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OpenStack Basics

Cloud concepts: Domain, User, Project, Role, Image, Flavor, Network, Instance, Volume, Key pairs

Before using the cloud, a user needs to be familiar with certain concepts. Most of these concepts are applicable to almost all clouds (OpenStack based private clouds, Amazon EC2, Microsoft Azure....):


The highest management layer in OpenStack are ''domains''. A domain is a collection of users, project, roles, role assignment and common configurations like quotas etc. The domain concept allows cloud providers to grant administrative privileges to certain users and thus enable them to manage their "own" cloud instance.

The domain concept also allows the creation of additional domains, e.g. for running a workshop with dedicated users. These domains can be created on request.


A user is an entity within a domain in OpenStack. User authentication and authorization in deNBI is done via Openid-Connect or via Shibboleth. Openid-Connect (oidc) and Shibboleth are "single sign-on" authentication protocols, which provide access to all denBI-Cloud locations. Oidc and Shibboleth are implemented by ELIXIR, which provides the denBI-Cloud a global authentication/authorization endpoint.


A project is a collection of resources like users, images, volumes etc. It is initially created by the deNBI and cloud administrators, where access to the project is granted to individual users. Resources are associated to a project and are subject to quotas on project level (e.g. number of instance or volume space).


A role authorizes a user to access a project and grants certain right to the user within the project scope. The standard roles in OpenStack are ''admin'' and ''member''; with the later one granting standard access and the first one giving administrative privileges within a project.

A user needs to be a member of a project (needs to have a role associated) to access the OpenStack web interface.


An image is the binary representation of a computer's hard disk and defines the content of a virtual machine. Images may be available in various different formats (raw, qcow2, vmdk, ...) and are used to populate the first hard disk of virtual machines. Ready-to-use images for various purposes are available for download in the internet, or can be created using a wide variety of tools.

Images may be associated to a project, or made publicly available for all projects. Upon uploading an image the minimal requirements, like hard disk size or available memory, can be specified to ensure that the image is used with the right amount of resources.


A flavor defines a virtual machine setup by defining parameters like hard disk size, available memory and CPU core number. The cloud provider can use flavors to optimize virtual machines for the available hardware. A flavor may also be used to restrict virtual machines to certain physical hosts, e.g. by requiring extra local storage or features like a GPU.

Flavors are pre-defined by denBI. For a list of available standard flavors, click here.


While a flavor defines the parameters of a single virtual machine, a network defines how virtual machines are connected to each other or to the internet. Each virtual machine needs at least one network connection.


An instance is a running virtual machine defined by an image (what to run), a flavor (which amount of resources) and a network (how to connect to it).


Hard disks of virtual machines are not persistent. After the virtual machine is terminated, its hard disk's content is deleted and not available anymore. This is the principle of an "ephemeral disk". A volume, in contrast, provides a persistent way to store data. It is created separately from a virtual machine and it can be attached to an instance at runtime. It may also be detached and attached to another instance at will. The volume acts a another block device from the point of view of the virtual machine, so users are not restricted to certain formats.

In the current configuration, the same volume cannot be attached to multiple virtual machines (in the same way a physical hard disk can only be connected to one compute). To share the content of volume, snapshots can be made and different snapshots can be attached to different instances.

Volumes are part of a project and cannot be shared, this means that the transfer of volumes or snapshots between projects is impossible.

Key pair

Key pairs are credentials defined by a user to access a virtual machine. It usually consists of a SSH key pair, with the public key being configured within an instance during startup and the private key being accessible to the user only. The user can use the key pair to access an instance via SSH over the network.

Without a key pair, a user can only access a virtual machine's console by using the console redirection feature of the OpenStack dashboard. This is not the recommended way. Many available images for virtual machines require the user to define a SSH key pair since no standard user account is configured.

Always define a SSH-Keypair before spinning up any instances.