openstack-manuals/doc/common/get-started-image-service.rst
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======================
Image service overview
======================
The Image service (glance) enables users to discover,
register, and retrieve virtual machine images. It offers a
:term:`REST <RESTful>` API that enables you to query virtual
machine image metadata and retrieve an actual image.
You can store virtual machine images made available through
the Image service in a variety of locations, from simple file
systems to object-storage systems like OpenStack Object Storage.
.. important::
For simplicity, this guide describes configuring the Image service to
use the ``file`` back end, which uploads and stores in a
directory on the controller node hosting the Image service. By
default, this directory is ``/var/lib/glance/images/``.
Before you proceed, ensure that the controller node has at least
several gigabytes of space available in this directory. Keep in
mind that since the ``file`` back end is often local to a controller
node, it is not typically suitable for a multi-node glance deployment.
For information on requirements for other back ends, see
`Configuration Reference
<https://docs.openstack.org/newton/config-reference/image.html>`__.
The OpenStack Image service is central to Infrastructure-as-a-Service
(IaaS) as shown in :ref:`get_started_conceptual_architecture`. It accepts API
requests for disk or server images, and metadata definitions from end users or
OpenStack Compute components. It also supports the storage of disk or server
images on various repository types, including OpenStack Object Storage.
A number of periodic processes run on the OpenStack Image service to
support caching. Replication services ensure consistency and
availability through the cluster. Other periodic processes include
auditors, updaters, and reapers.
The OpenStack Image service includes the following components:
glance-api
Accepts Image API calls for image discovery, retrieval, and storage.
glance-registry
Stores, processes, and retrieves metadata about images. Metadata
includes items such as size and type.
.. warning::
The registry is a private internal service meant for use by
OpenStack Image service. Do not expose this service to users.
Database
Stores image metadata and you can choose your database depending on
your preference. Most deployments use MySQL or SQLite.
Storage repository for image files
Various repository types are supported including normal file
systems (or any filesystem mounted on the glance-api controller
node), Object Storage, RADOS block devices, VMware datastore,
and HTTP. Note that some repositories will only support read-only
usage.
Metadata definition service
A common API for vendors, admins, services, and users to meaningfully
define their own custom metadata. This metadata can be used on
different types of resources like images, artifacts, volumes,
flavors, and aggregates. A definition includes the new property's key,
description, constraints, and the resource types which it can be
associated with.