In the Stacks

Glance: Inconspicuous

The Glance Image service handles a very important task in OpenStack, although it is quite inconspicuous. Because you can hardly expect users in clouds to install an operating system from scratch for each new VM, Glance provides prebuilt, preconfigured hard disk images. Users download these images to their virtual disks for new VMs and thus have an installed and running system within a few seconds. The cloud-init service can be used freely to transfer various configuration parameters to the VM.

As mentioned earlier, Glance comprises the API and registry. The service can handle multiple formats for virtual hard disks and connect directly to different storage systems, such as Ceph or GlusterFS, which then help Glance distribute its images.

Glance connects seamlessly with the Nova component. When a user configures and starts a new VM, they thus only need to select the appropriate operating system from a list – Nova and Glance then negotiate the details of commissioning in the background.

Nova: The Veteran

OpenStack Nova is the oldest OpenStack service and, in a way, the primordial soup from which OpenStack emerged in its present form. OpenStack was initially a joint venture between the Rackspace hosting service and NASA. Although NASA has discontinued its commitment to the cloud environment, it is still responsible for the first versions of Nova.

Simply put, Nova in OpenStack takes care of everything that has to do with the management of VMs: creating new VMs, deleting old VMs, and starting and stopping them, including responses to requests by users or admins. Nova consists of a wide range of services, with the API, the Scheduler, and the Compute component being the most important services.

The API fields all commands from users and components and processes them. If an incoming command requires that a new VM be created, the API first sends the command to the Scheduler. The Scheduler keeps track of the available compute nodes and the available resources and chooses a suitable host. On the host, Nova's Compute component then launches and starts up the new VM locally.

The OpenStack developers deserve praise for not reinventing the wheel in the Compute component, but relying on existing technology. For example, if you use KVM as the hypervisor, Nova Compute relies on libvirt to manage the VMs. Nova supports more than just KVM: Xen and various container technologies are also included, along with more commercial approaches like VMware ESX or Microsoft Hyper-V.

Neutron: Connected

Most VMs need access to a network. In contrast to conventional virtualization environments, simply using a bridge to loop through one of the host's networks in a cloud is not sufficient. In clouds of all types, and especially large-scale public clouds, the basic assumption is that VMs from several customers can run on the same host without interfering with one another – anything else would simply not scale.

SDN is therefore mandatory in a cloud. Various implementations of SDN are available on the market: Open vSwitch, along with several commercial solutions based on it (e.g., MidoNet, PLUMgrid), and alternatives such as OpenContrail.

OpenStack Neutron (Figure 3) bridges the gap between OpenStack and the various SDN flavors. Basically, Neutron is not much more than an API that has a plugin interface. A plugin for a specific SDN style can then be downloaded via the interface. Neutron itself has a few SDN implementations on board (e.g., Open vSwitch). The hosts in the setup then run various additional services (aka agents) associated with Neutron that store the appropriate SDN configuration locally.

Figure 3: In OpenStack, Neutron takes care of SDN and allows connections to various SDN solutions in the background.

Neutron is regarded as one of the most complex OpenStack services. Initially, in particular, the interrelations between the agents and services is difficult to comprehend. However, Neutron is now essential for complex setups, with practically no alternative. Development of the legacy Nova Network component, which took care of SDN and was part of Nova, stopped some time ago, and Nova Network will soon be disappearing from Nova.