openstack-manuals/doc/admin-guide/source/compute-arch.rst

===================
System architecture
===================

OpenStack Compute contains several main components.

-  The :term:`cloud controller` represents the global state and interacts with
   the other components. The ``API server`` acts as the web services
   front end for the cloud controller. The ``compute controller``
   provides compute server resources and usually also contains the
   Compute service.

-  The ``object store`` is an optional component that provides storage
   services; you can also use OpenStack Object Storage instead.

-  An ``auth manager`` provides authentication and authorization
   services when used with the Compute system; you can also use
   OpenStack Identity as a separate authentication service instead.

-  A ``volume controller`` provides fast and permanent block-level
   storage for the compute servers.

-  The ``network controller`` provides virtual networks to enable
   compute servers to interact with each other and with the public
   network. You can also use OpenStack Networking instead.

-  The ``scheduler`` is 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
received.

Hypervisors
~~~~~~~~~~~
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
<https://docs.openstack.org/developer/nova/support-matrix.html>`_.

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

-  `Baremetal <https://wiki.openstack.org/wiki/Ironic>`__

-  `Docker <https://www.docker.io>`__

-  `Hyper-V <http://www.microsoft.com/en-us/server-cloud/hyper-v-server/default.aspx>`__

-  `Kernel-based Virtual Machine
   (KVM) <http://www.linux-kvm.org/page/Main_Page>`__

-  `Linux Containers (LXC) <https://linuxcontainers.org/>`__

-  `Quick Emulator (QEMU) <http://wiki.qemu.org/Manual>`__

-  `User Mode Linux (UML) <http://user-mode-linux.sourceforge.net/>`__

-  `VMware
   vSphere <http://www.vmware.com/products/vsphere-hypervisor/support.html>`__

-  `Xen <http://www.xen.org/support/documentation.html>`__

For more information about hypervisors, see the
`Hypervisors <https://docs.openstack.org/ocata/config-reference/compute/hypervisors.html>`__
section in the OpenStack Configuration Reference.

Projects, users, and roles
~~~~~~~~~~~~~~~~~~~~~~~~~~
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:

-  Number of volumes that can be launched.

-  Number of processor cores and the amount of RAM that can be
   allocated.

-  Floating IP addresses assigned to any instance when it launches. This
   allows instances to have the same publicly accessible IP addresses.

-  Fixed IP addresses assigned to the same instance when it launches.
   This allows instances to have the same publicly or privately
   accessible IP addresses.

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.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.

.. 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: ephemeral storage
and persistent volume.

**Ephemeral storage**

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,
except ``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 ``virt_mkfs`` or
``default_ephemeral_format`` configuration options.

.. 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.

**Persistent volume**

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 <https://docs.openstack.org/ocata/config-reference/block-storage/block-storage-overview.html>`_
in the OpenStack Configuration Reference.

.. note::

   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.

EC2 compatibility API
~~~~~~~~~~~~~~~~~~~~~
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.

.. warning::

   Nova in tree EC2-compatible API is deprecated.
   The `ec2-api project <https://git.openstack.org/cgit/openstack/ec2-api/>`_
   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:

Euca2ools
    A popular open source command-line tool for interacting with the EC2
    API. This is convenient for multi-cloud environments where EC2 is
    the common API, or for transitioning from EC2-based clouds to
    OpenStack. For more information, see the `Eucalyptus
    Documentation <http://docs.hpcloud.com/eucalyptus>`__.

Hybridfox
    A Firefox browser add-on that provides a graphical interface to many
    popular public and private cloud technologies, including OpenStack.
    For more information, see the `hybridfox
    site <http://code.google.com/p/hybridfox/>`__.

boto
    Python library for interacting with Amazon Web Services. You can use
    this library to access OpenStack through the EC2 compatibility API.
    For more     information, see the `boto project page on
    GitHub <https://github.com/boto/boto>`__.

fog
    A Ruby cloud services library. It provides methods to interact
    with a large number of cloud and virtualization platforms, including
    OpenStack. For more information, see the `fog
    site <https://rubygems.org/gems/fog>`__.

php-opencloud
    A PHP SDK designed to work with most OpenStack-based cloud
    deployments, as well as Rackspace public cloud. For more
    information, see the `php-opencloud
    site <http://www.php-opencloud.com>`__.

Building blocks
~~~~~~~~~~~~~~~
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:

.. code-block:: console

   $ 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 for ``compute_extension:flavormanage`` in
``/etc/nova/policy.json`` on the ``compute-api`` server.

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

.. code-block:: console

   $ 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      |
   +-----+-----------+-------+------+-----------+-------+-----------+

Compute 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