What sort of information on my cloud do I get in my Chef cookbooks?

Every time an event is fired, and a Chef command set is sent to an instance, apart from the data required for that particular task, it always receives data describing the current cluster state. Below is an example in JSON.

You'll be getting a list of all instances in your cloud sorted by their roles, and information on application, deployment (when the event is deploy), and the instance itself.

{
  "deploy": {
    "myapp": {
      "database": {
        "adapter": "mysql",
        "username": "root",
        "database": "myapp",
        "host": null,
        "password": null
      },
     "memcached": {
        "host": "10.12.0.240",
        "port": 11211
      }
      "rails_env": "production",
      "application_type": "rails",
      "deploy_to": "/srv/www/myapp",
      "scm": {
        "repository": "git@github.com:peritor/myapp.git",
        "scm_type": "git",
        "revision": null,
        "ssh_key": "",
        "user": "",
        "password": ""
      }
    }
  },
  "recipes": [
    "rails::configure",
    "scalarium_cluster_state",
    "scalarium_custom_cookbooks"
  ],
  "scalarium_custom_cookbooks": {
    "enabled": false,
    "recipes": [

    ]
  },
  "scalarium": {
    "activity": "configure",
    "applications": [
       {
          "name": "MyApp.com",
          "slug_name": "myapp",
          "application_type": "rails"
       }
     ],
    "cluster": {
      "name": "MyGrid"
     },
    "instance": {
      "aws_instance_id": "i-d19760a7",
      "availability_zone": "us-west-1a",
      "public_dns_name": "ec2-79-125-70-118.eu-west-1.compute.amazonaws.com",
      "instance_type": "c1.medium",
      "id": "340d2bec55150ab085e933ba5565c422",
      "private_ip": "10.226.74.161",
      "ip": "79.125.70.118",
      "instance_type": "m1.large",
      "private_dns_name": "ip-10-226-74-161.eu-west-1.compute.internal",
      "hostname": "shortbread",
      "roles": [
        "couchdb",
        "rails-app"
      ],
      "architecture": "amd64"
    },
    "roles": {
      "rails-app": {
        "name": "Rails Application Server",
        "instances": {
          "shortbread": {
            "aws_instance_id": "i-d19760a7",
            "availability_zone": "us-west-1a",
            "public_dns_name": "ec2-79-125-70-118.eu-west-1.compute.amazonaws.com",
            "private_dns_name": "ip-10-226-74-161.eu-west-1.compute.internal",
            "instance_type": "m1.large",
            "elastic_ip": null,
            "private_ip": "10.226.74.161",
            "ip": "79.125.70.118",
            "status": "online",
            "backends": 16
          }
        }
      },
      "couchdb": {
        "name": "CouchDB",
        "instances": {
          "shortbread": {
            "public_dns_name": "ec2-79-125-70-118.eu-west-1.compute.amazonaws.com",
            "private_dns_name": "ip-10-226-74-161.eu-west-1.compute.internal",
            "instance_type": "m1.large",
            "elastic_ip": null,
            "private_ip": "10.226.74.161",
            "ip": "79.125.70.118",
            "status": "online",
            "backends": 16
          }
        }
      }
    },
    "deployment": null
  }
}

You can access the data in your custom cookbooks by accessing them like so:

node[:scalarium][:instance][:hostname] # => shortbread

The scalarium node is also available as a JSON file on file system for your custom scripts. The path of the JSON file is /var/lib/scalarium/cluster_state.json