OpenStack Compute contains several main components.
API serveracts as the web services front end for the cloud controller. The
compute controllerprovides compute server resources and usually also contains the Compute service.
object storeis an optional component that provides storage services; you can also use OpenStack Object Storage instead.
auth managerprovides authentication and authorization services when used with the Compute system; you can also use OpenStack Identity as a separate authentication service instead.
volume controllerprovides fast and permanent block-level storage for the compute servers.
network controllerprovides virtual networks to enable compute servers to interact with each other and with the public network. You can also use OpenStack Networking instead.
scheduleris used to select the most suitable compute controller to host an instance.
Compute uses a messaging-based,
shared nothing architecture. All
major components exist on multiple servers, including the compute,
volume, and network controllers, and the Object Storage or Image service.
The state of the entire system is stored in a database. The cloud
controller communicates with the internal object store using HTTP, but
it communicates with the scheduler, network controller, and volume
controller using Advanced Message Queuing Protocol (AMQP). To avoid
blocking a component while waiting for a response, Compute uses
asynchronous calls, with a callback that is triggered when a response is
Compute 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 and Xen-based hypervisors. For a detailed list of features and support across different hypervisors, see the Feature Support Matrix.
You can also orchestrate clouds using multiple hypervisors in different availability zones. Compute supports the following hypervisors:
For more information about hypervisors, see the Hypervisors section in the OpenStack Configuration Reference.
The Compute 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 Compute service. They consist of an
individual VLAN, and volumes, instances, images, keys, and users. A user
can specify the project by appending
project_id to their access key.
If no project is specified in the API request, Compute attempts to use a
project with the same ID as the user.
For projects, you can use quota controls to limit the:
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
policy.json 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.
Earlier versions of OpenStack used the term
tenant instead of
project. Because of this legacy terminology, some command-line tools
--tenant_id where you would normally expect to enter a
OpenStack provides two classes of block storage: ephemeral storage and persistent volume.
Ephemeral storage includes a root ephemeral volume and an additional ephemeral volume.
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 ephemeral root volume, all default types of flavors,
m1.tiny, which is the smallest one, provide an additional
ephemeral block device sized between 20 and 160 GB (a configurable value
to suit an environment). 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. OpenStack Compute
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 specify any
other filesystem type by using
default_ephemeral_format configuration options.
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.
A persistent volume is represented by a persistent virtualized block device independent of any particular instance, and provided by OpenStack Block Storage.
Only a single configured instance can access a persistent volume. Multiple instances cannot access a persistent volume. 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 GlusterFS. 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 in the OpenStack Configuration Reference.
A persistent volume does not provide concurrent access from multiple instances. That type of configuration requires a traditional network file system like NFS, or CIFS, or a cluster file system such as GlusterFS. These systems can be built within an OpenStack cluster, or provisioned outside of it, but OpenStack software does not provide these features.
In addition to the native compute API, OpenStack provides an EC2-compatible API. This API allows EC2 legacy workflows built for EC2 to work with OpenStack.
Nova in tree EC2-compatible API is deprecated. The ec2-api project is working to implement the EC2 API.
You can use numerous third-party tools and language-specific SDKs to interact with OpenStack clouds. You can use both native and compatibility APIs. Some of the more popular third-party tools are:
In OpenStack the base operating system is usually copied from an image stored in the OpenStack Image service. 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 volume system. 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:
ACTIVEare available for use.
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 for
/etc/nova/policy.json on the
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 | +-----+-----------+-------+------+-----------+-------+-----------+
These basic categories describe the service architecture and information about the cloud controller.
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.
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 workers manage computing instances on host machines. The API dispatches commands to compute workers to complete these tasks:
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: