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Re: [libvirt] [PATCH] docs: add a page describing support guarantees for libvirt features



On 11/16/2017 10:19 AM, Daniel P. Berrange wrote:
While we have collective knowledge about the support status of various
parts of libvirt, this has never been formally documented, leaving our
users to guess.

Signed-off-by: Daniel P. Berrange <berrange redhat com>
---
  docs/docs.html.in    |   3 +
  docs/support.html.in | 257 +++++++++++++++++++++++++++++++++++++++++++++++++++
  2 files changed, 260 insertions(+)
  create mode 100644 docs/support.html.in

diff --git a/docs/docs.html.in b/docs/docs.html.in
index 931da85424..a8d544f83f 100644
--- a/docs/docs.html.in
+++ b/docs/docs.html.in
@@ -110,6 +110,9 @@
          <dt><a href="drivers.html">Drivers</a></dt>
          <dd>Hypervisor specific driver information</dd>
+ <dt><a href="support.html">Support guarantees</a></dt>
+        <dd>Details of support status for various interfaces</dd>
+
          <dt><a href="hvsupport.html">Driver support</a></dt>
          <dd>matrix of API support per hypervisor per release</dd>
diff --git a/docs/support.html.in b/docs/support.html.in
new file mode 100644
index 0000000000..6f34a35eb7
--- /dev/null
+++ b/docs/support.html.in
@@ -0,0 +1,257 @@
+<?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
+      adminstrators. The intent is to help users understand what features they

administrators

+      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>
+      not guranteed to be preserved forever. The project reserves the right to remove

extra 'not'

+      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 unforseen interactions with libvirt's

unforeseen

+      operation.
+    </p>
+
+    <h2><a id="apierrors">Error reporting</a></h2>
+
+    <p>
+      Almost API calls are subject to failure and so will report error codes and

s/Almost/Most/

+      messages. Libvirt defines error codes for a wide variety of scenarios, some
+      are very targetted to specific problems, while others are general purpose

targeted ? Or is this like tunnelled vs tunneled :-).

+      for broad classes of problem. Over time the error codes reported are liable
+      to, usually changing from a generic error to a more specific error. Thus

s/liable to,/liable to change,/

+      applications should be careful about checking for &amp; taking action upon
+      specific error codes, as the behaviour is liable to 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 &amp; 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. An deployments that expects a particular value for a

s/An deployments/A deployment/

+      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 no supported. Likewise libvirt will

s/no/not/

+      make reasonable efforts to keep API calls working across hypervisor
+      releases even if the underlying implementation changes. In cases where
+      this is impossible, an 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 implementation that are liable to change

s/implementation/implementations/ ?

+      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
+      hypevisor 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>

Reviewed-by: Jim Fehlig <jfehlig suse com>

Regards,
Jim


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