Choosing between a VM and Docker-based environment

Semaphore supports two types of environments for running jobs. A Virtual Machine (VM) based environment and a Docker container based environment.

Both environment types have advantages and disadvantages compared to each other. Use the following guides to choose the best environment type for your project.

Sometimes the choice between Virtual Machines and Docker containers is not clear. Commonly asked questions include:

Virtual Machine based environments

Virtual machines are maintained by engineers at Semaphore. Every two weeks, a new release of the platform is released bundled with the latest software packages.

To use a virtual machine based environment for your jobs, use the following type of agent definition in your pipeline YAML file.

Linux based virtual machines

agent:
  machine:
    type: e1-standard-2
    os_image: ubuntu1804

Mac based virtual machines

agent:
  machine:
    type: a1-standard-4
    os_image: macos-mojave-xcode11

Read more about Machine Types and the Ubuntu 18.04, Mac OS XCode 10, Mac OS XCode 11 virtual machines images.

Docker-based environments

The Docker-based environment is a composable environment that allows the usage of one or more Docker containers to construct your test environment on Semaphore.

The recommended way of using Docker images is to build and maintain a custom-built image with the precise set of software that is necessary for your project.

Alternatively, if you are just starting out with Docker-based environments, you can use one of Semaphore's images to get started.

To use a container based environment, define at least one container in your agent definition in your pipeline YAML file.

agent:
  machine:
    type: e1-standard-2

  containers:
    - name: main
      image: semaphoreci/ruby:2.6.1

    - name: db
      image: postgres:9.6
      env_vars:
        - name: POSTGRES_PASSWORD
          value: keyboard-cat

    - name: cache
      image: redis:5.0

Read more about docker based environments.

When should you use Virtual Machines for your projects?

  • You want to have an up to date testing environment with the latest software packages. The Virtual Machines are regularly updated by engineers at Semaphore with the latest software packages.

  • You are building Docker images or spinning up local Kubernetes testing clusters in the CI/CD jobs. When working with Docker or Kubernetes directly, you usually need full control over sockets, network-devices, and subnets. Access to these is provided in VMs, but not in containers.

  • You are running or testing nested Virtual Machines. In these situations, you need full control over the operating system that only virtual machines provide.

When should you use Docker-based environments?

  • You want to have full control over the software installed in your test environment. Rolling software updates offered in the VMs are a great way to keep your test environment up to date, but they can also introduce unwanted issues into your test suite a new release is rolled out. If you build and maintain your own Docker images, you gain full control over the release cycle of your testing environment.

  • You want to have the same testing environment on Semaphore as in your local development environment. A docker image is a perfect choice in this case. With Docker images, you can guarantee the same software stack across development and build environments.

  • You need custom software that is not available in the VM images. Semaphore's Virtual Machines are packed with a wide variety of languages and tools, but it is impossible to pack everything. In the virtual machines, you can use apt-get to install them, but a more efficient way is to build a docker image that pre-installs everything necessary for your tests.

Can I use Docker images in Virtual Machines?

Yes. Running Docker images in Virtual Machines is one of the most common ways to start background services necessary for your jobs.

Instead of defining them in the container section in the agent, use the prologue commands to spin up your Docker images.

For example, to start RabbitMQ in a Virtual Machine, use the following snippet:

blocks:
  - name: Tests
    task:
      prologue:
        commands:
          - docker run -d --hostname my-rabbit --name rabbit -p 15672:15672 -p 5672:5672 rabbitmq:3-management
          - sleep 5 # give some time for Rabbit to start up

      jobs:
        - name: Rabbit Test
          commands:
            - make rabbit.based.tests

Can I build Docker-in-Docker?

It is possible to build Docker images in container based environments (the Docker socket is mounted and available). Still, it is not the recommended way to build Docker images on Semaphore.

Docker-in-Docker has a set of issues that you need to be aware of before choosing this approach.

Jérôme Petazzoni — the author of the feature that made it possible for Docker to run inside a Docker container — wrote a blog post saying not to do it. The use case he describes matches the use case of a CI Docker container that needs to run jobs inside other Docker containers.

A lot has changed since 2015 in the Docker world. However, some issues are still present.

We recommend using Virtual Machines for building Docker images.

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