GitLab Runner implements a number of executors that can be used to run your builds in different scenarios:
- Shell
- Docker
- Docker Machine and Docker Machine SSH (auto-scaling)
- Parallels
- VirtualBox
- SSH
- Kubernetes
The executors support different platforms and methodologies for building a project. The table below shows the key facts for each executor which will help you decide.
| Executor | Shell | Docker | Docker-SSH | VirtualBox | Parallels | SSH | Kubernetes |
|---|---|---|---|---|---|---|---|
| Clean build environment for every build | no | ✓ | ✓ | ✓ | ✓ | no | ✓ |
| Migrate runner machine | no | ✓ | ✓ | partial | partial | no | ✓ |
| Zero-configuration support for concurrent builds | no (1) | ✓ | ✓ | ✓ | ✓ | no | ✓ |
| Complicated build environments | no (2) | ✓ | ✓ | ✓ (3) | ✓ (3) | no | ✓ |
| Debugging build problems | easy | medium | medium | hard | hard | easy | medium |
- it's possible, but in most cases it is problematic if the build uses services installed on the build machine
- it requires to install all dependencies by hand
- for example using Vagrant
Shell is the simplest executor to configure. All required dependencies for your builds need to be installed manually on the machine that the Runner is installed.
A better way is to use Docker as it allows to have a clean build environment, with easy dependency management (all dependencies for building the project could be put in the Docker image). The Docker executor allows you to easily create a build environment with dependent services, like MySQL.
We usually don't advise to use Docker-SSH which is a special version of
the Docker executor. This executor allows you to connect to a Docker
container that runs the SSH daemon inside it. It can be useful if your
Docker image tries to replicate a full working system: it uses some process
management system (init), exposes the SSH daemon, and contains already
installed services. These kind of images are fat images, and are not generally
advised to be used by the Docker community.
We also offer two full system virtualization options: VirtualBox and Parallels. This type of executor allows you to use an already created virtual machine, which will be cloned and used to run your build. It can prove useful if you want to run your builds on different Operating Systems since it allows to create virtual machines with Windows, Linux, OSX or FreeBSD and make GitLab Runner to connect to the virtual machine and run the build on it. Its usage can also be useful to reduce the cost of infrastructure.
The SSH executor is added for completeness. It's the least supported executor from all of the already mentioned ones. It makes GitLab Runner to connect to some external server and run the builds there. We have some success stories from organizations using that executor, but generally we advise to use any of the above.