that are works in progress.
Advantages of continuous integration include:
- Regressions are caught quickly, so less time is spent tracking
them down since you know which change introduced the bug.
- There is always a working "latest version", rather than finding
out near release time that it turns out there are problems.
- Prevents non-working code from reaching the "mainline."
- Notifies developers immediately when code doesn't work.
- Automates deployment of your application so you always have a
running test server with the latest code, or even deploy straight
- Encourages developers to frequently share their work in
- Encourages developers to write automated tests.
Source code management
We manage our code in Git
repositories, although other source code management systems will
work as well.
Centralized source code
Developers must regularly "push" their code from their
workstations to a centralized repository. These can be managed
solutions, or privately hosted within a company's network,
depending on specific project needs. We most commonly use Github, Gitlab, and Atlassian Bitbucket.
The CI server "watches" for changes to the centralized
repository and automatically executes scripts for each change, and
provides notifications and reports based on whether those scripts
succeed or fail. We regularly use:
- Travis CI: tightly
integrated with Github, although it is a separate product operated
by a different company
- Gitlab CI: built
directly into Gitlab for a totally seamless experience when using
that tool for code repositories and issue management
- Jenkins CI: one of the older
CI servers, with a vast array of plugins that allow it to work with
in conceivable environment, although tight integration can be
tricky to configure
Bamboo: part of the Atlassian suite, and tightly integrated
with their other tools like Bitbucket and JIRA but also usable
Features of CI server
While every CI server is different, they offer a similar array
of features (some integrated directly, others via plugins).
The CI process can automatically deploy your project for
testing. This is always a good idea for a test environment, and
also used for automatically deploying to production from a
Gitlab CI also supports "review apps", which will deploy a copy
of the app for every branch, which is especially helpful for code
reviews since the reviewer can work with a "live" version of the
application built from the code that is under review, without first
needing to build their own copy.
Builds can each be run in their own throw-away Docker
containers. This means the build environments (e.g. operating
system, compilers, system libraries) are defined by Docker images,
and multiple projects with different requirements can be built on
the same machine and in parallel without interfering with each
Parallel and distributed
CI jobs are performed by "agents" (also known as "slaves" or
"runners"), and multiple agents can be connected to a single CI
server or project. This allows setting up a cluster of machines to
perform run builds.
Different projects or build stages within projects can be
configured to require certain attributes in the agent where it
runs. For example, a multi-platform project might have agents on
Windows, Linux, and macOS, and run the build and tests on all
Notification integrates with your existing communications tools,
such as e-mail and corporate chat services like Slack. Developers
are notified of build failures, successful deployments, and many
Various reports are generated that provide a snapshot of your
projects' health, so you can see at a glance whether there are
increased rates of regressions, and you can see exactly which
version of code is running where.
The CI server manages secret values such as deployment
credentials, so that the builds have access to them but they aren't
revealed otherwise. Access to the credentials can be restricted so
that only some build stages can access them, which lets you prevent
"untrusted" stages from having access to the credentials.
CI systems let you specify build "artifacts" which are saved by
a build stage and can then be accessed by later stages or outside
processes. For example, you might have a "build" stage that builds
an executable and saves it as an artifact, followed by "deploy"
stage that retrieves the artifact and deploys the executable to a
web server. These artifacts are saved so that you can go back to an
old build and retrieve its the file(s).
One disadvantage of building in an ephemeral container is that
build artifacts (e.g. intermediate files like object code) are lost
between builds, which would result in unnecessarily long build
times as the same code is built over and over. CI systems support
specifying certain paths and files to cache between builds, so that
those files will be restored in their original locations in the
There is some overlap between caching and artifacts, but caching
is meant for intermediate files to speed up subsequent builds,
while artifacts are meant to save the final results of a build
Continuous integration is a tool that can be wielded in many
ways. These are some of the principles we follow to make the best
use of it.
in the CI configuration
It can be tempting to use every feature and plugin of a CI
system to manage builds, but this is usually counterproductive. In
general, let the CI system handle the "where and when" of building,
but use your own scripts within the repository for the "how". This
lets developers use the same scripts locally and makes it easier to
switch to a different CI system in the future should that be
desirable. It also means as much of the build process as possible
is versioned along with the code which makes it easier to build
Write automated tests
While a CI system is still useful without automated tests, it
really shines when an excellent suite of unit and integration tests
is in place. You get quick feedback as soon as tests fail, and this
kind of feedback encourages developers to write more tests. This
also avoids code that fails tests from reaching the mainline.
Make builds and tests fast
If it takes too long to get feedback, it discourages regular use
of the CI system. Features like caching, artifacts, and distributed
builds mean you can avoid repeating unnecessary work and speed up
builds. For larger and more complex projects this can be difficult,
and sometimes it makes sense to split out more thorough integration
tests into a separate nightly process so that the main tests return
Commit, merge, and
push code regularly
Developers are encouraged to create feature branches for work in
progress and push to them regularly. This gives feedback from the
CI system regularly, and also makes collaboration easier.
Merging from the "mainline" branch to feature branches should be
frequent to avoid complex conflict resolutions. CI systems can also
be configured to run a feature branches build as though "mainline"
was merged so that developers know as soon as potential conflicts
and test failures arise, without having to perform the merge
Employing continuous integration makes your development team
more productive and your release process less stressful. FP
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