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Images and Roles in Scalr

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Images and Roles are the two fundamental features of Scalr that allow you to build reusable infrastructure components. We describe Images and Roles in Concepts and Terminology and are reproducing those descriptions here for your convenience.

Images (aka, AMI, template etc) are the main building blocks for building cloud infrastructure in all clouds. Images are used to create servers in a cloud. Scalr also indirectly makes use of Images stored in the clouds, but in order to do this it requires a record within it’s own internal meta data of every image that can be used. Thus an image in Scalr is simply a unique registration of an image that exists in a cloud. Images are used indirectly through Roles and Farm Roles within Scalr. There are three broad types of image.

  • Images provided by Scalr that include Scalr’s agent called Scalarizr. These Scalr images are ready made to provide the full functionality of Scalr as the agent enables control and monitoring functions. Scalr images are currently available in AWS, GCP and Azure.
  • Base images provided by the cloud provider, such as AWS’s AMI’s
  • Images users have created from customized servers with additional pre-installed software and configurations.

Base and user created images can easily be imported into Scalr and converted to scalarized images through the Scalr UI.

Roles provide an abstraction layer over the top of the images. Within Scalr a Role is the reusable building block for provisioning servers through Scalr. A Role can be associated with multiple similar images from multiple clouds. All similar images linked to a role will have the same base operating system and version, and the same pre-installed software and configuration. This may or may not include Scalarizr, but all images in a role must be the same in this respect. The effect of this abstraction is that any user can potentially chose which cloud to use when launching a farm/application and they will get exactly the same features and behaviour. A role can also include a set of orchestration rules and global variables which ensures consistent configuration of an server regardless of which Farm it is used in and which cloud it is deployed in.

Roles tie together:

  • One or more similar (i.e. with the same OS) Images, located in different clouds or locations.
  • Automation, through Orchestration Rules.
  • Configuration, through Role-Scope Global Variables.

Images and Roles can be configured at all three scopes within Scalr and are inherited by the lower scopes. Roles can link to Images from the same or higher scopes. Images and Roles can only be edited at the scope where they were created, but they can be cloned at lower scopes and the can be promoted from lower scopes to higher scopes.

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Please work through the sections listed below to understand the full details of working with Images and Roles.

Cloning, Promoting and Using

Additional Image and Role options are available from the right hand panel when an Image or Role is selected from the Image or Role list.

Option For Description
Clone CLONE Role Create a new role based on an existing Role
Promote PROMO Image, Role Promote to a higher scope to make more widely available
Use USE Role Add the Role directly to a Farm from the list