Compute

OpenStack Compute (Nova) is one of the original components of OpenStack. It provides the ability to provision a virtual machine, an application container, or a physical system, depending on configuration. All provisioning is image-based, and the OpenStack Image Service (Glance) is a prerequisite for the Compute service. Some kind of networking is also required to launch a compute instance.

Networking was originally provided by the Compute service in OpenStack, but the use of Nova Networking was deprecated in the Newton release of OpenStack, and it is no longer supported. Networking is provided by the Neutron service, which offers a wide range of functionality.

In OpenStack, we refer to provisioned compute nodes as instances and not virtual machines. Although this might seem like a matter of semantics, it's a useful device for a few reasons. The first reason is that it describes the deployment mechanism; all compute in OpenStack is the instantiation of a Glance image with a specified hardware template, the flavor.

The flavor describes the characteristics of the instantiated image, and it normally represents a number of cores of compute with a given amount of memory and storage. Storage may be provided by the Compute service or the block storage service, Cinder. Although quotas are defined to limit the amount of cores, memory, and storage available to a given user (the tenant), charge-back is traditionally established by the flavor (that is, instantiating a particular image on an m1-small flavor may cost a tenant a certain number of cents an hour).

The second reason that the term instance is useful is that virtual machines in OpenStack do not typically have the same life cycle as they do in traditional virtualization. Although we might expect virtual machines to have a multiyear life cycle like physical machines, we would expect instances to have a life cycle which is measured in days or weeks. Virtual machines are backed up and recovered, whereas instances are rescued or evacuated. Legacy virtualization platforms assume resizing and modifying behaviors are in place; cloud platforms such as OpenStack expect redeployment of virtual machines or adding additional capacity through additional instances, not adding additional resources to existing virtual machines.

The third reason that we find it useful to use the term instance is that the Compute service has evolved over the years to launch a number of different types of compute. Some OpenStack deployments may only launch physical machines, whereas others may launch a combination of physical, virtual, and container-based instances. The same construct applies, regardless of the compute provider.

Some of the lines between virtual machines and instances are becoming more blurred as more enterprise features are added to the OpenStack Compute service. Later on, we'll discuss some of the ways in which we can launch instances which act more like virtual machines for more traditional compute workloads.