mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-12-26 23:55:23 +00:00
259 lines
13 KiB
XML
259 lines
13 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE html>
|
|
<html xmlns="http://www.w3.org/1999/xhtml">
|
|
<body>
|
|
<h1>Support guarantees</h1>
|
|
|
|
<ul id="toc"></ul>
|
|
|
|
<p>
|
|
This document will outline the support status / guarantees around the
|
|
very interfaces that libvirt exposes to applications and/or system
|
|
administrators. The intent is to help users understand what features they
|
|
can rely upon in particular scenarios, and whether they are likely to
|
|
suffer disruption during upgrades.
|
|
</p>
|
|
|
|
<h2><a id="publicAPI">Primary public API</a></h2>
|
|
|
|
<p>
|
|
The main public API provided by <code>libvirt.so</code> and described
|
|
in <code>libvirt/libvirt.h</code> exposes the primary hypervisor
|
|
agnostic management interface of libvirt. This API has the strongest
|
|
guarantee of any part of libvirt with a promise to keep backwards
|
|
compatibility forever. Specific details are as follows:
|
|
</p>
|
|
|
|
<dl>
|
|
<dt>Functions</dt>
|
|
<dd>Functions will never be removed from the public API, and will
|
|
never have parameters added, removed or changed in their signature.
|
|
IOW they will be ABI compatible forever. The semantics implied by
|
|
a specific set of parameters passed to the function will remain
|
|
unchanged. Where a parameter accepts a bitset of feature flags, or
|
|
an enumerated value, further flags / enum values may be supported
|
|
in the future. Where a parameter accepts one of a set of related
|
|
constants, further constants may be supported in the future.
|
|
</dd>
|
|
<dt>Struct types</dt>
|
|
<dd>Once defined in a release, struct definitions will never have any
|
|
fields add, removed or changed in any way. Their size and layout is
|
|
fixed forever. If a struct name starts with an underscore, it is
|
|
considered acceptable to rename it. Applications should thus always
|
|
use the corresponding typedef in preference to the struct name.
|
|
</dd>
|
|
<dt>Union types</dt>
|
|
<dd>Once defined in a release, union definitions will never have any
|
|
existing fields removed or changed. New union choices may be added,
|
|
provided that they don't change the size of the existing union
|
|
definition. If a struct name starts with an underscore, it is
|
|
considered acceptable to rename it. Applications should thus always
|
|
use the corresponding typedef in preference to the struct name.
|
|
</dd>
|
|
<dt>Type definitions</dt>
|
|
<dd>Most custom data types used in the APIs have corresponding typedefs
|
|
provided for their stable names. The typedefs should always be used
|
|
in preference to the underlying data type name, as the latter are not
|
|
guaranteed to be stable.
|
|
</dd>
|
|
<dt>Enumerations</dt>
|
|
<dd>Once defined in a release, existing enumeration values will never
|
|
be removed or renamed. New enumeration values may be introduced at
|
|
any time. Every enumeration will have a '_LAST' value which indicates
|
|
the current highest enumeration value, which may increase with new
|
|
releases. If an enumeration name starts with an underscore, it is
|
|
considered acceptable to rename it. Applications should thus always
|
|
use the corresponding typedef in preference to the enum name.
|
|
</dd>
|
|
<dt>Constants</dt>
|
|
<dd>Once defined in a release, existing constants will never be removed
|
|
or have their value changed. Most constants are grouped into related
|
|
sets, and within each set, new constants may be introduced. APIs which
|
|
use the constants may thus accept or return new constant values over
|
|
time.
|
|
</dd>
|
|
<dt>Symbol versions</dt>
|
|
<dd>Where the platform library format permits, APIs defined in libvirt.so
|
|
library will have version information associated. Each API will be
|
|
tagged with the version in which it was introduced, and this won't
|
|
be changed thereafter.
|
|
</dd>
|
|
</dl>
|
|
|
|
<h2><a id="hvAPI">Hypervisor specific APIs</a></h2>
|
|
|
|
<p>
|
|
A number of hypervisor drivers provide additional libraries with hypervisor
|
|
specific APIs, extending the core libvirt API. These add-on libraries follow
|
|
the same general principles described above, however, they are <strong>not</strong>
|
|
guaranteed to be preserved forever. The project reserves the right to remove
|
|
hypervisor specific APIs in any new release, or to change their semantics.
|
|
That said the project will endeavour to maintain API compatibility for as long
|
|
as is practical.
|
|
</p>
|
|
|
|
<p>
|
|
Use of some hypervisor specific APIs may result in the running guest being
|
|
marked as "tainted" if the API is at risk of having unexpected interactions
|
|
with normal libvirt operations. An application which chooses to make use of
|
|
hypervisor specific APIs should validate their operation with each new release
|
|
of libvirt and each new release of the underlying hypervisor. The semantics
|
|
may change in unexpected ways, or have unforeseen interactions with libvirt's
|
|
operation.
|
|
</p>
|
|
|
|
<h2><a id="apierrors">Error reporting</a></h2>
|
|
|
|
<p>
|
|
Most API calls are subject to failure and so will report error codes and
|
|
messages. Libvirt defines error codes for a wide variety of scenarios, some
|
|
represent very specific problems, while others are general purpose for
|
|
broad classes of problem. Over time the error codes reported are liable
|
|
to change, usually changing from a generic error to a more specific error.
|
|
Thus applications should be careful about checking for & taking action
|
|
upon specific error codes, as their behaviour may change across releases.
|
|
</p>
|
|
|
|
<h2><a id="xmlschema">XML schemas</a></h2>
|
|
|
|
<p>
|
|
The main objects exposed via the primary libvirt public API are usually
|
|
configured via XML documents following specific schemas. The XML schemas
|
|
are considered to be stable formats, whose compatibility will be maintained
|
|
forever. Specific details are as follows:
|
|
</p>
|
|
|
|
<dl>
|
|
<dt>Attributes</dt>
|
|
<dd>Attributes defined on an XML element will never be removed or
|
|
renamed. New attributes may be defined. If the set of valid values
|
|
for an attribute are determined by an enumeration, the permitted
|
|
values will never be removed or renamed, only new values defined.
|
|
None the less, specific hypervisors may reject usage of certain
|
|
values according to their feature set.
|
|
</dd>
|
|
<dt>Elements</dt>
|
|
<dd>Elements defined will never be removed or renamed. New child
|
|
elements may be defined at any time. In places where only a
|
|
single instance of a named XML element is used, future versions
|
|
may be extended to permit multiple instances of the named XML
|
|
element to be used. An element which currently has no content
|
|
may later gain child elements.
|
|
</dd>
|
|
</dl>
|
|
|
|
<p>
|
|
Some hypervisor drivers may choose to allow use of hypervisor specific
|
|
extensions to the XML documents. These extensions will always be
|
|
contained within a hypervisor specific XML namespace. There is generally
|
|
no guarantee of long term support for the hypervisor specific extensions
|
|
across releases, though the project will endeavour to preserve them as
|
|
long as is possible. Applications choosing to use hypervisor specific
|
|
extensions should validate their operation against new libvirt or
|
|
hypervisor releases.
|
|
</p>
|
|
|
|
<h2><a id="configfiles">Configuration files</a></h2>
|
|
|
|
<p>
|
|
A number of programs / daemons provided libvirt rely on host filesystem
|
|
configuration files. These configuration files are accompanied by augeas
|
|
lens for easy manipulation by applications. There is in general no
|
|
guarantee that parameters available in the configuration file will be
|
|
preserved across releases, though the project will endeavour to preserve
|
|
them as long as is possible. If a configuration option is dropped from
|
|
the file, the augeas lens will retain the ability to read that configuration
|
|
parameter, so that it is able to read & update historically modified
|
|
files.
|
|
|
|
The default configuration files ship with all parameters commented out
|
|
such that a deployment relies on the built-in defaults of the application
|
|
in question. There is no guarantee that the defaults will remain the same
|
|
across releases. A deployment that expects a particular value for a
|
|
configuration parameter should consider defining it explicitly, instead
|
|
of relying on the defaults.
|
|
</p>
|
|
|
|
<h2><a id="hvdrivers">Hypervisor drivers</a></h2>
|
|
|
|
<p>
|
|
The libvirt project provides support for a wide variety of hypervisor
|
|
drivers. These drivers target certain versions of the hypervisor's
|
|
underlying management APIs. In general libvirt aims to work with any
|
|
hypervisor version that is still broadly supported by its vendor.
|
|
When a vendor discontinues support for a particular hypervisor
|
|
version it will be dropped by libvirt. Libvirt may choose to drop
|
|
support for a particular hypervisor version prior to the vendor
|
|
ending support, if it deems that the likely usage is too small to
|
|
justify the ongoing maintenance cost.
|
|
</p>
|
|
<p>
|
|
Each hypervisor release will implement a distinct subset of features
|
|
that can be expressed in the libvirt APIs and XML formats. While the
|
|
XML schema syntax will be stable across releases, libvirt is unable
|
|
to promise that it will always be able to support usage of the same
|
|
features across hypervisor releases. Where a hypervisor changes the
|
|
way a feature is implemented, the project will endeavour to adapt
|
|
to the new implementation to provide the same semantics. In cases
|
|
where the feature is discontinued by the hypervisor, libvirt will
|
|
return an error indicating it is not supported. Likewise libvirt will
|
|
make reasonable efforts to keep API calls working across hypervisor
|
|
releases even if the underlying implementation changes. In cases where
|
|
this is impossible, a suitable error will be reported. The list of
|
|
APIs which have implementations <a href="hvsupport.html">is detailed separately</a>.
|
|
</p>
|
|
|
|
<h2><a id="rpcproto">RPC protocol</a></h2>
|
|
|
|
<p>
|
|
For some hypervisor drivers, the libvirt.so library communicates with
|
|
separate libvirt daemons to perform work. This communication takes
|
|
place over a binary RPC protocol defined by libvirt. The protocol uses
|
|
the XDR format for data encoding, and the message packet format is
|
|
defined in libvirt source code.
|
|
</p>
|
|
<p>
|
|
Applications are encouraged to use the primary libvirt.so library which
|
|
transparently talks to the daemons, so that they are not exposed to the
|
|
hypervisor driver specific details. None the less, the RPC protocol
|
|
associated with the libvirtd is considered to be a long term stable ABI.
|
|
It will only ever have new messages added to it, existing messages will
|
|
not be removed, nor have their contents changed. Thus if an application
|
|
does wish to provide its own client side implementation of the RPC
|
|
protocol this is supported, with the caveat that the application will
|
|
loose the ability to work with certain hypervisors libvirt supports.
|
|
The project reserves the right to define new authentication and encryption
|
|
options for the protocol, and the defaults used in this area may change
|
|
over time. This is particularly true of the TLS ciphers permitted. Thus
|
|
applications choosing to implement the RPC protocol must be prepared to
|
|
track support for new security options. If defaults are changed, however,
|
|
it will generally be possible to reconfigure the daemon to use the old
|
|
defaults, albeit with possible implications for system security.
|
|
</p>
|
|
|
|
<p>
|
|
Other daemons besides, libvirtd, also use the same RPC protocol, but
|
|
with different message types defined. These RPC protocols are all
|
|
considered to be private implementations that are liable to change
|
|
at any time. Applications must not attempt to talk to these other
|
|
daemons directly.
|
|
</p>
|
|
|
|
<h2><a id="virsh">virsh client</a></h2>
|
|
|
|
<p>
|
|
The virsh program provides a simple client to interact with an arbitrary libvirt
|
|
hypervisor connection. Since it uses the primary public API of libvirt, it should
|
|
generally inherit the guarantees associated with that API, and with the hypervisor
|
|
driver. The commands that virsh exposes, and the arguments they accept are all
|
|
considered to be long term stable. Existing commands and arguments will not be
|
|
removed or renamed. New commands and arguments may be added in new releases.
|
|
The text output format produced by virsh commands is not generally guaranteed to
|
|
be stable if it contains compound data (eg formatted tables or lists). Commands
|
|
which output single data items (ie an object name, or an XML document), can be
|
|
treated as having stable format.
|
|
</p>
|
|
|
|
</body>
|
|
</html>
|