AWS ECS – data persistency test

To experiment with container services, I created a 2-container Wordpress app to run on AWS ECS

•  The first container “mysql” runs a mysql image from docker hub, and creates a database for use by WorkPress app
•  The second container “wordpress” runs a WordPress image from docker hub. It has a link to “mysql” container since the app needs to access its database. In addition, the container defines port mapping which maps its port 80 to port 80 on the host, thus making the app available.

It works nicely. ECS is fast. I can get a WordPress app up and running in minutes, available on the internet. However, when I stop and restart the service, all configuration and content is lost. This is expected, since by default, container data is not persistent.

ECS supports data persistency by way of container volumes. In container definition, the volume is mounted to the “containerPath” which is the content to be preserved. In volume definition, “sourcePath” references a location on the host to store the volume data. With volume backed by sourcePath, even if the containers are shut down, the app data on container volume is preserved.

In the sample 2-container WordPress app, the database container saves app database in /var/lib/mysql to /ecs/database on the host, the app container saves app configuration in /var/www/html to /ecs/config on the host.

Volume name	sourcePath (stored location on host)	containerPath (stored content on container)
vol-database	/ecs/database	/var/lib/mysql
vol-config	/ecs/config	/var/www/html

Initially this feature didn’t work. When configured from the GUI, the sourcePath variable was not taken, and no error was given. I posted on the forum. If it was a bug, it has since been corrected. When containers get restarted, the content of the blog is preserved. It is a simple and powerful technique for preserve data with apps on ECS.

Here is the original AWS post on container volume: https://aws.amazon.com/blogs/aws/ec2-container-service-ecs-update-access-private-docker-repos-mount-volumes-in-containers/

Attached JSON code for the 2-container WordPress app.

{

  "family": "ECS-webapp-test-persistency",

  "containerDefinitions": [

    {

      "name": "mysql",

      "image": "seanxwang/mysqlwp:v1",

      "cpu": 10,

      "memory": 300,

      "entryPoint": [],

      "environment": [

        {

          "name": "MYSQL_DATABASE",

          "value": "wordpress"

        },

        {

          "name": "MYSQL_PASSWORD",

          "value": "wordpresspwd"

        },

        {

          "name": "MYSQL_USER",

          "value": "wordpress"

        },

        {

          "name": "MYSQL_ROOT_PASSWORD",

          "value": "wordpressdocker"

        }

      ],

      "command": [],

      "portMappings": [],

      "volumesFrom": [],

      "links": [],

      "mountPoints": [

        {

          "sourceVolume": "vol-data",

          "containerPath": "/var/lib/mysql",

          "readOnly": false

        }

      ],

      "essential": true

    },

    {

      "name": "wordpress",

      "image": "seanxwang/wordpress:v1",

      "cpu": 10,

      "memory": 300,

      "entryPoint": [],

      "environment": [],

      "command": [],

      "portMappings": [

        {

          "hostPort": 80,

          "containerPort": 80,

          "protocol": "tcp"

        }

      ],

      "volumesFrom": [],

      "links": [

        "mysql"

      ],

      "mountPoints": [

        {

          "sourceVolume": "vol-config",

          "containerPath": "/var/www/html",

          "readOnly": false

        }

      ],

      "essential": true

    }

  ],

  "volumes": [

    {

      "name": "vol-data",

      "host": {

        "sourcePath": "/ecs/database"

      }

    },

    {

      "name": "vol-config",

      "host": {

        "sourcePath": "/ecs/config"

      }

    }

  ]

}