Open Stack

Hi folks,

Ability to manage multiple versions of application packages and their
dependencies was always an important item in Murano roadmap, however we
still don't have a clear spec for this feature.
Yesterday we hosted a small design session to come up with a plan on what
can be done in Liberty timeframe to have proper versioning for MuranoPL
classes and packages. Stan Lagun, Kirill Zaitsev and myself participated
offline, some other muranoers joined remotely. Thanks to everybody who
joined us.

TL;DR: it turns out that now we have a clear plan which will allow us to
achieve proper versioning of the packages and classes, and we'll try to
implement the most important parts of it in Liberty.

Here is the detailed outcome of the session:


   1. We'll use the standard Semantic Versioning format
   ('Major.Minor.Patch[-dev-build.label[+metadata.label]]') to version our
   packages: changes which break backwards-compatibility should increment the
   major segment, non-breaking new features increment the minor segment and
   all non-breaking bugfixes increment the patch segment. The developers
   should be carefull with the "new features" part: if you add a new method to
   a class, it may be considered a breaking change if the existing subclasses
   of this class have the same method already declared. We still assume that
   such changes should lead to increment of 'minor' segment, however it is up
   to best judgement of developers in particular case: if the risk of such
   method override is really high it may worth to increment the 'major'
   segment. Proper guideline on the versioning rules will be published closer
   to L release.

   2. A new field 'Version' is introduced into package manifest file which
   should define package version in complete semver format. The field itself
   is optional (so existing apps are not broken), if it is not specified the
   package is assumed to have version '0.0.0'

   3. The existing 'Require' block of Application manifest will be used to
   specify the package dependencies. Currently it is a yaml-based dictionary,
   with the keys set to fully-qualified names of the dependency packages and
   the values set to the version of those dependencies. Currently this block
   is used only for integration with apps stored at apps.openstack.org. It
   is suggested to use this block in the deployment process as well, and
   extend its semantics.
   The version of the dependency specified there should also follow the
   semver notation, however it may be specified in the shortened format, i.e.
   without specifying the 'patch' or 'patch' and 'minior' components. In this
   case the dependency will be specified as a range of allowed versions. For
   example, a dependency version 1.2 will mean a (1.2.0 >= version < 1.3)
   range.
   If the version of a dependency is not specified (like in this existing
   app - [1]) then we assume the version "0" - i.e. the last available
   pre-release version of a package.

   4. Murano core library is also a package which has its own version. The
   current one is assumed to have a version 0.1.0, the one which is going to
   be released in L will be probably called 0.2.0. The lib is still quickly
   evolving, so we are not releasing a 1.0.0 until we are sure that we are not
   going to have any breaking changes anytime soon.
   As with any other package it will be possible to have several versions
   of the Core Library installed in Murano at the same moment of time.

   5. There is no mandatory need to add the the dependency on the core
   library to the "Requires" block of each application, as it is added there
   implicitly. However, this implicit dependency will have a version "0" -
   i.e. will reference the latest pre-release version of the Core Library
   available. So it is still better to pin the core library requirement to a
   particular version to make sure that your app does not break if we
   introduce any breaking change into the core lib.

   6. All classes defined in a package are assumed to have a version
   identical to the version of the package.

   7. Murano Extension Plugins (i.e python packages which declare
   setuptools-entrypoints in 'io.murano.extensions' namespace) also will have
   similar versioning semantics: they will have a fully qualified name
   (defined by the setuptools' package name) and a version (also defined by
   setuptools), an will get an ability to specify their own dependencies if
   needed. From the class-loader perspective the MuranoPL classes defined in
   the plugins are no difference from the classes defined in a regular package.

   8. We are going to store murano packages as Glance V3 Artifacts
   Repository, naturally mapping package's FQN and version to artifact's name
   and version.
   The package dependencies will be stored in Glance as cross-artifact
   dynamic dependencies (i.e. dependencies not on a particular artifact but on
   the last artifact matching the given name and the version range query) as
   soon as that feature is implemented in Glance (currently only static
   dependencies are implemented there). Until that, the dependencies will be
   stored as a regular list of strings, and the Murano engine will process it
   and query Glance to fetch the packages.

   9. In L cycle we are not going to show multiple versions of the same app
   in Murano dashboard: only the last one will be shown if the multiple
   versions are present. This is to minimize the changes at Dashboard side: in
   future releases we'll add the ability to select the proper version.
   The generation of the object model by dynamic UI also remains intact.

   10. However, the structure of the object model isself gets changed: in
   the "?" block of each object two new fields appear: "package" and
   "version", which correspond to the FQN and the version of the package which
   contain the class of the given object. UI leaves these fields as Nones when
   it generates the OM, and the engine computes them in a regular way: queries
   the package repository for the most recent version of a package which
   contains the class with a given name, and saves information about its name
   and version. This values get persisted in an Object Model when it gets
   serialized after the deployment. As a result, the versions of the
   components are fixed once the environment is deployed, so if some packages
   get updated afterwards, the existing components remain pinned to their
   initial version. As a result, the environment may get several components of
   the same type but different versions.

   11. When the Object Model is validated after the deserialization, the
   behavior of "$.class()" contract is changed. During its validation the
   value passed to the appropriate property or argument should be of a type
   which is declared either in a current package (or in the another version of
   the current package, given that the major component of the versions is the
   same) or in one of the packages satisfying the requirements of the current
   one. I.e. it becomes impossible to reference any class from the
   unreferenced package.

   12. When inheriting some other class using the 'Extends' attribute, the
   ancestor class should be defined either in the current package or in one of
   the packages satisfying the requirements of the current one.

   13. (creepy advanced stuff) It may turn out that in case of the multiple
   inheritance a single class will attempt to inherit from two different
   versions of a same class. An exception should be thrown in this case,
   unless there is a possibility to find a version of this class which
   satisfies all parties.

*For example: classA inherits classB from packageX and classC from
packageY. Both classB and classC inherit from classD from packageZ, however
packageX depends on the version 1.2.0 of packageZ, while packageY depends
on the version 1.3.0. This leads to a situation when classA transitively
inherits classD of both versions 1.2 and 1.3. So, an exception will be
thrown. However, if packageY's dependency would be just "1" (which means
any of the 1.x.x family) the conflict would be resolved and a 1.2 would be
used as it satisfies both inheritance chains.*



So, all the above cover most of our present needs for MuranoPL package and
class versioning.
Also, we already have a way which allows us to properly version the format
of MuranoPL language (a "Format" key in application manifests) and
UI-definition files ("Version" key in that files). This basically allows us
to target the packages for a minimum version of Murano / Murano Dashboard.


I hope this rather lengthly email is useful. Stan Lagun has taken an action
item to frame all the above into a more formal spec.



[1]
https://github.com/openstack/murano-apps/blob/master/MySQL/package/manifest.yaml#L26


--
Regards,
Alexander Tivelkov
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