Continuous integration (CI) automatically builds your code, runs tests on a variety of different platforms, and deploys all manner of builds and documentation as desired. Typically, CI is run when new code is proposed (e.g. through GitHub Pull Requests) or committed to the repository. CI is useful for catching bugs before they reach your end users, and for running tests automatically, including on platforms that are not available to every developer.
CI can be broken down into several stages. Most CI should at least build the code and then run unit tests. The build stage takes the source code and performs compilation and dependency resolution/installation for the next stage. Compiled languages like C++ and Rust require this step to turn the source code into executables. Interpreted languages like Python or R do not usually need this step, as their code is not compiled, but they still typically need to install dependencies. The unit test stage runs a series of tests to ensure that the code is working as expected without syntactical or logical errors. Most, if not all, codes should have unit tests. Some codes where reproducibility of results is highly sought for (especially in the scientific field) should also include a regression test stage, in which the results of the code are compared against previously computed values. Regression tests can take significantly longer than unit tests and may need to be relegated to very infrequent CI runs, or handled through separate means. Lastly, a deploy stage can take the compiled and verified code and push it to an appropriate branch or service to make it available. Deployment can also include documentation pages, API’s, and experimental/nightly builds.
GitHub itself now provides a CI service for its repositories called “GitHub Actions” which can be configured to run with most repositories. However, there are also many other CI services, most of which have webhooks for integration with GitHub. There are also CI services for non-GitHub based code repositories.
Examples of CI Software/Services