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From "Carsten Ziegeler" <cziege...@s-und-n.de>
Subject [vote] Versioning Guide
Date Thu, 22 Apr 2004 20:03:29 GMT
Appended is the first version of the versioning guide. I 
incorporated all changes/comments that have been made to
it on the list (at least that's what I hope I did...).

Now, this is a first version that we can expand over time.
In addition some native speakers or people that are able
to write understandable english (so not me) should review
the guide once we added it to our cvs.

So, I think we should vote now for the general tendency
outlined in the guide and not on every detail. If the guide
is accepted, we add it to our CVS, update it, change
details that perhaps might not be appropriate/correct and 
then try to follow it as good as we can.

Here is my +1 (of course) :)

The Cocoon Versioning Manifest (CVM)
------------------------------------
This document covers how the Cocoon project is versioned. 
Since Cocoon is a framework, it is very important to define a stable API for
users and developers. However, we also need to move Cocoon forward,
technologically. To balance these two needs, a strict policy of versioning
is required, which users can rely upon to understand the limitations,
restrictions, and the changes that can occur from one release to the next.

Basics
------
Versions are denoted using a standard triplet of integers: 
MAJOR.MINOR.PATCH. The basic intent is that MAJOR versions are incompatible,
large-scale upgrades of the API. MINOR versions retain usage and extension
compatibility with older minor versions, and changes in the PATCH level are
perfectly compatible, forwards and backwards.

Following the main design principle of Cocoon, the pyramid of contracts, we
distinguish between users and developers of Cocoon.
A very rough distinction between them is that a user writes the application
using Cocoon without coding Java. There is one exception to this rule: flow
script - the java script is also written by the user.
A developer codes Java and provides additional functionality for the user,
so a developer extends Cocoon.

Therefore we distinguish between usage compatibility (user API) and
extension compatibility (developer API). Both compatibility levels cover
some kind of "source" compatibility. Cocoon does not provide binary
compatibility. But as Cocoon is distributed as a source release that you
have to compile anyway, it's saver to compile your own application code (if
any) using the version of Cocoon that your application runs on.
[ADD SOME REASONS WHY WE RECOMMENT RECOMPILATION ANYWAY]

Usage Compatibility
-------------------

'Usage' compatibility guarantees that an application written by a Cocoon
user is compatible. All files developed by a typical Cocoon user like xml
files, sitemaps, stylesheets (elements and namespace declarations) keep on
being picked up by the machinery and are dealt with correctly (sitemap
semantics, generator/transformer-picked up elements, config file
entries...). 
In fact this should cover everything (including flow script) but except own
Java code.

Applications that write against a particular version will remain usage
compatible against later versions, until the major number changes.
Writing an application against a version means that this application does
not use any deprecated API of that version. Therefore minor version changes
are only usage compatible from one minor version to the direct following
one. For example 2.2 is usage compatible to 2.1.
But 2.3 is not necessary usage compatible to 2.1, although it is usage
compatible to 2.2. As long as you don't use deprecated API (and the API you
rely on does not get deprecated), your application is usage compatible
across all minor versions.

Example: 
- a feature is introduced in 2.0 and used by the application.
- it is deprecated in 2.2.
- it will be removed in 2.3.

If the time inbetween two minor releases is very short, then it might happen
that deprecated
parts will be kept in the following minor release and are removed in a
follow-up minor
release. This is in order to ensure that users have time to update their
applications
if they use the deprecated parts.

However, if an application uses an API which has become available in a
particular minor version, it (obviously) will no longer operate against
previous minor versions.

Extension Compatibility
-----------------------

'extension' compatibility guarantees that own extensions to what Cocoon
provides (own Java classes that interface directly with API in the Cocoon
distribution) compile and operate.

Applications that write against a particular version will remain extension
compatible against later versions until the major or the minor number
changes (Please note the difference to the usage compatibility). However,
the Cocoon developers take care that even if the minor number changes, most
of the own code still works and operates properly. Incompatible changes
between minor versions are kept to a minimum. Frequent new releases of
Cocoon ensure that developers have a smooth transition path.

If an interface/class changes in an incompatible way between minor version
changes, the Cocoon documentation will contain a detailed update guide that
contains a solution for every incompatible change.
So following this guide should make the update smoothly.


Deprecation and Exceptions
--------------------------

To continue the Cocoon development and to keep up with the innovations,
parts of Cocoon might get deprecated; this includes parts of the user
API and also parts of the developer API.

If a part of the user API is deprecated, this will be flagged through 
run-time warnings that appear in the logs but remain supported. This
indicates that an upcoming minor (or major) release will no longer 
support this.

If a part of the developer API is deprecated it will be removed with 
the next major, minor or patch release. However, the need for removing
deprecated stuff between two patch releases is really very rare and
will only happen if the cost of keeping it is much higher than the
cost that might occur for updating the application. 

For developers there is one exception to this rule: private API. Cocoon
has some internal classes and interfaces that are not meant to be used
by a Cocoon developer (someone extending Cocoon). These pieces of
Java code are clearly marked in the Javadocs and should not be used.
They might change even between a patch version change in an incompatible
way without providing a workaround!

Blocks and Block Stability
--------------------------
Cocoon currently allows optional functionality to be included or excluded
using a simple system called blocks, in which the functionality is included
or excluded at compile time.

[NB. This is a precursor to a more complete block system which is currently
under development.]

A block can have one of three statuses: unstable, stable or deprecated. An
unstable block has an API that can change without notice. A stable block is
subject to the same versioning process as described in this document.
Similarly, when the entire functionality of a block is deprecated, it will
be handled in the same way as any other deprecated code within Cocoon.


External Libraries
------------------
Cocoon uses a set of external libraries (like for example Avalon, 
Xalan or Xerces). Inbetween any release, even patch releases,
the versions of the external libraries might be updated to any version.
Cocoon only updates external libraries if there are good reasons
for it, like important bug fixes or new features that will be
used by the new Cocoon version.

Therefore if your application is written against a special API of an 
external library it might be that this API of the external library 
changes inbetween two Cocoon versions and therefore your application 
does not work properly anymore (or even does not compile anymore).
Unfortunately, this issue is out of the scope of Cocoon.

The documentation of Cocoon contains a list of the libraries and the
version changes between libraries from one Cocoon release to another.


Examples
--------
Here are some examples to demonstrate the compatibility:

Original Version    New Version    Usage Compatible    Extension Compatible
2.2.3                  2.2.4          Yes                      Yes
2.2.3                  2.3.1          Yes                      No
2.2.3                  3.0.0          No                       No

Note: while some of the cells say "no", it is possible that the versions 
may be compatible, depending very precisely upon the particular APIs 
used by the application.


Versioning and Repositories
---------------------------
Cocoon is very innovative and new features are added very frequently. This
results in new functionality that justify a minor version change on its own.
In addition, to move Cocoon forward technologically, some features have
to be removed/deprecated which of course results in a minor version change
as well.

So, Cocoon will potentially release several versions with minor version
changes in a year and only a few patch releases. Of course, if a patch
release is required, for example if a potential security issue is found
etc., the patch release will be made available asap.

This high innovention has - at least in theory - the price of maintaining
several branches at once in order to be able to apply patches to older
versions. 
To reduce this to a minimum, Cocoon uses one repository per major version.
The HEAD of the repository always contains the latest version. If a new
minor release is required, the version in the repository is directly changed
to this minor version (e.g. from 2.1.4 to 2.2). The current state is
tagged and if the need for a 2.1.5 release arises, a branch is created.

However, due to the compatibility rules explained above, there should only
be a reason for a patch version if major problems occur (like security
issues). In any other case, the Cocoon community expects the applications
to migrate to the new minor version release which should be fairly simple.

In addition Cocoon has a sandbox repository to test new features.


-------------------------------------------------------------------

Carsten 

Carsten Ziegeler 
Open Source Group, S&N AG
http://www.osoco.net/weblogs/rael/


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