Vulkan-Docs/doc/specs/vulkan/style/extensions.txt

// Copyright (c) 2015-2016 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html

[[extensions]]
= Layers & Extensions

This chapter describes required and recommended processes for writing formal
extensions and layers for the Vulkan API. It is concerned with processes and
registration, while fine-grained naming conventions are included in the
<<naming,API Naming Conventions chapter>>. Prior to revision 1.0.19 of the
<<Vulkan API Specification>>, most of the content in this chapter existed as
part of Appendix C of that document.

[NOTE]
.Note
====
The mechanism and process of specifying extensions is subject to change, as
we receive feedback from authors and further requirements of documentation
tooling. This document will be updated as changes are made.
====


== Introduction

The Khronos extension registries and extension naming conventions serve
several purposes:

  * Avoiding naming collisions between extensions developed by mutually
    unaware parties, both in the extension names themselves, as well as
    their token, command, and type names.
  * Allocating enumerant values for tokens added by extensions
  * Creating a defined order between extensions. Extensions with higher
    numbers may have dependencies upon extensions with lower numbers, and
    must define any relevant interactions with lower-numbered extensions.
  * Provides a central repository for documentation and header changes
    associated with extensions

Vulkan's design and general software development trends introduces two
new paradigms that require rethinking the existing mechanisms:

  * Layers, and with them a focus on a more open ecosystem where non-Khronos
    members are expected to extend a Khronos API using the Layer mechanism.
  * Namespaced constants (enumerations) that do not necessarily draw from a
    single global set of token values.


== General Rules/Guidelines

Some general rules to simplify the specific rules below:

  * Extensions and layers must each have a globally unique name.
  * All commands and tokens must have a globally unique name.
  * Extensions can expose new commands, types, and/or tokens, but layers
    must not.
  ** However, layers can expose their own extensions, which in turn are
     allowed to expose new commands and tokens.
  * All extensions must be registered with Khronos.
  * Extensions must be strictly additive and backwards-compatible. That is,
    extensions must not remove existing functionality, or change existing
    default behaviors. A Vulkan implementation may support any
    combination of extensions, but applications written using only the core
    API, or a subset of the supported extensions, must continue to work in
    such an implementation without changes in behavior.

[[extensions-naming-conventions]]
== Extension and Layer Naming Conventions

Extensions and layers have formal _names_. These names are used in a variety
of places:

  * When specifying extensions and layers to enable in the API.
  * As a preprocessor symbol in the +vulkan.h+ header file indicating that
    an extension interface is defined at compile time.
  * To control building the Vulkan Specification from asciidoc source
    containing many extension, by explicitly enabling inclusion of one or
    more extensions.

[NOTE]
.Note
====
In version 1.0.24 and earlier versions of the Vulkan Specification,
extensions were maintained in separate git branches from the core
Specification source. This proved to be unwieldy and difficult to maintain
as the core was repeatedly updated, and as of version 1.0.25, we have moved
to a ``single-branch'' model in which most extensions can optionally be
included or not from a single set of source documents.

It is possible that some vendors will choose to continue maintaining their
extension branches, and the existing extension branches will not be removed
from Github.
====

There is a rigid syntax for these names:

  * Extensions are named with the syntax: +VK_AUTHOR_<name>+.
  * Layers are named with the syntax: +VK_LAYER_{AUTHOR|FQDN}_<name>+.

Both extensions and layer names include a +VK_+ prefix, as described in the
<<naming-preprocessor,Preprocessor Defines>> section above. In addition,
layers add a +LAYER_+ prefix.

Extension and layer names must both be valid C language identifiers.


[[extensions-naming-conventions-name-strings]]
=== Extension and Layer Name Strings

The +<name>+ portion of extension and layer names is a concise name
describing the purpose or functionality of the extension or layer. The
underscore (`_`) character is used as a delimiter between words. Every
character of the name must be in lower case.


=== Author IDs

Extension and layer names also contain an _author ID_, indicated by +AUTHOR+
above, identifying the author of the extension/layer. This ID is a short,
capitalized string identifying an author, such as a Khronos member
developing Vulkan implementations for their devices, or a non-Khronos
developer creating Vulkan layers. Author IDs must be registered with
Khronos.

Some authors have platform communities they wish to distinguish between, and
can register additional author IDs for that purpose. For example, Google
has separate Android and Chrome communities.

Details on how to register an author ID are provided below. Layer authors
not wishing to register an author ID with Khronos can instead use a
fully-qualified domain name (FQDN) as the ID. The FQDN should be a domain
name owned by the author. FQDNs cannot be used for extensions, only for
layers.

  * The following are examples of extension and layer names, demonstrating
    the above syntax:
  ** Extension names all use the base prefix +VK_+.
  ** Khronos-ratified extensions add the reserved author ID +KHR+, and
     will use the prefix +VK_KHR_+.
  ** The following author IDs are reserved and must not be used:
  *** +VK+ - To avoid confusion with the top-level +VK_+ prefix.
  *** +VULKAN+ - To avoid confusion with the name of the Vulkan API.
  *** +LAYER+ - To avoid confusion with the higher-level ``LAYER'' prefix.
  *** +KHRONOS+ - To avoid confusion with the Khronos organization.
  ** Multi-author extensions that have not been ratified by Khronos (those
     developed via cooperation between, and intended to be supported by two
     or more registered authors) add the special author ID +EXT+ to the base
     prefix, and will use the prefix +VK_EXT_+.
  ** Traditional author-specific extensions developed by one author (or one
     author in cooperation with non-authors) add the author ID to the
     base prefix. For example, NVIDIA will use the prefix +VK_NV_+, and
     Valve will use the prefix +VK_VALVE_+. Some authors can have
     additional registered author IDs for special purposes. For
     example, an Android extension developed by Google - but part of an
     Android open-source community project, and so not a proprietary Google
     extension - will use the author ID +ANDROID+.
  ** Layer names follow the same conventions as extensions, but use the base
     prefix +VK_LAYER_+.
  ** Because layers need not be registered with Khronos, an alternative
     mechanism is needed to allow creating unique layer names without
     registering an author ID. Layer authors that prefer not to register an
     author ID can instead use a fully-qualified domain name (FQDN) in
     reverse-order as an author ID, replacing +.+ (period) with `_`
     (underscore) characters. The restriction that layer names must be valid
     C identifiers means that some FQDNs cannot be used as part of layer
     names.

[source, c]
.Example
----
// Khronos extension name
VK_KHR_mirror_clamp_to_edge

// Multivendor extension name
VK_EXT_debug_marker

// Vendor extension name using author ID NV
VK_NV_glsl_shader

// Vendor layer name using author ID LUNARG
VK_LAYER_LUNARG_vktrace

// Layer name using the FQDN www.3dxcl.invalid instead of an author ID
VK_LAYER_invalid_3dxcl_www
----

[NOTE]
.Note
====
To avoid linking to a nonexistent domain, the reserved TLD +.invalid+ is
used in the example above.
====


[[extensions-naming]]
== Extension Command, Type, and Token Naming Conventions

Extensions may add new commands, types, and tokens, or collectively
``objects'', to the Vulkan API. These objects are given globally unique
names by appending the author ID defined above for the extension name
as described in the <<naming-extension-identifiers, Extension Identifier
Naming Conventions>> section above.


[[extensions-api-registry]]
== The Vulkan Registry

The canonical definition of the Vulkan APIs is kept in an XML file known
as the *Vulkan registry*. The registry is kept in +src/spec/vk.xml+ in
the branch of the vulkan project containing the most recently released core
API specification. The registry contains reserved author IDs, core and
extension interface definitions, definitions of individual commands and
structures, and other information which must be agreed on by all
implementations. The registry is used to maintain a single, consistent
global namespace for the registered entities, to generate the
Khronos-supplied +vulkan.h+, and to create a variety of related
documentation used in generating the API specification and reference pages.


[[extensions-author-ID]]
== Registering an Author ID with Khronos

Previous Khronos APIs could only officially be modified by Khronos members.
In an effort to build a more flexible platform, Vulkan allows non-Khronos
developers to extend and modify the API via layers and extensions in the
same manner as Khronos members. However, extensions must still be
registered with Khronos. A mechanism for non-members to register layers and
extensions is provided.

Extension authors will be able to create an account on Github and register
an author ID with Khronos through the +<<KhronosGroup/Vulkan-Docs>>+ project.
The author ID must be used for any extensions that author registers. The
same mechanism will be used to request registration of extensions or layers
with Khronos, as described below.

To reserve an author ID, propose a merge request against
<<extensions-api-registry,+vk.xml+>> in the +1.0+ branch. The merge must
add a +<tag>+ XML tag and fill in the +name+, +author+ and +contact+
attributes with the requested author ID, the author's formal name (e.g.
company or project name), and contact email address, respectively. The
author ID will only be reserved once this merge request is accepted.

Please do not try to reserve author IDs which clearly belong to another
existing company or software project which may wish to develop Vulkan
extensions or layers in the future, as a matter of courtesy and respect.
Khronos may decline to register author IDs that are not requested in good
faith.


[[extensions-vendor-id]]
== Registering a Vendor ID with Khronos

Vulkan implementers must report a valid vendor ID for their implementation
when queried by fname:vkGetPhysicalDeviceProperties, as described in the
``Devices and Queues'' section of the <<Vulkan API Specification>>. If
there is no valid PCI vendor ID defined for the physical device,
implementations must obtain a Khronos vendor ID.

Khronos vendor IDs are reserved in a similar fashion to
<<extensions-author-ID,author IDs>>. While vendor IDs are not
directly related to API extensions, the reservation process is very similar
and so is described in this section.

To reserve an Khronos vendor ID, you must first have a Khronos author ID.
Propose a merge request against <<extensions-api-registry,+vk.xml+>> in the
+1.0+ branch. The merge must add a +<vendorid>+ tag and fill in the +name+
and +id+ attributes. The +name+ attribute must be set to the author ID. The
+id+ attribute must be the first sequentially available ID in the list of
+<vendorid>+ tags. The vendor ID will be reserved only once this merge
request has been accepted.

Please do not try to reserve vendor IDs unless you are making a good faith
effort to develop a Vulkan implementation and require one for that
purpose.


== Registering Extensions and Layers

Extensions must be registered with Khronos. Layers may be registered, and
registration is strongly recommended. Registration means:

  * Receiving an extension number.
  * Adding the extension or layer name to the list in +vk.xml+ and appearing
    on the Khronos registry website, which will link to associated
    documentation hosted on Khronos.
  * For extensions which add to the Vulkan API, including definitions of
    those additions to +vk.xml+.

Registration for Khronos members is handled by filing a merge request in the
internal gitlab repository against the branch containing the core
specification against which the extension or layer will be written. The
merge must modify +vk.xml+ to define extension names, API interfaces, and
related information. Registration is not complete until the registry
maintainer has validated and accepted the merge.

Since this process could in principle be completely automated, this
suggests a scalable mechanism for accepting registration of non-Khronos
extensions. Non-Khronos members who want to create extensions must register
with Khronos by creating a github account, and registering their author
ID and/or FQDNs to that account. They can then submit new extension
registration requests by proposing merges to +vk.xml+. On acceptance of the
merge, the extension will be registered, though its specification need not
be checked into the Khronos github repository at that point.

The registration process can be split into several steps to accommodate
extension number assignment prior to extension publication:

  * Acquire an extension number. This is done by proposing a merge request
    against +vk.xml+ similarly to how <<extensions-author-ID,author
    IDs are reserved>>. The merge should add a new +<extension>+ tag
    at the end of the file with attributes specifying the proposed extension
    +name+, the next unused sequential extension +number+, the +author+ and
    +contact+ information (if different than that already specified for the
    author ID used in the extension name), and finally, specifying
    +supported="disabled"+. The extension number will be reserved only once
    this merge request is accepted into the +1.0+ branch.
  * Develop and test the extension using the registered extension number.
  * Publish the extension to Khronos using the previously registered
    extension number, by submitting merge requests to the +1.0+ branch
    defining the changes specific to the extension. Changes to both the
    specification source, and to +vk.xml+ will be needed.
  ** Extension changes to the specification source must be protected by
     asciidoc conditionals as described in the
     <<extensions-documenting,Documenting Extensions>> section.
  ** Changes to +vk.xml+ must define the extension interfaces in the
     +<extension>+ block, and must also change the +supported+ attribute
     value of the +<extension>+ to +supported="vulkan"+.
  ** When publishing an extension, mark it as enabled by proposing a merge
     to the +1.0+ branch changing the +supported+ attribute value of the
     +<extension>+ to +supported="vulkan"+. This should be completely
     automated and under the control of the publishers, to allow them to
     align publication on Khronos with product releases. However, complete
     automation might be difficult, since steps such as regenerating and
     validating +vulkan.h+ are involved. Once the merge is accepted and the
     corresponding updated header with the new extension interface is
     committed to the +1.0+ branch, publication is complete.
  ** Publishing on the <<KhronosGroup/Vulkan-Docs,Khronos public github
     repository>> is preferred whenever possible. Khronos members may
     instead create branches on Khronos' internal gitlab server, but those
     branches will eventually be mirrored to github.
  ** Once the merge request defining an extension has been accepted into the
     +1.0+ branch, publication is complete - although it may not be visible
     on Github until the next regular core Specification update is pushed
     out.
  * It is still possible to publish a separate branch of the repository with
    appropriate changes relative to the core Vulkan API branch instead, but
    this approach is deprecated and discouraged. If this is done, all
    changes to +vk.xml+ must still be made in the +1.0+ branch.

ifdef::editing-notes[]
[NOTE]
.editing-note
====
TODO: This section is subject to change and not complete yet, but in broad
is how we expect extension registration and specifications to work. The
process will be refined as members and outside authors define further
extensions.
====
endif::editing-notes[]


[[extensions-documenting]]
== Documenting Extensions

Extensions are documented as modifications to the Vulkan specification.
Changes specific to an extension are protected by asciidoc conditionals. The
changes are only visible in generated documentation when the Specification
is built with an asciidoc attribute of that name defined.

[NOTE]
.Note
====
This is referred to as the ``single-branch'' model, in contrast to the
earlier model where each extension lived in a separate branch from the +1.0+
core Specification source.
====

For example, the +VK_KHR_surface+ extension is now documented in the +1.0+
branch of the Github +<<KhronosGroup/Vulkan-Docs>>+ project. However,
specifications generated from this branch will only include the extension
when the Makefile is invoked appropriately.

Most language defining extensions can be localized either into a small
number of asciidoc include files which are conditionally included in the
core specification chapters and appendices, or into parts of +vk.xml+
defining the extension interfaces.

[NOTE]
.Note
====
We do not yet fully document an example of including an extension in the
single-branch model. Instead, authors should refer to the +1.0+ branch and
search for the names of existing extensions, such as +VK_KHR_surface+, as
examples. Some aspects of the changes for this example extension are
described below.
====

The WSI extensions were used to help prototype what changes need to be made
for extensions, which include (but may not be limited to) the following:

  * All extensions should add an appendix to the Vulkan specification. This
    can be modeled after what was done for the +VK_KHR_surface+ extension in
    +doc/specs/vulkan/appendices/VK_KHR_surface/wsi.txt+, which contains
    metainformation about the extension (as well as code examples, and
    revision history). This example is complicated because +VK_KHR_surface+
    is a base extension on which many window system-specific extensions are
    layered.
  * Each extension's appendix file is included by adding an +include+
    statement to the +doc/specs/vulkan/vkspec.txt+ file. The +include+
    statement must be protected by appropriate asciidoc conditionals. For
    example:
+
--
.Example Markup
----
\ifdef::VK_KHR_surface[]
\include::appendices/VK_KHR_surface/wsi.txt[]
\endif::VK_KHR_surface[]
----
--
  * If an extension is more of an addition to the Vulkan specification, the
    extension should add a chapter to the Vulkan specification. For example,
    in the case of the +VK_KHR_surface+ example a new chapter is added in
    +vkspec.txt+ by including at the end of the core Specification chapters:
+
--
.Example Markup
----
\ifdef::VK_KHR_surface[]
\include::chapters/VK_KHR_surface/wsi.txt[]
\endif::VK_KHR_surface[]
----
--
  * In every other place where the extension modifies the core
    Specification, modify the core files and protect the modifications with
    the same asciidoc conditionals. For example, +VK_KHR_surface+ adds new
    error codes to Vulkan. These are added to +chapters/fundamentals.txt+ in
    the ``Return Codes'' section as follows:
+
--
.Example Markup
----
... list of existing error codes
\ifdef::VK_KHR_surface[]
\include::VK_KHR_surface/VkResultErrorDescriptions_surface.txt[]
\endif::VK_KHR_surface[]
----
--
  * For vendor extensions, changes made to existing core Specification
    source files and to +vk.xml+ all fall under the Contributor License
    Agreement. Vendors may use their own copyright on new files they add to
    the +1.0+ branch, although that copyright must be compatible with the
    Specification copyright.


== Assigning Extension Token Values

Extensions can define their own enumeration types and assign any values to
their enumerants that they like. Each enumeration has a private namespace,
so collisions are not a problem. However, when extending existing
enumeration objects with new values, care must be taken to preserve global
uniqueness of values. Enumerations which define new bits in a bitmask are
treated specially as described in
<<extensions-reserving-bitmask-values,Reserving Bitmask Values>> below.

Each extension is assigned a range of values that can be used to create
globally-unique enum values. Most values will be negative numbers, but
positive numbers are also reserved. The ability to create both positive and
negative extension values is necessary to enable extending enumerations such
as etext:VkResult that assign special meaning to negative and positive
values. Therefore, 1000 positive and 1000 negative values are reserved for
each extension. Extensions must not define enum values outside their
reserved range without explicit permission from the owner of those values
(e.g. from the author of another extension whose range is infringed on, or
from the Khronos Registrar if the values do not belong to any extension's
range).

[NOTE]
.Note
====
Typically, extensions use a unique offset for each enumeration constant they
add, yielding 1000 distinct token values per extension. Since each
enumeration object has its own namespace, if an extension needs to add many
enumeration constant values, it can reuse offsets on a per-type basis.
====

The information needed to add new values to the XML are as follows:

  * The **extension name** (e.g. +VK_KHR_swapchain+) that is adding the new
    enumeration constant.
  * The existing enumeration **type** being extended (e.g.
    stext:VkStructureType).
  * The name of the new enumeration **token** being added (e.g.
    etext:VK_STRUCTURE_TYPE_SWAPCHAIN_CREATE_INFO_KHR).
  * The **offset**, which is an integer between 0 and 999 relative to the
    base being used for the extension.
  * The **direction** may be specified to indicate a negative value
    (+dir="-"+) when needed for negative etext:VkResult values indicating
    errors, like etext:VK_ERROR_SURFACE_LOST_KHR. The default direction is
    positive, if not specified.

Implicit is the registered number of an extension, which is used to create a
range of unused values offset against a global extension base value.
Individual enumerant values are calculated as offsets in that range. Values
are calculated as follows:

  * base_value = 1000000000
  * range_size = 1000
  * enum_offset(extension_number,offset) = base_value +
    (extension_number - 1) × range_size + offset
  * Positive values: enum_offset(extension_number,offset)
  * Negative values: -enum_offset(extension_number,offset)

The exact syntax for specifying extension enumerant values is defined in the
+readme.pdf+ specifying the format of +vk.xml+, and extension authors can
also refer to existing extensions for examples.


[[extensions-reserving-bitmask-values]]
=== Reserving Bitmask Values

Enumerants which define bitmask values are a special case, since there are
only a small number of unused bits available for extensions. For core Vulkan
API and KHR extension bitmask types, reservations must be approved by a
vote of the Vulkan Working Group. For EXT and vendor extension bitmask
types, reservations must be approved by the listed contact of the
extension. Bits are not reserved, and must not be used in a published
implementation or specification until the reservation is merged into
<<extensions-api-registry,+vk.xml+>> by the registry maintainer.


== Required Extension Tokens

In addition to any tokens specific to the functionality of an extension,
all extensions must define two additional tokens.

  * +VK_EXTNAME_SPEC_VERSION+ is an integer constant which is the revision of
    the extension named +VK_extname+ (+EXTNAME+ is all upper-case, while
    extname is the capitalization of the actual extension name) in
    +vulkan.h+. This value begins at 1 with the initial version of an
    extension specification, and is incremented when significant changes
    (bugfixes or added functionality) are made. Note that the revision of an
    extension defined in +vulkan.h+ and the revision supported by the
    Vulkan implementation (the pname:specVersion field of the
    slink:VkExtensionProperties structure corresponding to the extension and
    returned by one of the <<extended-functionality-extensions,extension
    queries>>) may differ. In such cases, only the functionality and
    behavior of the lowest-numbered revision can be used.
  * +VK_EXTNAME_EXTENSION_NAME+ is a string constant which is the name of the
    extension.

For example, for the WSI extension +VK_KHR_surface+, at the time of writing
the following definitions were in effect:

[source,c]
---------------------------------------------------
#define VK_KHR_SURFACE_SPEC_VERSION 24
#define VK_KHR_SURFACE_EXTENSION_NAME "VK_KHR_surface"
---------------------------------------------------


== Extension Objects, Enums, and Typedefs

Expanding on previous discussion, extensions can add values to existing
enums; and can add their own commands, enums, typedefs, etc. This is done
by adding to <<extensions-api-registry,+vk.xml+>>. All such additions will
be included in the +vulkan.h+ header supplied by Khronos.

[NOTE]
.Note
====
Application developers are encouraged to be careful when using +switch+
statements with Vulkan API enums. This is because extensions can add new
values to existing enums. The use of a +default:+ statement, within a
+switch+, may avoid future compilation issues.
====


[[extension-function_prototypes]]
== Extension Function Prototypes

Function pointer declarations and function prototypes for all core Vulkan
API commands are included in the +vulkan.h+ file. These come from the
official XML specification of the Vulkan API hosted by Khronos.

Function pointer declarations are also included in the +vulkan.h+ file for
all commands defined by registered extensions. Function prototypes for
extensions may be included in +vulkan.h+. Extension commands that are part
of the Vulkan ABI must be flagged in the XML. Function prototypes will
be included in +vulkan.h+ for all extension commands that are part of the
Vulkan ABI.

An extension can be considered platform specific, in which case its
interfaces in +vulkan.h+ are protected by #ifdefs. This is orthogonal to
whether an extension command is considered to be part of the Vulkan ABI.

The initial set of WSI extension commands (i.e. for +VK_KHR_surface+,
+VK_KHR_swapchain+, and +VK_KHR_*_surface+) are considered to be part of the
Vulkan ABI. Function prototypes for these WSI commands are included in
the +vulkan.h+ provided by Khronos, though the platform-specific portions of
+vulkan.h+ are protected by #ifdefs.

[NOTE]
.Note
====
Based on feedback from implementers, Khronos expects that the Android,
Linux, and Windows Vulkan SDKs will include our +vulkan.h+ and export
the supported WSI functions for those platforms from their loader
libraries. Other implementations can make different choices for their
headers and loader libraries, but are encouraged to be consistent with
these implementations.
====


== Accessing Extension Functions from Programs

flink:vkGetInstanceProcAddr and flink:vkGetDeviceProcAddr can be used in
order to obtain function pointer addresses for core and extension commands
(per the description in the ``Command Function Pointers'' section of the
<<Vulkan API Specification>>). Different Vulkan API loaders can choose to
statically
export functions for some or all of the core Vulkan API commands, and
can statically export functions for some or all extension commands. If a
loader statically exports a function, an application can link against that
function without needing to call one of the ftext:vkGet*ProcAddr commands.

[NOTE]
.Note
====
The Vulkan API loader for Android, Linux, and Windows exports functions for all
core Vulkan API commands, and for a set of WSI extension commands that
are applicable to those operating systems (see Vulkan loader documentation for
the relevant platform/OS for details).  The
WSI functions are considered special, because they are required for many
applications.
====


[[extensions-interactions]]
== Extension Interactions

Extensions modifying the behavior of existing commands should provide
additional parameters by using the pname:pNext field of an existing
structure, pointing to a new structure defined by the extension, as
described in the ``Valid Usage'' section of the
<<Vulkan API Specification>>. Extension
structures defined by multiple extensions affecting the same structure can
be chained together in this fashion. Any structure which can be chained
in this fashion must begin with the following two members:

["source","{basebackend@docbook:c++:cpp}",title=""]
------------------------------------------------------------------------------
VkStructureType        sType;
const void*            pNext;
------------------------------------------------------------------------------

It is in principle possible for extensions to provide additional parameters
through alternate means, such as passing a handle parameter to a structure
with a pname:sType defined by the extension, but this approach is
discouraged and should not be used.

When chaining multiple extensions to a structure, the implementation will
process the chain starting with the base parameter and proceeding through
each successive chained structure in turn. Extensions should be defined to
accept any order of chaining, and must define their interactions with other
extensions such that the results are deterministic. If an extension needs a
specific ordering of its extension structure with respect to other
extensions in a chain to provide deterministic results, it must define the
required ordering and expected behavior as part of its specification.