Dockers

Dockers 101 – Series 8 of N – Stateful Containers and Importing and Exporting Containers

Mohd Naeem
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Photo: panumas nikhomkhai / Unsplash · Royalty-free
  • Stateless vs Stateful containers
    • Stateless – they don’t need to maintain the state of an application
      • e.g The TicTacToe game container we created is a simple game. We just wanted that when the container image is downloaded then the game should run. But we are not maintaining any users, their scores or anything like that.
    • Stateful – they need the application state to be maintained on some storage volume e.g in a database we are storing the users, scores, history of the games etc.
  • Approaches for Stateful containers
    • -v <host-dir>:<container-dir> parameter option
      • -v host-dir:container-dir option instructs the docker to map a host directory to a container directory. It can be a good option for some scenarios but not an effective solution. What if the container is run from another docker where the host directory does not exist?
    • Using Data Containers
      • they are responsible for storing data
      • but they don’t run like other containers
      • they hold the data/volume and are referenced by other containers who want to use this volume
  • Data containers in action
    • Lets use Busybox(one of the smaller Linux distributions) we will use this container to hold our data and to be referenced by other containers
    • We will use docker create command to create a new container and pass -v parameter to create a container folder
    • We will then copy the configuration file from host folder to container folder
    • Now with the new data container created, we will use this container to reference/mount on a Ubuntu container using command –volumes-from
    • We will see how in the Ubuntu container since out container is mounted as volume, we can see the config file there.
    • This data container can be exported and imported too.

    • # create a config file
      echo “test=true” >> config.conf # create a container by a specific name , with v option to create a folder in the container
      # (busybox is very small container)
      docker create -v /config –name naeemsDataContainer busybox # copy data from local to the container
      docker cp config.conf naeemsDataContainer:/config/ # run an ubuntu container, referencing the container naeemsDataContainer using command –volumes-from
      docker run –volumes-from naeemsDataContainer ubuntu ls /config # export the container
      docker export naeemsDataContainer > naeemsDataContainer.tar # import the container
      docker import naeemsDataContainer.tar # check the docker images and see the imported image (you will see naeemsDataContainer – a data container)
      docker images # check docker containers and see the running container
      # (you will not see naeemsDataContainer, as it actually does not run, it is just a mount volume for other containers)
      docker ps -a

    • Capture
    • Capture