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<h1>Terminology and goals</h1>
<p>To avoid ambiguity about the terms used, here are the definitions
for some of the specific concepts used in libvirt documentation:</p>
<ul>
<li>a <strong>node</strong> is a single physical machine</li>
<li>an <strong>hypervisor</strong> is a layer of software allowing to
virtualize a node in a set of virtual machines with possibly different
configurations than the node itself</li>
<li>a <strong>domain</strong> is an instance of an operating system
(or subsystem in the case of container virtualization) running on a
virtualized machine provided by the hypervisor</li>
</ul>
<p class="image">
<img alt="Hypervisor and domains running on a node" src="node.gif"/>
</p>
<p>Now we can define the goal of libvirt: <b> to provide a common and
stable layer sufficient to securely manage domains on a node, possibly
remote</b>.</p>
<p> As a result, libvirt should provide all APIs needed to do the
management, such as: provision, create, modify, monitor, control, migrate
and stop the domains - within the limits of the support of the hypervisor
for those operations.
Not all hypervisors provide the same operations; but if an operation is
useful for domain management of even one specific hypervisor it is worth
providing in libvirt.
Multiple nodes
may be accessed with libvirt simultaneously, but the APIs are limited to
single node operations. Node resource operations which are needed
for the management and provisioning of domains are also in the scope of
the libvirt API, such as interface setup, firewall rules, storage management
and general provisioning APIs. Libvirt will also provide the state
monitoring APIs needed to implement management policies, obviously
checking domain state but also exposing local node resource consumption.
</p>
<p>This implies the following sub-goals:</p>
<ul>
<li>All API can be carried remotely though secure APIs</li>
<li>While most API will be generic in term of hypervisor or Host OS,
some API may be targeted to a single virtualization environment
as long as the semantic for the operations from a domain management
perspective is clear</li>
<li>the API should allow to do efficiently and cleanly all the operations
needed to manage domains on a node, including resource provisioning and
setup</li>
<li>the API will not try to provide high level virtualization policies or
multi-nodes management features like load balancing, but the API should be
sufficient so they can be implemented on top of libvirt</li>
<li>stability of the API is a big concern, libvirt should isolate
applications from the frequent changes expected at the lower level of the
virtualization framework</li>
<li>the node being managed may be on a different physical machine than
the management program using libvirt, to this effect libvirt supports
remote access, but should only do so by using secure protocols.</li>
<li>libvirt will provide APIs to enumerate, monitor and use the resources
available on the managed node, including CPUs, memory, storage, networking,
and NUMA partitions.</li>
</ul>
<p>So libvirt is intended to be a building block for higher level
management tools and for applications focusing on virtualization of a
single node (the only exception being domain migration between node
capabilities which involves more than one node).</p>
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