Persisting and sharing data across Docker containers

To share data volumes between containers within and across hosts in a grid, to preserve containers' data volumes across containers life-cycles and dynamic placements in a grid, and to use the automatic data volume provisioning using IBM Spectrum Scale, you need to configure data volumes to use.

Procedure

Follow these steps to create, configure, and use data volumes:

  1. Create and mount a data volume that can be accessed from the Docker hosts in a grid.
  2. Configure a volume attribute in the application template to include the container directory (path inside the container) of the data volume, the host directory (path accessible to the host) of the data volume, and optionally the access permission type and a name of an environment variable that is set inside the container and contains the path of the data volume inside the container.
  3. Configure the relevant applications in the container to use the data volume for data storage. You can configure application instances to use the environment variable that is specified in the previous step to obtain the path of the data volume inside the container.
    For example, here is an application template with data volume defined: 
    
    apache:
        type: IBM::ASC::DockerContainer
        properties:
            imagename: tutum/apache-php
            registryurl: registry.hub.docker.com
            command: /run.sh              
            cpu:
                shares: 1024
                set: 0
            mem:
                mem_limit_mb: 300
            ports:
                - containerport: 80
                  hostport: 8001
                  ip:
            user: root
            volumes:
                - containerpath: /app
                  hostpath: /opt/web
                  permissions: ro
                  volumeenvvarname: DataVolumeEnvVAR_apache

dataUpdate:
        type: IBM::ASC::DockerContainer
        properties:
            imagename: image:v2
            registryurl:
            command: /run.sh
            user: www-data                
            volumes:
                - containerpath: /tmp
                  hostpath: /opt/web
                  permissions: rw
                  volumeenvvarname: DataVolumeEnvVAR_dataUpdate

dockerpod1:
        type: IBM::ASC::DockerPod
        properties:
            sharedproperties:
                network:
                    type: bridge
            containers:
                - { get_resource: apache }
                - { get_resource: dataUpdate }
    The Apache container displays web pages from /opt/web, which is mapped into the container directory /app. The permission is read-only.

    The dataUpdate container updates the web pages from /opt/web, which is mapped into the container directory /tmp_new. The permission is read/write.

    The dataUpdate container has a script /command/change.sh that overwrites the data in /tmp_new, which is accessible via port 8001 on the host that the Apache container runs on.

    When both containers stop, the files in /opt/web persist the changes that are made from the dataUpdate container.