Resources, Projects, and Domains

OpenStack API resources do not have unique names. For some resource types, names have to be unique at the project or domain level, but resources of the same type from different domains or projects can use the same name. For some resource types, the name is not expected to be unique at all, and multiple instances of the same resource, on the same project, could use the same name.

OpenStack API resources are uniquely identified by an UUID assigned by its service. All OpenStack API calls which create new resource instances return the UUID of the new resource, and this is the only guaranteed way of referencing the resource later on. While you can change the name of a resource, you cannot change its UUID.

Most OpenStack client commands will accept names of resources and retrieve their UUIDs from the specified project and domain scopes, but if they cannot, you are expected to retry the command providing the UUIDs of API resources you need.

When one API resource references other API resources, it is always by UUID, never by name. This means API resource names could change at any time.

Some OpenStack API resources also accept zero or more tags, which you use for arbitrary grouping of resources. OpenStack services do not give any meaning to tags, but they help Operators manage clusters with a large number of resources.

In the previous chapter, we learned that Keystone authenticates the identity of a user, but Keystone itself makes no authorization decisions: these belong to individual OpenStack services.

Keystone does populate access tokens with data, such as the originating domain and project of a user, and the list of groups and roles a user is a member of. OpenStack services can use any of that information to make authentication decisions, and they are expected to follow a few conventions, among them:

To understand the difference between admin and member roles, consider the conflicting needs of granting some autonomy to OpenStack Operators, while denying them the ability to perform dangerous operations. For example, a user might have permission to create server instances and networks inside a project, but the same user cannot change resource quotas on that project, nor grant access to resources to other users.

The admin, member, and reader roles exist by convention. It is up to individual OpenStack services to assign specific meaning to each of those roles, that is: which APIs they can invoke and using which parameters. For some services, the distinction between admin and member might make no sense and those roles would be equivalent. For other services, the reader role may not be able to view sensitive attributes of a resource instance.

It is possible, but uncommon, to create custom roles in OpenStack clusters. The reason being that you cannot define the permissions (or rules) for custom roles using OpenStack APIs. You would have to manage services directly on the control plane to provide them with policy files. Most organizations stay within the three standard roles.

As a best practice, avoid assigning roles directly to users, but assign them to groups and let users get roles based on their group membership.

It is possible to create and manage users and groups as Keystone API resources, but it is not recommended. Those local users and groups are nice for small scenarios, like test clusters and proofs-of-concept, but the recommended approach is delegating user and group management to external enterprise identity systems based on the LDAP or OIDC standards.

One Keystone domain can refer to only one authentication backend. This is a reason some organizations create user domains and resource domains: so they can create multiple user domains, each referring to a different identity system.

Traditional data centers may be used to the LDAP technology, used by identity management systems like Microsoft Active Directory (MSAD) and Red Hat Enterprise Linux Identity Management (RHEL IdM) but this technology is losing ground for identity management systems compatible with OpenID Connect (OIDC), which are based on the OAuth standard.

Nowadays, it is advisable to prefer OIDC identity systems over LDAP systems, especially for sharing authentication backends with modern cloud-native and containerized IT infrastructure applications, such as CI/CD systems. Some organizations stick with LDAP systems because most operating systems cannot authenticate local users using OIDC, but most of the time there is no reason to make OpenStack compute nodes members of your MSAD or RHEL IdM authentication domain.

Keystone and some other OpenStack services recognize the system scope meaning an entire cluster. OpenStack APIs which operate at the system scope, outside of any domain and project, are very rare, but granting a user or group with the admin role at the system scope is an effective way of making them superusers for the totality of a cluster.

For some services there may be a special project and/or a special domain, prepopulated with shared resources, and with admin or member restricted to only a few users, plus readers assigned by default to all users. This looks like system scope but it is only a domain and project reserved for OpenStack Administrators to share resources with other users and projects. Organizations can use a similar approach for avoiding duplicating resources on multiple projects and domains.

API resource quotas affect the number of instances you can create, of a given API resource type, inside a project. That class of quota limits the number of server instances, floating ips, virtual networks, volumes, and so on.

Compute resource quotas affect the quantity, or capacity, of a compute resource that a project can consume, aggregating the consumption from all server instances in the project. That class of quota limits the total quantity of vCPUs, total memory aggregated from all server instances, total disk space from all volumes, and so on.

Remember that quotas are defined and enforced by each OpenStack service themselves, and this allows for some inconsistencies and also for purposeful exceptions. One such exception is Swift, which provides both API resource and compute resource storage quotas per storage account.

Object storage accounts enable external applications to save and retrieve objects without directly authenticating to Keystone. They can represent individual users of Swift object storage instead of individual applications.

Object storage quotas limit the number of containers or buckets, the number of objects, and the total space occupied by aggregating all objects owned by the account.

Because most applications are bursty, meaning their actual usage of compute resources vary over time, and most times it happens during short periods of time, alternating with other short periods of idleness, OpenStack enables overcommit of compute resources by default.

An OpenStack Administrator can configure different overcommit levels for different classes of compute nodes, but the idea is that, if a compute node has an overcommit factor of 2.0 and 16 cores, it can run server instances adding up to 32 vCPUs.

It is not frequent to configure overcommit of other classes of compute resources, such as memory and GPUs because they tend to be used with a more consistent, non-bursty pattern.