2017-11-16 17:11:55 +00:00
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Support guarantees</h1>
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<ul id="toc"></ul>
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<p>
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This document will outline the support status / guarantees around the
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very interfaces that libvirt exposes to applications and/or system
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administrators. The intent is to help users understand what features they
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can rely upon in particular scenarios, and whether they are likely to
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suffer disruption during upgrades.
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</p>
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<h2><a id="publicAPI">Primary public API</a></h2>
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<p>
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The main public API provided by <code>libvirt.so</code> and described
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in <code>libvirt/libvirt.h</code> exposes the primary hypervisor
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agnostic management interface of libvirt. This API has the strongest
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guarantee of any part of libvirt with a promise to keep backwards
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compatibility forever. Specific details are as follows:
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</p>
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<dl>
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<dt>Functions</dt>
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<dd>Functions will never be removed from the public API, and will
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never have parameters added, removed or changed in their signature.
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IOW they will be ABI compatible forever. The semantics implied by
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a specific set of parameters passed to the function will remain
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unchanged. Where a parameter accepts a bitset of feature flags, or
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an enumerated value, further flags / enum values may be supported
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in the future. Where a parameter accepts one of a set of related
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constants, further constants may be supported in the future.
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</dd>
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<dt>Struct types</dt>
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<dd>Once defined in a release, struct definitions will never have any
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fields add, removed or changed in any way. Their size and layout is
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fixed forever. If a struct name starts with an underscore, it is
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considered acceptable to rename it. Applications should thus always
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use the corresponding typedef in preference to the struct name.
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</dd>
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<dt>Union types</dt>
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<dd>Once defined in a release, union definitions will never have any
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existing fields removed or changed. New union choices may be added,
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provided that they don't change the size of the existing union
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definition. If a struct name starts with an underscore, it is
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considered acceptable to rename it. Applications should thus always
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use the corresponding typedef in preference to the struct name.
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</dd>
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<dt>Type definitions</dt>
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<dd>Most custom data types used in the APIs have corresponding typedefs
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provided for their stable names. The typedefs should always be used
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in preference to the underlying data type name, as the latter are not
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guaranteed to be stable.
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</dd>
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<dt>Enumerations</dt>
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<dd>Once defined in a release, existing enumeration values will never
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be removed or renamed. New enumeration values may be introduced at
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any time. Every enumeration will have a '_LAST' value which indicates
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the current highest enumeration value, which may increase with new
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releases. If an enumeration name starts with an underscore, it is
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considered acceptable to rename it. Applications should thus always
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use the corresponding typedef in preference to the enum name.
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</dd>
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<dt>Constants</dt>
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<dd>Once defined in a release, existing constants will never be removed
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or have their value changed. Most constants are grouped into related
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sets, and within each set, new constants may be introduced. APIs which
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use the constants may thus accept or return new constant values over
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time.
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</dd>
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<dt>Symbol versions</dt>
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<dd>Where the platform library format permits, APIs defined in libvirt.so
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library will have version information associated. Each API will be
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tagged with the version in which it was introduced, and this won't
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be changed thereafter.
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</dd>
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</dl>
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<h2><a id="hvAPI">Hypervisor specific APIs</a></h2>
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<p>
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A number of hypervisor drivers provide additional libraries with hypervisor
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specific APIs, extending the core libvirt API. These add-on libraries follow
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the same general principles described above, however, they are <strong>not</strong>
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guaranteed to be preserved forever. The project reserves the right to remove
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hypervisor specific APIs in any new release, or to change their semantics.
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That said the project will endeavour to maintain API compatibility for as long
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as is practical.
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</p>
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<p>
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Use of some hypervisor specific APIs may result in the running guest being
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marked as "tainted" if the API is at risk of having unexpected interactions
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with normal libvirt operations. An application which chooses to make use of
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hypervisor specific APIs should validate their operation with each new release
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of libvirt and each new release of the underlying hypervisor. The semantics
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may change in unexpected ways, or have unforeseen interactions with libvirt's
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operation.
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</p>
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<h2><a id="apierrors">Error reporting</a></h2>
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<p>
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Most API calls are subject to failure and so will report error codes and
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messages. Libvirt defines error codes for a wide variety of scenarios, some
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represent very specific problems, while others are general purpose for
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broad classes of problem. Over time the error codes reported are liable
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to change, usually changing from a generic error to a more specific error.
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Thus applications should be careful about checking for & taking action
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upon specific error codes, as their behaviour may change across releases.
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</p>
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<h2><a id="xmlschema">XML schemas</a></h2>
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<p>
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The main objects exposed via the primary libvirt public API are usually
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configured via XML documents following specific schemas. The XML schemas
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are considered to be stable formats, whose compatibility will be maintained
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forever. Specific details are as follows:
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</p>
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<dl>
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<dt>Attributes</dt>
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<dd>Attributes defined on an XML element will never be removed or
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renamed. New attributes may be defined. If the set of valid values
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for an attribute are determined by an enumeration, the permitted
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values will never be removed or renamed, only new values defined.
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None the less, specific hypervisors may reject usage of certain
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values according to their feature set.
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</dd>
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<dt>Elements</dt>
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<dd>Elements defined will never be removed or renamed. New child
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elements may be defined at any time. In places where only a
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single instance of a named XML element is used, future versions
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may be extended to permit multiple instances of the named XML
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element to be used. An element which currently has no content
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may later gain child elements.
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</dd>
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</dl>
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<p>
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Some hypervisor drivers may choose to allow use of hypervisor specific
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extensions to the XML documents. These extensions will always be
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contained within a hypervisor specific XML namespace. There is generally
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no guarantee of long term support for the hypervisor specific extensions
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across releases, though the project will endeavour to preserve them as
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long as is possible. Applications choosing to use hypervisor specific
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extensions should validate their operation against new libvirt or
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hypervisor releases.
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</p>
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<h2><a id="configfiles">Configuration files</a></h2>
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<p>
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A number of programs / daemons provided libvirt rely on host filesystem
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configuration files. These configuration files are accompanied by augeas
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lens for easy manipulation by applications. There is in general no
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guarantee that parameters available in the configuration file will be
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preserved across releases, though the project will endeavour to preserve
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them as long as is possible. If a configuration option is dropped from
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the file, the augeas lens will retain the ability to read that configuration
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parameter, so that it is able to read & update historically modified
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files.
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The default configuration files ship with all parameters commented out
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such that a deployment relies on the built-in defaults of the application
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in question. There is no guarantee that the defaults will remain the same
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across releases. A deployment that expects a particular value for a
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configuration parameter should consider defining it explicitly, instead
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of relying on the defaults.
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</p>
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<h2><a id="hvdrivers">Hypervisor drivers</a></h2>
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<p>
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The libvirt project provides support for a wide variety of hypervisor
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drivers. These drivers target certain versions of the hypervisor's
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underlying management APIs. In general libvirt aims to work with any
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hypervisor version that is still broadly supported by its vendor.
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When a vendor discontinues support for a particular hypervisor
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version it will be dropped by libvirt. Libvirt may choose to drop
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support for a particular hypervisor version prior to the vendor
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ending support, if it deems that the likely usage is too small to
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justify the ongoing maintenance cost.
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</p>
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<p>
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Each hypervisor release will implement a distinct subset of features
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that can be expressed in the libvirt APIs and XML formats. While the
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XML schema syntax will be stable across releases, libvirt is unable
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to promise that it will always be able to support usage of the same
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features across hypervisor releases. Where a hypervisor changes the
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way a feature is implemented, the project will endeavour to adapt
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to the new implementation to provide the same semantics. In cases
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where the feature is discontinued by the hypervisor, libvirt will
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return an error indicating it is not supported. Likewise libvirt will
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make reasonable efforts to keep API calls working across hypervisor
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releases even if the underlying implementation changes. In cases where
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2018-05-03 11:14:19 +00:00
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this is impossible, a suitable error will be reported. The list of
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2017-11-16 17:11:55 +00:00
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APIs which have implementations <a href="hvsupport.html">is detailed separately</a>.
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</p>
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<h2><a id="rpcproto">RPC protocol</a></h2>
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<p>
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For some hypervisor drivers, the libvirt.so library communicates with
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separate libvirt daemons to perform work. This communication takes
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place over a binary RPC protocol defined by libvirt. The protocol uses
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the XDR format for data encoding, and the message packet format is
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defined in libvirt source code.
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</p>
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<p>
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Applications are encouraged to use the primary libvirt.so library which
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transparently talks to the daemons, so that they are not exposed to the
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hypervisor driver specific details. None the less, the RPC protocol
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associated with the libvirtd is considered to be a long term stable ABI.
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It will only ever have new messages added to it, existing messages will
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not be removed, nor have their contents changed. Thus if an application
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does wish to provide its own client side implementation of the RPC
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protocol this is supported, with the caveat that the application will
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loose the ability to work with certain hypervisors libvirt supports.
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The project reserves the right to define new authentication and encryption
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options for the protocol, and the defaults used in this area may change
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over time. This is particularly true of the TLS ciphers permitted. Thus
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applications choosing to implement the RPC protocol must be prepared to
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track support for new security options. If defaults are changed, however,
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it will generally be possible to reconfigure the daemon to use the old
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defaults, albeit with possible implications for system security.
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</p>
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<p>
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Other daemons besides, libvirtd, also use the same RPC protocol, but
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with different message types defined. These RPC protocols are all
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considered to be private implementations that are liable to change
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at any time. Applications must not attempt to talk to these other
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daemons directly.
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</p>
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<h2><a id="virsh">virsh client</a></h2>
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<p>
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The virsh program provides a simple client to interact with an arbitrary libvirt
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hypervisor connection. Since it uses the primary public API of libvirt, it should
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generally inherit the guarantees associated with that API, and with the hypervisor
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driver. The commands that virsh exposes, and the arguments they accept are all
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considered to be long term stable. Existing commands and arguments will not be
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removed or renamed. New commands and arguments may be added in new releases.
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The text output format produced by virsh commands is not generally guaranteed to
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be stable if it contains compound data (eg formatted tables or lists). Commands
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which output single data items (ie an object name, or an XML document), can be
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treated as having stable format.
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</p>
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</body>
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</html>
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