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 AMQP and Nova.

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 conductor.

To horizontally expand Nova deployments, we have a deployment sharding concept called cells. All deployments contain at least one cell. For more information, refer to Cells (v2).

Components

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

../_images/architecture.svg

  • 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 instance.

  • Compute: Manages communication with hypervisor and virtual machines.

  • Conductor: Handles requests that need coordination (build/resize), acts as a database proxy, or handles object conversions.

  • :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.

Hypervisors

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 Feature Support Matrix.

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

For more information about hypervisors, see 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 key.

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 address.

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 resources.

Note

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 types.

Note

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 that 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:

ID

Automatically generated UUID of the image

Name

Free form, human-readable name for image

Status

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

Server

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 Nova Policies.

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 process.

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