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Taikun OCP Guide

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Nova System Architecture

Nova comprises multiple server processes, each performing different
functions. The user-facing interface is a REST API, while internally
Nova components communicate via an RPC message passing mechanism.

The API servers process REST requests, which typically involve
database reads/writes, optionally sending RPC messages to other Nova
services, and generating responses to the REST calls. RPC messaging is
done via the oslo.messaging library, an abstraction on
top of message queues. Nova uses a messaging-based, “shared nothing”
architecture and most of the major nova components can be run on
multiple servers, and have a manager that is listening for RPC messages.
The one major exception is the compute service, where a single process
runs on the hypervisor it is managing (except when using the VMware or
Ironic drivers). The manager also, optionally, has periodic tasks. For
more details on our RPC system, refer to /reference/rpc.

Nova uses traditional SQL databases to store information. These are
(logically) shared between multiple components. To aid upgrade, the
database is accessed through an object layer that ensures an upgraded
control plane can still communicate with a compute nodes running the
previous release. To make this possible, services running on the compute
node proxy database requests over RPC to a central manager called the

To horizontally expand Nova deployments, we have a deployment
sharding concept called cells <cell>. All deployments contain at least
one cell. For more information, refer to /admin/cells.


Below you will find a helpful explanation of the key components of a
typical Nova deployment.


  • DB: SQL database for data storage.
  • API: Component that receives HTTP requests,
    converts commands and communicates with other components via the
    oslo.messaging queue or HTTP.
  • Scheduler: Decides which host gets each
  • Compute: Manages communication with hypervisor and
    virtual machines.
  • Conductor: Handles requests that need coordination
    (build/resize), acts as a database proxy, or handles object
  • :placement-doc:`Placement <>`: Tracks
    resource provider inventories and usages.

While all services are designed to be horizontally scalable, you
should have significantly more computes than anything else.


Nova controls hypervisors through an API server. Selecting the best
hypervisor to use can be difficult, and you must take budget, resource
constraints, supported features, and required technical specifications
into account. However, the majority of OpenStack development is done on
systems using KVM-based hypervisors. For a detailed list of features and
support across different hypervisors, see /user/support-matrix.

You can also orchestrate clouds using multiple hypervisors in
different availability zones. Nova supports the following

For more information about hypervisors, see /admin/configuration/hypervisors section in the Nova
Configuration Reference.

Projects, users, and roles

To begin using Nova, you must create a user with the Identity service <>.

The Nova system is designed to be used by different consumers in the
form of projects on a shared system, and role-based access assignments.
Roles control the actions that a user is allowed to perform.

Projects are isolated resource containers that form the principal
organizational structure within the Nova service. They typically consist
of networks, volumes, instances, images, keys, and users. A user can
specify the project by appending project_id to their access

For projects, you can use quota controls to limit the number of
processor cores and the amount of RAM that can be allocated. Other
projects also allow quotas on their own resources. For example, neutron
allows you to manage the amount of
networks that can be created within a project.

Roles control the actions a user is allowed to perform. By default,
most actions do not require a particular role, but you can configure
them by editing the policy.yaml file for user roles. For
example, a rule can be defined so that a user must have the
admin role in order to be able to allocate a public IP

A project limits users’ access to particular images. Each user is
assigned a user name and password. Keypairs granting access to an
instance are enabled for each user, but quotas are set, so that each
project can control resource consumption across available hardware


Earlier versions of OpenStack used the term tenant
instead of project. Because of this legacy terminology,
some command-line tools use --tenant_id where you would
normally expect to enter a project ID.

Block storage

OpenStack provides two classes of block storage: storage that is
provisioned by Nova itself, and storage that is managed by the block
storage service, Cinder.

Nova-provisioned block storage

Nova provides the ability to create a root disk and an optional
“ephemeral” volume. The root disk will always be present unless the
instance is a Boot From Volume instance.

The root disk is associated with an instance, and exists only for the
life of this very instance. Generally, it is used to store an instance’s
root file system, persists across the guest operating system reboots,
and is removed on an instance deletion. The amount of the root ephemeral
volume is defined by the flavor of an instance.

In addition to the root volume, flavors can provide an additional
ephemeral block device. It is represented as a raw block device with no
partition table or file system. A cloud-aware operating system can
discover, format, and mount such a storage device. Nova defines the
default file system for different operating systems as ext4 for Linux
distributions, VFAT for non-Linux and non-Windows operating systems, and
NTFS for Windows. However, it is possible to configure other filesystem


For example, the cloud-init package included into an
Ubuntu’s stock cloud image, by default, formats this space as an ext4
file system and mounts it on /mnt. This is a cloud-init
feature, and is not an OpenStack mechanism. OpenStack only provisions
the raw storage.

Cinder-provisioned block storage

The OpenStack Block Storage service, Cinder, provides persistent
volumes hat are represented by a persistent virtualized block device
independent of any particular instance.

Persistent volumes can be accessed by a single instance or attached
to multiple instances. This type of configuration requires a traditional
network file system to allow multiple instances accessing the persistent
volume. It also requires a traditional network file system like NFS,
CIFS, or a cluster file system such as Ceph. These systems can be built
within an OpenStack cluster, or provisioned outside of it, but OpenStack
software does not provide these features.

You can configure a persistent volume as bootable and use it to
provide a persistent virtual instance similar to the traditional
non-cloud-based virtualization system. It is still possible for the
resulting instance to keep ephemeral storage, depending on the flavor
selected. In this case, the root file system can be on the persistent
volume, and its state is maintained, even if the instance is shut down.
For more information about this type of configuration, see Introduction to the Block Storage service

Building blocks

In OpenStack the base operating system is usually copied from an
image stored in the OpenStack Image service, glance. This is the most
common case and results in an ephemeral instance that starts from a
known template state and loses all accumulated states on virtual machine
deletion. It is also possible to put an operating system on a persistent
volume in the OpenStack Block Storage service. This gives a more
traditional persistent system that accumulates states which are
preserved on the OpenStack Block Storage volume across the deletion and
re-creation of the virtual machine. To get a list of available images on
your system, run:

$ openstack image list
| ID                                   | Name                        | Status |
| aee1d242-730f-431f-88c1-87630c0f07ba | Ubuntu 14.04 cloudimg amd64 | active |
| 0b27baa1-0ca6-49a7-b3f4-48388e440245 | Ubuntu 14.10 cloudimg amd64 | active |
| df8d56fc-9cea-4dfd-a8d3-28764de3cb08 | jenkins                     | active |

The displayed image attributes are:


Automatically generated UUID of the image


Free form, human-readable name for image


The status of the image. Images marked ACTIVE are
available for use.


For images that are created as snapshots of running instances, this
is the UUID of the instance the snapshot derives from. For uploaded
images, this field is blank.

Virtual hardware templates are called flavors. By
default, these are configurable by admin users, however, that behavior
can be changed by redefining the access controls
policy.yaml on the nova-api server. For more
information, refer to /configuration/policy.

For a list of flavors that are available on your system:

$ openstack flavor list
| ID  | Name      |   RAM | Disk | Ephemeral | VCPUs | Is_Public |
| 1   | m1.tiny   |   512 |    1 |         0 |     1 | True      |
| 2   | m1.small  |  2048 |   20 |         0 |     1 | True      |
| 3   | m1.medium |  4096 |   40 |         0 |     2 | True      |
| 4   | m1.large  |  8192 |   80 |         0 |     4 | True      |
| 5   | m1.xlarge | 16384 |  160 |         0 |     8 | True      |

Nova service architecture

These basic categories describe the service architecture and
information about the cloud controller.

API server

At the heart of the cloud framework is an API server, which makes
command and control of the hypervisor, storage, and networking
programmatically available to users.

The API endpoints are basic HTTP web services which handle
authentication, authorization, and basic command and control functions
using various API interfaces under the Amazon, Rackspace, and related
models. This enables API compatibility with multiple existing tool sets
created for interaction with offerings from other vendors. This broad
compatibility prevents vendor lock-in.

Message queue

A messaging queue brokers the interaction between compute nodes
(processing), the networking controllers (software which controls
network infrastructure), API endpoints, the scheduler (determines which
physical hardware to allocate to a virtual resource), and similar
components. Communication to and from the cloud controller is handled by
HTTP requests through multiple API endpoints.

A typical message passing event begins with the API server receiving
a request from a user. The API server authenticates the user and ensures
that they are permitted to issue the subject command. The availability
of objects implicated in the request is evaluated and, if available, the
request is routed to the queuing engine for the relevant workers.
Workers continually listen to the queue based on their role, and
occasionally their type host name. When an applicable work request
arrives on the queue, the worker takes assignment of the task and begins
executing it. Upon completion, a response is dispatched to the queue
which is received by the API server and relayed to the originating user.
Database entries are queried, added, or removed as necessary during the

Compute worker

Compute workers manage computing instances on host machines. The API
dispatches commands to compute workers to complete these tasks:

  • Run instances
  • Delete instances (Terminate instances)
  • Reboot instances
  • Attach volumes
  • Detach volumes
  • Get console output

Network Controller

The Network Controller manages the networking resources on host
machines. The API server dispatches commands through the message queue,
which are subsequently processed by Network Controllers. Specific
operations include:

  • Allocating fixed IP addresses
  • Configuring VLANs for projects
  • Configuring networks for compute nodes


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