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What is continuous integration?

Continuous integration (CI) is the practice of running an automated pipeline of scripts to build and test a project after every change.

This allows maintainers to identify bugs early in the development cycle, ensuring that all code that is pushed into the main development branch is compliant with the requirements of the project.

Continuous deployment

Continuous deployment (CD) takes this another step further by automating the process of deploying an application to production after every change.

If configured correctly (this is the default), CI pipelines will run for every merge request, meaning the modified code can be build and tested before changes are accepted into the repository.

The fork-branch-merge-request presented earlier can then be augmented to include CI at all stages

GitLab workflow with CI

How to enable CI on GitLab

All Gitlab CI/CD is configured with a YAML-format file called .gitlab-ci.yml in the root of the project repository.


Running a simple script

A basic example could just check that your script runs without failing. Consider the example from

Simple Gitlab-CI configuration

image: python

    - python

Here, a single job called test is configured to run python If that command exits with code 0, the job passes, otherwise it fails.

image: python

We use image: python to declare to GitLab-CI that this job should run inside a python container.

The default is image: docker:latest.

An example of this pipeline running is here:

Building a (python) package

A more complicated example is available on the package branch of the same gitlab-example repository:

Here the full YAML configuration is:

Building and testing a Python package

  - build
  - test

image: python

  stage: build
    - python sdist --dist-dir .
      - "hello-world-*.tar.*"

.test: &test
  stage: test
    - build
    - python -m pip install hello-world-*.tar.*
    - hello-world

  extends: .test
  image: python:3.9

  extends: .test
  image: python:3.10

Here we define a more complicated pipeline with two stages:

graph TD; build-->py39["test:3.9"]; build-->py310["test:3.10"];

The build job uses artifacts to store the output of its job (the built tarball) so that it can be used in later jobs.

This configuration also uses a template for the test jobs, called .test, and uses the extends keyword to reuse that configuration for the test:3.9 and test:3.10 jobs.

An example of this pipeline running is here:

Other examples

gwdatafind/gwdatafind (python package with automated tests): :

lscsoft/bayeswave (cmake build with binary packaging and tests): :

lscsoft/lalsuite (multi-package, multi-distribution package suite with scheduled nightly jobs): :

emfollow/gwcelery (python package with documentation, tests, and continuous deployment to multiple locations): :

Gitlab-CI Templates

The IGWN Computing and Software Working Group maintain a suite of Gitlab-CI templates that can be used to trivialise basic operations in a number of projects. Please see

For example, we can simplify the above Building a (python) package example by using the .python:build template for the build` job:

  - project: computing/gitlab-ci-templates
    file: python.yml

  - build
  - test

    - .python:build
  stage: build


Here the entire job configuration is delegated to the template. This has the benefit of simplifying this YAML file, and also meaning that any improvements in build techniques applied by Computing and Software to the job templates are automatically included in any pipelines.

You can see a full example of the gitlab-ci-templates used to configure an end-to-end CI pipeline in a newer version of the .gitlab-ci.yml for gwdatafind/gwdatafind (pipeline).

GitLab Pages

GitLab supports building static web content with CI and hosting them through Gitlab with an extension called Pages.

An example of this can be seen for this webpage:

For more details, see

Runners and tags

In Gitlab-CI a runner is the name given to the machine that actual executes the jobs in a CI/CD pipeline. Each pipeline can run jobs on multiple runners, and each runner can run jobs from multiple pipelines concurrently (depending on its configuration).

Linux (default)

By default on the IGWN GitLab instance, jobs will run on a Linux x86_64 runner machine that supports the docker executor. If you do not specify a tag then the build will be randomly assigned to any of the Linux runners. There are site specific tags to ensure that your build runs at either UWM, using the uwm tag, or at Caltech, using the cit tag. By default each build will have access to 4 CPU cores and 10Gb RAM. If you require more memory than this then the highmem tag can be used, this will give you build access to 12 CPU cores and 25Gb RAM. Further details regarding these runners can be seen in the table below:

Site Slots Cores RAM Architecure CPU Tag
UWM 15 4 10Gb x86_64 Intel Xeon Silver 4116 uwm
CIT 32 4 10Gb x86_64 AMD EPYC 7402 cit
CIT 4 12 25Gb x86_64 AMD EPYC 7402 highmem
CNAF 12 4 8Gb x86_64 AMD EPYC cnaf

For these runners you need to specify which docker container to use for your build, if a container image is not specified (using the image keyword in your .gitlab-ci.yaml file) then the docker:latest image will be used. This is a very basic Linux image with docker available that is only really suitable for building docker containers.


There are a total of 6 macOS runners using both x86_64 and ARM64 CPUs, details of these machines are available in the table below:

Site Slots Cores RAM Architecure CPU Tag Operating System
CIT 2 4 8Gb x86_64 Intel (Haswell) macos_catalina_x86_64 macOS 10.15 (Catalina)
CIT 2 4 8Gb x86_64 Intel (Haswell) macos_bigsur_x86_64 macOS 11 (Big Sur)
CIT 2 8 16Gb ARM64 M1 macos_bigsur_arm64 macOS 11 (Big Sur)

macOS version-independent tags

If you do not have a preference for what version of macOS is used you can also use the tags macos_x86_64 and macos_arm64 to match to any runner with an x86_64 or ARM64 CPU respectively. For compatibility with the previous configuration the tag macos can also be used to match to any of the x86_64 macOS machines. Use of this tag is deprecated and the tag will be removed at some point.

Alternative Architectures

The UWM runner is also configured to allow the running of containers using alternative architectures using the multiarch cross building images. No extra user configuration is needed, you just need to specify a container based on one of the multiarch base images and then the runner will automatically run this using the appropriate QEMU emulator. We have prebuilt arm64 and ppc64le images of the igwn/base containers, igwn/base:arm64-conda and igwn/base:ppc64le-conda. These have Conda installed with the appropriate channels configured.

Only supported on the UWM runner

Running these alternative architecture containers is only supported on the UWM runner, so jobs that utilise these images must be tagged uwm.

Building you own

If you wish to build your own alternative architecture containers you must use of the the multiarch containers as the base, as these have the appropriate configuration for running under QEMU.

Advanced concepts


MATLAB can be used in Gitlab CI only from a runner machine that can access a license file or a network license manager.

Finding the License server information

The license information for the LIGO-Caltech MATLAB license can be discovered directly from any of the LDAS-CIT login hosts as follows:

cat /ldcg/matlab_r2021b/licenses/network.lic | grep ^SERVER | awk -v RS='\r?\n' '{ print $4 "@" $2 }'

Replace the version as required

The MATLAB version (r2021b in the above command) should be replaced with whatever version is configured in your .gitlab-ci.yml file (see below for an example).

The output of this command should then be stored in a masked or protected CI/CD variable with the name MLM_LICENSE_FILE.

Do not publish the license information

The MLM_LICENSE_FILE must not be configured as a normal (plaintext) CI variable, or the value stored in an unprotected manner in any repository.

Configuring .gitlab-ci.yml

The YAML configuration for a MATLAB job would then look something like this:

  image: mathworks/matlab:r2021b
    - cit
    - matlab -batch test

For more information on the MATLAB docker images, see here.

Multi-project pipelines

GitLab CI allows specifying triggers in one project that will launch a CI/CD pipeline in a downstream project. For example:

  stage: test
  script: make check

  stage: deploy
  trigger: my/otherproject

In this example, after the test job succeeds in the test stage, the downstream job starts. GitLab then creates a downstream pipeline in the my/otherproject project.

This is used in Computing and Software to rebuild downstream Docker containers if the base container is rebuilt, see here.

See Multi-project pipelines in the official GitLab Docs for full details.