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From Michael Busch <busch...@gmail.com>
Subject Proposal for changing Lucene's backwards-compatibility policy
Date Fri, 16 Oct 2009 06:57:37 GMT
Hello Lucene users:

In the past we have discussed our backwards-compatibility policy
frequently on the Lucene developer mailinglist and we are thinking about
making some significant changes. In this mail I'd like to outline the
proposed changes to get some feedback from the user community.

Our current backwards-compatibility policy regarding API changes
states that we can only make changes that break
backwards-compatibility in major releases (3.0, 4.0, etc.); the next
major release is the upcoming 3.0.

Given how often we made major releases in the past in Lucene this
means that deprecated APIs need to stay in Lucene for a very long
time. E.g. if we deprecate an API in 3.1 we'll have to wait until 4.0
before we can remove it. This means that the code gets very cluttered
and adding new features gets somewhat more difficult, as attention has
to be paid to properly support the old *and* new APIs for a quite long
time.

The current policy also leads to delaying a last minor release before
a major release (e.g. 2.9), because the developers consider it as the
last chance for a long time to introduce new APIs and deprecate old ones.

The proposal now is to change this policy in a way, so that an API can
only be removed if it was deprecated in at least one release, which
can be a major *or* minor release. E.g. if we deprecate an API and
release it with 3.1, we can remove it with the 3.2 release.

The obvious downside of this proposal is that a simple jar drop-in
replacement will not be possible anymore with almost every Lucene release
(excluding bugfix releases, e.g. 2.9.0->2.9.1). However, you can be
sure that if you're using a non-deprecated API it will be in the next
release.

Note that of course these proposed changes do not affect
backwards-compatibility with old index formats. I.e. it will still be
possible to read all 3.X indexes with any Lucene 4.X version.

Our main goal is to find the right balance between
backwards-compatibility support for all the Lucene users out there and
fast and productive development of new features.

The developers haven't come to an agreement on this proposal yet.
Potentionally giving up the drop-in replacement promise that Lucene
could make in the past is the main reason for the struggle the developers
are in and why we'd like to ask the user community for feedback to
help us make a decision. After we gathered some feedback here we will
call a vote on the development mailinglist where the committers have
to officially decide whether to make these changes or not.

So please tell us which you prefer as a back compatibility policy for
Lucene:
A) best effort drop-in back compatibility for minor version numbers
(e.g. v3.5 will be compatible with v3.2)
B) best effort drop-in back compatibility for the next minor version
number only, and deprecations may be removed after one minor release
(e.g. v3.3 will be compat with v3.2, but not v3.4)

Note that in any case the changes will take affect *after* the 3.0
release.

On behalf of the Lucene developers,
  Michael Busch

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