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Accounts own resources. This is extremely important so I'll state it again: Accounts own resources. So while a domain generally does not, an account does. If you delete an account all resources associated with it (instances, volumes, snapshots, etc) will be removed as well. Usage is also tracked at an account level. So for billing or chargeback purposes, if the usage module is enabled, reporting is available for resources used at an account level.

Projects

Projects are similar to accounts but unique in one special aspect. Projects can share control of resources amongst multiple accounts. The resources themselves (instances, volumes, snapshots, etc) are owned by the project and are allowed to be manipulated but multiple accounts within the same domain. So if there was a joint project being worked on by multiple departments within an organization a project could be created and could invite other accounts (departments in the organization) to take part in the project. With a project, one account must be delegated as the project administrator. A project admin has the ability to invite and revoke access to other accounts within the domain with regard to access on that project. A project admin only has control of the project and has no other authority over other accounts (e.g. it cannot impose account-level restrictions such as limits on the number of instances, volumes, snapshots, etc permitted), only over which accounts can access the project. While there can only be one project admin, it can be moved between accounts without affecting anything because all resources created by the project are owned by the project and not the individual accounts that are participating in it.