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The Marathon adapter, in combination with the Panamax remote agent, enables the deployment of a Panamax template to a Mesosphere cluster.

Template Translation

To understand how the Marathon Adapter translates a Panamax Template into a deployment, let's look at an example template:

name: Wordpress with MySQL
images:
- name: WP
  source: centurylink/wordpress:3.9.1
  environment:
    - variable: DB_PASSWORD
      value: pass@word01
    - variable: DB_NAME
      value: wordpress
  links:
  - service: DB
    alias: DB_1
  ports:
  - host_port: 8000
    container_port: 80
- name: DB
  source: centurylink/mysql:5.5
  environment:
    - variable: MYSQL_ROOT_PASSWORD
      value: pass@word01
  ports:
  - host_port: 3306
    container_port: 3306

The Panamax Remote Agent

The Panamax Remote Agent will receive the template with its deployment counts and overridden environment variables and pass that configuration to the adapter. The adapter processes this request and will translate it into deployment requests.

Grouping

The Marathon Adapter creates a unique group name identifier and prefixes all services with this group name when they are translated into application deployments. Therefore, all template deployments are isolated from other template deployments. When all the applications within a group have been removed the group is also removed.

Scaling

During the deployment process, the user is provided an opportunity to scale the services being deployed. For example, if a user has a cluster of 3 nodes the user might choose to scale the WordPress service across all 3 nodes. The user may also override environment variables defined in the template so that during deployment, the values of these keys can be specific to the target deployment environment.

A dependent service such as MySQL in the template above can only exist on a single node unless the application uses a proxy service in front of the dependency. If a dependent service is scaled to more than one, it will automatically be limited to only one instance (a singleton service).

This approach was taken so that a template could be deployed on any Mesosphere cluster without assumptions about the cluster having DNS or proxy services for service discovery and routing.

Service discovery

Mesosphere has documented a suggested architecture for service discovery: Mesosphere Service Discovery

However, the marathon adapter does not assume the cluster was configured using this architecture. The adapter manages application deployments by transitioning a deployment through three states.

Requirements

By analyzing the links stanza in the template request, the adapter can determine if the given service is dependent upon another service. A deployment will remain in this state until its service requirements have been met. In the template example above, the WP service is dependent upon the DB service. Therefore, it cannot be deployed until it knows where the DB service has been deployed.

This step will result in the following 2 environment variables being set on the dependent container:

alias_SERVICE_HOST IP address derived from the value associated with the etcd key of the same name created when the dependency's container is created, e.g. DB_1_SERVICE_HOST=172.17.8.101

alias_PORT Full URL for the dependency's container, e.g. DB_1_PORT=tcp://172.17.8.101:3306. These values correspond to the lowest exposed port binding rule. If the child container in the link relationship exposes more than one port only the lowest numbered port will be used for the Fleet unit configuration.

Additionally, for each port binding rule, the following 4 variables will be set:

alias_PORT_num_protocol Full URL for the dependency's container, e.g. DB_1_PORT_3306_TCP=tcp://172.17.8.101:3306

alias_PORT_num_protocol_PROTO Protocol (tcp or udp) for the dependency's container, e.g. DB_1_PORT_3306_TCP_PROTO=tcp

alias_PORT_num_protocol_PORT Exposed port number for the dependency's container, e.g. DB_1_PORT_3306_TCP_PORT=3306

alias_PORT_num_protocol_ADDR IP address for the dependency's container, e.g. DB_1_PORT_3306_TCP_ADDR=172.17.8.101

Deployment

The deployment state executes the commands to deploy the application into the Mesosphere cluster.

Post Deployment

Once a service has been deployed it advertises its location and configuration so that services which may depend upon it can complete their requirement step.