Migrating a Service to Docker

Migrating an existing service to Docker can offer numerous benefits, such as improved consistency, easier scaling, and better resource utilization. However, the migration process requires careful planning and execution to ensure a smooth transition.

Step 1: Update the Docker Image

The first step in the migration process is to update the Docker image. This should be done at the beginning of the Dockerfile to ensure that the image is built on the latest base image and has all the necessary updates and security patches.

Step 2: Move Cron Files to cron-hssunified Package

If your service relies on cron jobs, it's important to move all the cron files to the cron-hssunified package. This allows for better organization and management of cron tasks within the containerized environment.

Step 3: Move Collectd Files to collectd-configs Package

Similarly, if your service uses Collectd for metrics collection, move all the Collectd configuration files to the collectd-configs package. This ensures that the container has access to the necessary configuration files for metrics collection.

Step 4: Verify Accessibility of Required Host Resources

Ensure that the container has access to all the required host resources, such as files, directories, or network resources. This is crucial for the proper functioning of the service within the container.

Step 5: Verify Accessibility of Produced Data

Verify that any data produced by the container is accessible by other containers or host services that need it. This may require setting up appropriate volume mounts or network communication channels.

Step 6: Verify Resource Permissions

Pay special attention to resource permissions, as the user and group IDs (UIDs and GIDs) may differ between the host and the container. Ensure that the container has the necessary permissions to access and modify the required resources.

Step 7: Set Container Resource Limits

Set appropriate resource limits for the container, such as CPU and memory limits, to prevent resource contention and ensure optimal performance. This can be done using the --cpu and --memory flags in the docker run command or by specifying the limits in the Docker Compose file.

Step 8: Update Docker Compose Image Name

In the Docker Compose file, update the image name to refer to the production image instead of using an exact hash. This allows for easier updates and maintenance of the containerized service.

Step 9: Manage Permissions within the Container

Each container should manage its own resource permissions. Move any permission management tasks specific to the service into the appropriate container to ensure proper isolation and maintainability.

Step 10: Stop Non-Dockerized Service

Before installing the Dockerized package, ensure that the non-Dockerized service is stopped. If there is uncertainty, perform a sv stop and cleanup during the installation of the Dockerized package.

Step 11: Create Non-Standard Paths

If the service relies on any non-standard paths on the host system, create those paths in the %post section of the package installation script.

Step 12: Run docker-updater

In the %post section, run the docker-updater.sh script to update the Docker configuration:

bashCopy/usr/local/sbin/docker-updater.sh || :

Similarly, run docker-updater.sh in the %postun section to handle package removal:

bashCopytest ".$1" = ".0" && {
    /usr/local/sbin/docker-updater.sh
} || :

Step 13: Clean Up Temporary Packages and Files

As the last step of the Docker image build process, remove any temporary packages and files that are no longer needed. This helps keep the image size small and reduces the attack surface.

Step 14: Clean Up Package Manager Cache

After all the operations are complete, clean up the package manager cache using yum clean all or apt-get clean, depending on the base image's package manager. This frees up disk space within the container.

Step 15: Configure Logging

In the Docker Compose file, configure the service to send logs to syslog using the following configuration:

yamlCopylogging:
  driver: syslog
  options:
    tag: "<YOUR_SERVICE_NAME>"

Replace <YOUR_SERVICE_NAME> with the actual name of your service. This ensures that the logs from the containerized service are properly captured and managed by the host's logging system.